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Performance Optimization in Visual Basic .NET

Visual Studio .NET 2003
 

Gordon Brown
Microsoft Corporation

September 2002

Summary: This article is intended to help developers improve the run-time performance of their Visual Basic .NET applications. (21 printed pages)

Contents

Introduction
Data Types
Declarations
Operations
Procedures
Other Optimizations
Conclusions

Introduction

One of the primary goals of Visual Basic® .NET is to perform faster than previous versions. But performance still depends on how you design your program. This article describes some important considerations that can help you optimize your application's performance. The remainder of this introduction reviews the conditions and assumptions underlying these optimization recommendations.

Intermediate Language

Visual Basic .NET and C#™ both compile to Microsoft intermediate language (MSIL). Equivalent source code in the two languages usually compiles to the same MSIL code and results in the same performance for your application. Performance should not be a criterion in choosing between the two languages. For more information, see Compiling to MSIL.

Execution Frequency

Some of the recommendations in this article might represent a negligible difference within one statement, but the performance gain can be greatly amplified inside a loop or a frequently called procedure. Therefore, code blocks that are executed many times are good candidates for optimization.

Bottlenecks

The most productive approach to optimization is to identify the bottlenecks, or slow places, in your application and work to improve them. Common bottlenecks are long loops and accesses to databases. Micro-optimizing every expression and procedure call is not an efficient use of development effort.

Application Dependency

Performance is highly dependent on the characteristics of each individual application. There are no guarantees. The recommendations in this article are guidelines only. It is possible that you might need to make some adjustments to optimize your particular application.

Platform

Visual Studio® .NET is optimized for the recommended system hardware configuration, both for the integrated development environment (IDE) and for the runtime. If you have less than the recommended amount of RAM, your performance is likely to suffer. This is especially true if you are running large or multiple applications. For more information, see Locating Readme Files to confirm that your system satisfies the prerequisites.

Testing Conditions

Some specialized testing for this article was performed using Visual Studio .NET 2002 on Microsoft® Windows® 2000 Professional, running on a 600 MHz Pentium III with 256 MB of RAM. The specialized testing consisted of tight loops that did nothing more than exercise the code elements being compared. In other words, the loops contained the code elements and nothing else. Therefore, the timing differences represent the most extreme cases, and you should not expect such large differences in a normal application.

Preliminary Information

This article is based on preliminary information. The recommendations are subject to change as experience provides updated and more refined information. Also, some of the underlying considerations might change in future releases.

Data Types

The data types you choose for your variables, properties, procedure arguments, and procedure return values can affect the performance of your application.

Value Types and Reference Types

Value types hold their data within their own allocated memory. Because each instance of a value type is isolated and cannot be accessed through more than one variable, value types can be held and managed on the stack.

Reference types hold only a pointer to the memory location that stores the data. Because more than one variable can point to the data, reference types must be held and managed on the heap.

Heap management is harder on performance than stack allocation. There is an overhead for heap allocation, object access, and garbage collection (GC). This means that a small value type is a better choice than a reference type when you do not need the flexibility of a reference type. However, value types lose this advantage as they become larger. For example, it takes more time to make an assignment with a five-byte value type than with a reference type.

The performance effect of data type choice can range from unnoticeable to as much as 30 percent in favor of small value types. Note that it also depends on other factors such as hardware platform, system loading, and data size. For more information, see Value Types and Reference Types.

Object Type and Late Binding

A reference type variable declared as Object can point to data of any data type. This flexibility, however, can compromise performance because an Object variable is always late bound.

A reference type variable is early bound if it is declared to be of a specific class, such as Form. This allows the Visual Basic compiler to perform certain optimizations at compile time, such as type checking and member lookup. When you access members on an early-bound object variable at run time, the compiler has already done much of the management work.

A variable is late bound if it is declared as type Object or without an explicit data type. When your code accesses members on such a variable, the common language runtime is obliged to perform type checking and member lookup at run time. Note that, if Option Explicit is Off, a variable declared without an explicit data type is of type Object.

Early-bound objects have significantly better performance than late-bound objects. They also make your code easier to read and maintain, and they reduce the number of run-time errors. This means you should declare your object variables using a specific class type whenever you know it at development time. For more information, see Early and Late Binding.

You should avoid using the Object type when it is not necessary. In addition to being subject to late binding, Object variables that point to data of a value type consume additional memory, both for the pointer and for an additional copy of the data. For more information, see Object Type.

Data Type Width

The most efficient data types are those that use the native data width of the run-time platform. On current platforms, the data width is 32 bits, for both the computer and the operating system.

Consequently, Integer is currently the most efficient data type in Visual Basic .NET. Next best are Long, Short, and Byte, in that order of efficiency. You can improve the performance of Short and Byte by turning off integer overflow checking, for example by setting the RemoveIntegerChecks property, but this incurs the risk of incorrect calculations due to undetected overflows. You cannot toggle this checking on and off during run time; you can only set its value for the next build of your application.

If you need fractional values, the best choice is Double, because the floating-point processors of current platforms perform all operations in double precision. Next best are Single and Decimal, in that order of efficiency.

Units of Representation

In some cases your application can use integral data types (Integer, Long, Short, Byte) in place of fractional types. This is often true when you have a choice of units. For example, if you are representing the size of an image, you can use pixels instead of inches or centimeters. Because the number of pixels in an image is always a whole number, you can store it in an Integer variable. If you chose inches, you would probably need to deal with fractional values, so you would need a Double variable, which is not as efficient. The smallest unit of representation can usually be integral, whereas the larger units are typically fractional. If you perform many operations with these units, an integral data type can improve performance.

Boxing and Unboxing

Boxing is the extra processing the common language runtime must do when you treat a value type as a reference type. Boxing is necessary, for example, if you declare an Integer variable and then assign it to an Object variable or pass it to a procedure that takes an Object argument. In this case, the common language runtime must box the variable to convert it to type Object. It copies the variable, embeds the copy in a newly allocated object, and stores its type information.

If you subsequently assign the boxed variable to a variable declared as a value type, the common language runtime must unbox it, that is, copy the data from the heap instance into the value type variable. Furthermore, the boxed variable must be managed on the heap, whether or not it is ever unboxed.

Boxing and unboxing cause very significant performance degradation. If your application frequently treats a value type variable as an object, it is better to initially declare it as a reference type. An alternative is to box the variable once, retain the boxed version as long as it is being used, and then unbox it when the value type is needed again.

You can eliminate inadvertent boxing by setting Option Strict On. This helps you find places where you unintentionally box a value type, and it forces you to use explicit conversion, which is often more efficient than boxing. Note, however, that you cannot bypass boxing by using explicit conversion. CObj(<value type>) and CType(<value type>, Object) both box the value type.

Arrays

Avoid using a larger rank than necessary. The fewer dimensions an array has, the more efficiently it performs. The difference is most significant between one- and two-dimensional arrays, because the common language runtime optimizes for one dimension.

Jagged arrays (arrays of arrays) are currently more efficient than rectangular (multidimensional) arrays. In other words, A(9)(9) performs more efficiently than A(9,9). This is because jagged arrays can profit from the optimization for one-dimensional arrays. The difference can exceed 30 percent.

Note that jagged arrays are not compliant with the common language specification (CLS). This means you should not expose jagged arrays from any class you want CLS-compliant code to consume. For more information, see Arrays.

ArrayList

The ArrayList class in the System.Collections namespace supports a dynamic array, which changes its size as required. To use it, you declare a variable with the ArrayList data type instead of using the standard array declaration. You can then call its Add, AddRange, Insert, InsertRange, Remove, and RemoveRange methods to add and delete elements.

If your array changes size frequently and you need to retain the values of existing elements, the ArrayList object can give you better performance than the ReDim statement with the Preserve keyword. The disadvantage of ArrayList is that all its members are of type Object and are therefore late bound. Whether the advantage over ReDim compensates for the disadvantage of late binding depends on your individual application. You should be prepared to try both approaches and compare performance. For more information, see ArrayList Class.

Declarations

As previously discussed in the Object Type and Late Binding section, early binding, which is faster than late binding, also provides better error checking. You should declare your object variables using the most specific, suitable class type. As an example of this, consider the partial inheritance hierarchy of these classes in the System.Windows.Forms namespace:

Object
   Control
      Button
      Label
      Form

Suppose you use an object variable in such a way that every object you assign to it is a Control, and most but not all are of type Form. Although Form is a more specific type than Control, you cannot declare the variable as type System.Windows.Forms.Form, because it might need to take some objects of type Button or Label. You should declare the variable as type System.Windows.Forms.Control, because that is the most specific type that can accept every object assigned to it. For more information, see System.Windows.Forms Namespace.

Properties, Variables, and Constants

Variables perform faster than properties. A variable access generates a simple memory fetch or store. A property access requires a call to the Get or Set method on that property, which sometimes does extra processing in addition to fetching or storing the value. Note that the compiler implements WithEvents variables as properties, so they do not share the performance advantage of other variables over properties.

Constants perform faster than variables because their values are compiled into the code. A constant access does not even require a memory fetch, except for constants of type Date or Decimal.

Option Settings

Option Explicit On forces you to declare all your variables, which makes your code easier to read and maintain. Be sure to use the As clause in every declaration, including procedure arguments. If you do not specify As, your variables and arguments take data type Object, which is usually not the optimal type. Using As improves performance because it moves type inference from run time to compile time. For more information, see Option Explicit Statement.

Option Strict On disallows implicit narrowing, requires the As clause in every declaration, and disallows late binding regardless of the Option Explicit setting. You can still perform narrowing type conversions, but you must use explicit conversion keywords such as CInt and CType. Explicit declaration improves performance because it protects your code from inadvertent late binding. For more information, see Option Strict Statement.

Option Compare Binary specifies that strings are to be compared and sorted based on the binary representation of their characters, without considering equivalent characters such as uppercase/lowercase pairs. You should use binary comparison whenever your application's logic permits it. It improves performance because the code does not need to deal with case insensitivity, or with groups of characters considered alphabetically the same in a given culture.

Object Collections and Object Arrays

When you have a set of related objects that you handle similarly, you can put them in an array of objects, or you can create a collection with the objects as members. The following considerations can help you choose between these schemes:

  • The common language runtime can optimize the code for an array, while every access into a collection requires one or more method calls. Therefore, arrays are usually preferable when they support all the operations you need to perform.
  • For indexed accesses, arrays are never slower, and are usually faster, than collections.
  • For keyed accesses, you should use a collection. Arrays do not support access using a key field, so you would have to write code to search through the elements of the array for the key.
  • For insertions and deletions, collections are usually preferable. Arrays do not directly support adding and removing elements. If you are inserting or deleting at the end of an array, you must use the ReDim statement, which reduces performance. To insert or delete anywhere else, you must use an ArrayList object instead of a standard array. By contrast, insertions and deletions are straightforward operations in a collection, and they are equally fast regardless of the position of the elements involved.

Disk File I/O

Visual Basic .NET offers three principal ways of accessing disk files:

  • Traditional Visual Basic runtime functions such as FileOpen and WriteLine
  • The FileSystemObject (FSO) object model of the Microsoft Scripting Runtime
  • The .NET Framework object model in the System.IO namespace

The traditional runtime file functions are provided for compatibility with earlier versions of Visual Basic. FSO is provided for compatibility with scripting languages, and for applications that require its functionality. Each of these models is implemented as a set of wrapper objects that call members of classes in the System.IO namespace. Therefore, they are not as efficient as using System.IO directly. In addition, FSO requires your application to implement COM interop, which incurs marshaling overhead.

You can improve your performance by using System.IO classes such as the following:

  • Path, Directory, and File for processing at the drive and folder level
  • FileStream for general reading and writing
  • BinaryReader and BinaryWriter for files with binary or unknown data
  • StreamReader and StreamWriter for text files
  • BufferedStream to supply buffering for an I/O stream

In general, FileStream provides the most efficient disk performance. It does its own buffering and its own disk operations, without wrapping anything other than the Windows I/O procedures. However, if disk I/O is not a bottleneck in your application, one of the other classes might be more convenient. For example, you might prefer StreamReader and StreamWriter if you are dealing only with text and the disk performance is not critical.

Buffers

Declare your buffers to be of reasonable size, particularly when using BufferedStream or FileStream. Usually the ideal size is a multiple of 4 KB, although this could vary depending on the application. Buffers smaller than 4 KB can degrade performance by causing too many I/O operations for a given amount of data. Buffers that are too large can consume more memory than necessary to achieve a given performance improvement. Depending on the hardware, the reasonable upper limit can vary from 8 KB to 64 KB.

Asynchronous I/O

If your application is disk bound, you can take advantage of the disk latency to perform other tasks while waiting for an I/O transfer to complete. To do this, use the asynchronous I/O available with the FileStream class. This approach can require more source code, but it optimizes your run-time performance because some of the code executes while a read or write operation is in progress. If you are transferring large amounts of data, or if your disk latency is significant, the improvement can be considerable.

Operations

Integer arithmetic is much faster than floating point or decimal. In calculations where you do not need decimal points or fractional values, declare all your variables and constants as integral data types, preferably Integer. Keep in mind, however, that converting them to and from floating point degrades performance.

Operators

When dividing integral values, use the integer division (\ operator) when you only need the quotient and not the remainder. The \ operator can be more than ten times as fast as the / operator.

Assignment Operators

Assignment operators such as += are more concise than their constituent operators (separate = and +), and they can make your code easier to read. If you are operating on an expression instead of a simple variable, for example an array element, you can achieve a significant performance improvement with assignment operators. This is because the expression, for example MyArray(SubscriptFunction(Arg1, Arg2)), has to be evaluated only once.

Concatenations

You should use the concatenation operator (&) instead of the plus operator (+) to concatenate strings. They are equivalent only if both operands are of type String. When this is not the case, the + operator becomes late bound and must perform type checking and conversions. The & operator is designed specifically for strings.

Boolean Tests

When you test a Boolean variable in an If statement, it is easier to read if you specify only the variable in the test rather than using the = operator to compare it to True. Although there is no significant performance difference, the second test in the following example represents better programming practice:

If BoolVariable = True Then   ' Unnecessary specification of True.
' ...
End If
If BoolVariable Then   ' More compact.
' ...
End If

Short-Circuiting

When possible, you should use the short-circuiting Boolean operators, AndAlso and OrElse. These can save time by bypassing the evaluation of one expression depending on the result of the other. In the case of AndAlso, if the result of the expression on the left is False, the final result is already determined and the expression on the right is not evaluated. Similarly, OrElse bypasses the expression on the right if the one on the left evaluates to True. Note also that the Case statement can short-circuit a list of multiple expressions and ranges, if it finds a match value before the end of the list.

Member Access

Some member accesses (. operator) call a method or a property that returns an object. In the System.Windows.Forms.Form class, for example, the Controls property returns a ControlCollection object. Such an access entails object creation, heap allocation and management, and garbage collection (GC). If you make this kind of member access in a loop, you create a new object every time, resulting in slower performance. If your intention is to deal with the same object in each loop iteration, this might also be a logic error, because every access of this type creates a different object.

Avoiding Requalification

If you make many references to members of an element that is qualified, such as MyForm.Controls.Item(Subscript), you can improve performance by using the With ... End With construction:

With MyForm.Controls.Item(Subscript)   ' Evaluate the qualification once.
   .Name = "Control number " & CStr(Subscript)
   .Text = .Name
   ' Access other members of MyForm.Controls.Item(Subscript)
   .Refresh()
End With

The preceding code evaluates MyForm.Controls.Item(Subscript) only once. It can run more than twice as fast as requalifying every member access. However, if the element is not qualified, for example AnswerForm or Me, there is no performance improvement using With ... End With. For more information, see With...End With Statements.

Type Conversions

Two class types have an inheritance relationship when one is derived from the other. If you have objects of each of these types and you need to convert one to the type of the other, you can use the DirectCast keyword instead of the CType keyword. DirectCast can have somewhat better performance because it does not use run-time helper functions. Note that DirectCast throws an InvalidCastException error if there is no inheritance relationship between the two types. For more information, see DirectCast.

Property Caching

If you repeatedly access a property, for example within a loop that is executed a large number of times, you can improve performance by caching the property value. To do this, you assign the property value to a variable before entering the loop, and you then use the variable during the loop. If necessary, you can assign the variable value back to the property when the loop has completed.

If accesses to repeatedly used properties represent a significant part of the code in the loop, caching them can allow your loop to run as much as three times as fast.

You might not be able to cache a property, however, if its Get and Set methods do extra processing in addition to fetching or storing the value.

Exception Handling

Traditional Visual Basic error handling uses the On Error GoTo and On Error Resume Next statements. These are not always easy to design, and the resulting source code is often convoluted and difficult to read and maintain.

Visual Basic .NET offers structured exception handling with the Try ... Catch ... Finally statements. This is based on a control structure that is flexible and easy to read. Structured exception handling can check a given block of code for several different exceptions and handle each one differently.

Both approaches carry some performance overhead. Using the On Error Resume Next statement obliges the compiler to generate additional intermediate language (IL) for every source statement in the block following the On Error Resume Next statement. A Catch block changes the state of the Err object on entry and on exit. Preliminary testing indicates that the performance is roughly equivalent for both approaches when the block has fewer than 20 source statements. However, blocks of several hundred statements should perform better with Try and Catch, because On Error Resume Next generates more IL for otherwise identical source code.

If you do not have a compelling reason to use On Error statements, you should use structured exception handling. For more information, see Structured Exception Handling.

Throwing Exceptions

Although structured exception handling is useful, you should use it exclusively for exceptions. An exception is not necessarily an error, but it should be something that happens infrequently and is not expected in normal operation. Throwing exceptions takes more processing time than testing and branching, for example using a Select construction or a While loop. Exceptions also make your code harder to read when used for normal flow control. You should not use them as a way of branching or returning values.

Catching Exceptions

Try ... Catch ... Finally constructions incur very little performance overhead unless an exception is thrown. In other words, creating an exception handler does not degrade performance, and you should not hesitate to use structured exception handling when you expect the exception to happen rarely.

Strings

Instances of the String class are immutable. Consequently, every time you change a String variable, you leave the existing String object allocated and create a new one. This can cause a high memory and performance overhead if you manipulate the same String variable many times. The most common manipulation is concatenation, but String methods such as Insert and PadRight also generate new instances.

StringBuilder

The StringBuilder class in the System.Text namespace supports a mutable string, which retains the same instance after modification. To use it, you declare a string variable with the StringBuilder data type instead of String. You can then call its Append, Insert, Remove, and Replace methods to manipulate the string.

If you do a large number of concatenations or other alterations, the StringBuilder class can perform up to three times as fast as the String data type. If desired, you can use the ToString method to copy the final string data to a String object when your manipulations are finished.

However, if you do not expect to manipulate the same instance very often, String is a better choice. This is because StringBuilder has one-time overhead that String does not. At creation time, the StringBuilder constructor takes more time than the String constructor. At conclusion, you must call ToString in most cases. For more information, see StringBuilder Class.

Concatenation

Sometimes you can combine all your string modifications into a single statement, for example:

MyString = PrefixString & ": " & MyString & " -- " & SuffixString

A statement like the one in the preceding example creates a new string only once, and there is no late binding because it uses the & operator.

When you concatenate string constants in a statement, Visual Basic combines them at compile time. This generates the final, resulting string in the intermediate language (IL), which improves performance at run time. Note that the result of the ChrW function can be used as a constant if its argument is a constant.

String Functions

The Format function does a large amount of checking, type conversion, and other processing, including formatting according to the current culture. If you do not need any of this special functionality, use the appropriate ToString method instead. The ToString methods are faster than the CStr conversion keyword because CStr does additional parsing before calling ToString.

The Asc and Chr functions work with single-byte character set (SBCS) and double-byte character set (DBCS) code points. They must consult the code page for the current thread and then translate characters into and out of Unicode. The AscW and ChrW functions are more efficient, because they work exclusively within Unicode and are independent of the culture and code page settings for the current thread.

Procedures

There is a trade-off between calling a procedure from within a loop and placing the body of the procedure inside the loop. If you include the procedure code inside the loop, you avoid the overhead of the call mechanism. However, other places in your application cannot access that code. Also, if you duplicate the code elsewhere, you make maintenance more difficult, and you run the risk of update synchronization errors.

Defining the procedure outside the loop makes the loop code easier to read, and it also makes the procedure available from other places in your application. The call overhead is not important if the procedure is large. However, the overhead can become very significant if the procedure does only one small task, for example accessing a member of a class object. In such a case, you achieve better performance by simply accessing the member directly inside the loop.

Procedure Size

Procedures are subject to just-in-time (JIT) compilation. A procedure is not compiled until the first time it is called. The JIT compiler attempts to perform a number of optimizations on a procedure while compiling it, such as generating inline code for small procedure calls. Very large procedures are not able to benefit from such optimizations. As a guideline, a procedure containing more than about 1000 lines of code is less likely to profit from JIT optimization.

Calling and Returning

In previous versions of Visual Basic, it could be faster to call a procedure from its own module than from another module, and faster to call a procedure from its own project than from another project. In Visual Basic .NET, neither of these makes a difference, so the location of a procedure in relation to the calling code is not a performance criterion.

Use the Return statement whenever your logic permits it. For more information, see Return Statement. The compiler can optimize the code better than if you use Exit Function, Exit Property, or Exit Sub, or allow the End Function, End Get, End Set, or End Sub statement to generate a return.

Virtual Calls

A virtual call is a call to an overridable procedure. Note that this depends only on whether the procedure is declared by using the Overridable keyword, not on whether any overrides have been defined. When you make a virtual call, the common language runtime must inspect the run-time type of the object to determine which override to invoke. By contrast, a nonvirtual call can obtain all the required information from the compile-time type.

From a performance standpoint, virtual calls take approximately twice as much time as nonvirtual calls. This difference is especially pronounced with a value type, for example when you call a procedure on a structure. (A structure cannot declare Overridable members, but it inherits Equals, GetHashCode, and ToString from Object, and it can implement Overridable members of interfaces.) You should define Overridable procedures only when there is a clear architectural advantage, and you should limit your calls as much as possible to NotOverridable procedures.

Procedure Arguments

When you pass an argument to a procedure by using the ByRef keyword, Visual Basic copies only a pointer to the underlying variable, whether that variable is a value type or a reference type. When you pass an argument ByVal, the contents of the underlying variable are copied. For a reference type, these contents consist only of a pointer to the object itself. For a value type, however, they consist of all the variable's data.

The performance difference between ByRef and ByVal is usually insignificant, especially for reference types. For value types, the difference depends on the data width of the type. Most value types have a data width that is about the size of a pointer, and in these cases the performance is equivalent. However, a large value type, such as a lengthy structure, can be more efficient to pass ByRef to avoid copying all the data. On the other hand, when you pass a value type of optimal size (Integer or Double), ByVal can be preferable because the compiler can often optimize the calling code, for example by holding the argument in a register.

Because the performance difference between ByVal and ByRef is usually not important, you can consider other criteria when choosing between the two passing mechanisms. The advantage of passing an argument ByRef is that the procedure can return a value to the calling code by modifying the contents of the variable you pass to the argument. The advantage of passing an argument ByVal is that it protects a variable from being changed by the procedure.

In the absence of a compelling reason to pass an argument ByRef, you should pass it ByVal. For more information, see Argument Passing ByVal and ByRef.

Loops

Minimize the number of loops inside a Try block, and minimize the number of Try blocks inside a loop. A long loop could amplify the overhead of structured exception handling.

There has been speculation regarding whether For, Do, or While loops are more efficient. Preliminary testing does not reveal any significant or consistent difference between them. Therefore, performance is not a consideration when choosing among these types of loops.

Collections

When you traverse a collection, you can use either a For loop or a For Each loop. If you expect to add, delete, or rearrange elements of the collection during the traversal, a For loop can produce more reliable results. It also allows you to determine the order of traversal. Furthermore, a collection derived from the CollectionBase class in the System.Collections namespace throws an exception if you attempt to change its members during a For Each loop.

A For Each loop gives control of the collection to the enumerator object returned by the IEnumerable.GetEnumerator method. This means that you cannot necessarily predict the order of traversal. For Each is useful when you are not able to access a collection's members using the Item property.

The performance difference between For and For Each loops does not appear to be significant.

Adding Members

When you call a collection's Add method, avoid using the Before and After arguments. When you specify a position in the collection with Before or After, you oblige the collection to find another member before it can add your new member.

If you are using a collection that has an AddRange method, use it in preference over Add. AddRange adds an entire list or collection in one call. Several collection classes expose the AddRange method, for example the ArrayList class in the System.Collections namespace and the ComboBox.ObjectCollection class in the System.Windows.Forms namespace.

Threading

If your application spends a high percentage of its time waiting for some of its operations to complete, consider using asynchronous processing, making use of methods of the Thread class in the System.Threading namespace. This can be useful when waiting for user responses, as well as when reading and writing to persistent media. Asynchronous processing and threading entail additional coding, but they can make a significant performance difference.

Be aware, however, that threading does carry overhead and must be used carefully. A thread with a short lifetime is inherently inefficient, and context switching takes a significant amount of execution time. You should use the minimum number of long-term threads, and switch between them as rarely as you can.

Multiple Buffering

If your application reads and writes extensively and you use asynchronous I/O, multiple buffering might be profitable. In multiple buffering, you allocate two or more buffers for a file you are reading or writing. While waiting for I/O to complete in one buffer, you can process the data in another. Multiple buffering is also called swing buffering. It is commonly called double buffering when you use two buffers.

Interprocess Calls

You should minimize interprocess calls, remote calls, and calls across application domains because of the overhead for marshaling. This is particularly true for calls across a COM interop boundary, that is, between managed code and unmanaged code (COM). When you need to make such calls, try to combine them into a few "chunky" calls instead of many "chatty" calls. A "chunky" call performs several tasks, such as initializing all the fields on an object. A "chatty" call does only one short task before returning. For more information, see Programming with Application Domains and Assemblies.

Blittable Data Types

Blittable types, which have the same representation in both managed and unmanaged memory, can be copied across the managed/unmanaged boundary without conversion. When your application makes COM interop calls, try to use only blittable data types for the arguments. The blittable types in Visual Basic .NET are Byte, Short, Integer, Long, Single, and Double. The common language runtime types System.SByte, System.UInt16, System.UInt32, and System.UInt64 are also blittable. For more information, see Blittable and Non-Blittable Types.

Some composite data types can also be blittable. A structure with all blittable members is itself blittable. A class is not automatically blittable even if all its members are, but you can still improve marshaling performance by using only blittable members. You can also set the ExplicitLayout member of the TypeAttributes enumeration in the System.Reflection namespace. This forces class members to be marshaled at the specified offsets without any realignments by the common language runtime.

Marshaling

Marshaling represents a significant part of interprocess calls, and lessening it can improve performance. Blittable data types and explicit member layout are among the most effective ways to minimize marshaling overhead. Using structures instead of classes whenever possible usually speeds up performance.

Managed code can also use the platform invoke functionality to call unmanaged functions implemented in dynamic-link libraries (DLLs), such as those in the Win32 API. To use platform invoke, you declare each external reference with a Declare statement, using the Function and Lib keywords. Calling through Declare statements can be more efficient than calling COM objects. For more information, see Consuming Unmanaged DLL Functions.

Other Optimizations

Compiling and Precompiling

Your source code is compiled to Microsoft intermediate language (MSIL) by the Visual Basic compiler. The MSIL resides in your application's .exe file, which is read by the just-in-time (JIT) compiler of the common language runtime. The JIT compiler normally compiles each procedure's MSIL to the platform's native code the first time that procedure is called.

It can be useful to compile frequently used procedures even before they are called. The integrated development environment (IDE) does this precompilation process for the standard libraries of Visual Basic, and it puts the native code versions in a special section of the global assembly cache (GAC). This saves time by making JIT compilation unnecessary for Visual Basic runtime functions.

In some cases, certain procedures in your application can be good candidates for precompilation. For example, Windows Forms applications typically use many shared libraries and call many procedures at startup time. You might be able to improve performance if you precompile such procedures.

You can precompile parts of your application by using ngen.exe during installation. Note that ngen.exe does not call the JIT compiler, but instead performs its own compilation. For more information, see Native Image Generator (Ngen.exe). You should be aware of the following considerations before you precompile any of your code:

  • You can precompile only client-side applications.
  • You cannot precompile ASP.NET applications at all.
  • Precompilation is intended to improve your application performance only at startup time.
  • The performance might not improve very noticeably. You should try your application both with and without precompilation and compare the timings.
  • Precompilation can slightly reduce the run-time performance of a frequently called procedure. This is because ngen.exe cannot make some of the optimizations that the JIT compiler can make at run time.

DLLs

Loading a dynamic-link library (DLL) takes a considerable amount of execution time. Bringing in a DLL only to call one or two procedures is highly inefficient. You should try to generate the smallest possible number of DLLs, even if this makes them relatively large. This means your application should use as few projects as possible and large solutions.

Debug and Retail Builds

You can improve performance by compiling to a retail build instead of to a debug build. This enables compiler optimizations, which make the resulting intermediate language (IL) smaller and faster. However, these optimizations reorder code, making debugging more difficult. You might want to compile to a debug build while your application is still under development.

Optimizing Display Speed

In a Windows Forms application, sometimes you want to display important forms and controls as quickly as possible. To optimize this, consider the following:

  • Avoid unnecessary repainting of controls. One helpful approach is to hide controls while you are setting their properties.
  • When you need to repaint a control or any display object, try to repaint only the newly exposed areas of an object. This reduces the time the user waits to see a completed display.
  • There is no equivalent in Visual Basic .NET to the Image control of previous versions. You must use PictureBox controls to display most graphics. The AutoRedraw function is also no longer supported. For more information, see Introduction to the Windows Forms PictureBox Control.

Optimizing Perceived Display Speed

When you do not want the user to believe that your application has stopped running, you can try to optimize the perceived display speed. The following suggestions might help:

  • Use progress indicators, such as the ProgressBar control, available in the ProgressBar class in the System.Windows.Forms namespace. This assures the user that your application is still running. For more information, see Introduction to the Windows Forms ProgressBar Control.
  • During short operations that require one second or less, you can turn the mouse pointer into an hourglass by using the MousePointer property of the Masked Edit control.
  • Preload critical data before your application needs it. This includes forms and controls, along with other data items. Although it still takes the same amount of time to load these items, you reduce the time that users wait for them to appear when they need to see them.
  • When you have preloaded forms or controls, keep them hidden. This also minimizes the amount of painting necessary.
  • While your application is waiting for user input, use threads and timers to do small tasks in the background. This can help prepare data for display when the user requests it.
  • It is often useful to maximize the speed of the early displays of your application, that is, those it displays when it first loads. The following points are worth considering:
    • Keep your early forms and controls as simple as possible to reduce loading and initialization time.
    • Call Me.Show as one of the first lines of code in each form load event.
    • Avoid loading modules that are not needed immediately. Be careful to avoid calling procedures that force such premature loads.
  • If your display includes animation or changes a display element often, use double or multiple buffering to prepare the next image while the current one is being painted. The ControlStyles enumeration in the System.Windows.Forms namespace applies to many controls, and the DoubleBuffer member can help prevent flickering.

Reducing Memory Consumption

It is important to keep the amount of your executable code to a minimum. You might also need to reduce the memory requirements of your application's data. Such reductions often improve performance, because smaller executables usually run faster, and eliminating memory swaps increases execution speed. In this respect, you might be able to profit from the following recommendations:

  • Minimize the number of forms that are loaded simultaneously. Delay the loading of a form until it is needed, unless you wish to optimize the perceived display speed. When you are finished with a form, unload it and set the variable to Nothing.
  • Use as few controls as possible on a form. Use the smallest and simplest controls that do what you need. For example, try to use labels instead of text boxes.
  • Keep related procedures in the same module. This minimizes the loading of modules.
  • Avoid using bigger data types than you need, especially in arrays.
  • When you are finished with a variable of a large data type, set it to Nothing. This applies especially to strings, arrays, and other potentially large objects.
  • If you are finished with some elements of an array but need to keep others, use ReDim to release the unneeded memory to garbage collection (GC).

Conclusions

When you are designing and writing a Visual Basic .NET application to optimize performance, the following are important points to keep in mind:

  • Concentrate your optimization efforts on code that runs within loops and frequently called procedures.
  • Find the slowest places in your application and optimize them to achieve performance that is acceptable to the user.
  • Use strongly typed variables, and arrange for early binding on object variables whenever possible.
  • Set Option Strict On and Option Explicit On.
  • Minimize memory usage.
  • Compile to a retail build when you do not need a debug build.
  • Plan your application with large solutions and as few projects as possible.
  • Measure your performance, rather than simply assuming that one technique is more efficient than another.

Disciplined coding, with proper design of the overall logic, is the first step in optimizing performance. Tuning is of little help if your application is not well designed from the top down.

Additional Resources

The following resources, which are part of the Visual Studio .NET documentation, provide further information relating to the concepts and programming elements discussed in this article.

Data Types

Value Types
Describes the concept of value types in the .NET Framework.
Value Type Usage Guidelines
Provides the .NET Framework's general guidelines for proper usage of value types.
Data Types
Defines data type and describes the various types available in Visual Basic .NET.
RemoveIntegerChecks Property
Specifies whether integer arithmetic errors should cause exceptions at run time.
Implicit and Explicit Conversions
Describes implicit and explicit data type conversion, and the conversion keywords.
ReDim Statement
Reallocates storage space for an array variable.

Declarations

Option Compare Statement
Declares the comparison method to use when comparing string data.
Collections in Visual Basic .NET
Provides an overview of collections, discussing the Collection class, zero-based and one-based collections, index and key values, and how to add and remove items.
Processing Drives, Folders, and Files
Introduces the three principal ways of accessing disk files with Visual Basic .NET.
System.IO Namespace
Contains classes and other types that allow synchronous and asynchronous reading and writing on data streams and files.
FileStream Class
Exposes a stream around a file, supporting both synchronous and asynchronous read and write operations.
BufferedStream Class
Implements buffered reading and writing around an existing stream.
Multithreading in Visual Basic .NET
Describes how to perform multiple tasks simultaneously.

Operations

Boolean Expressions
Discusses expressions that evaluate to the Boolean values True or False, short-circuiting operators, and parenthetical expressions.
Logical Operators
Provides information on operators that compare Boolean expressions and return a Boolean result.
ControlCollection Class
Implements a collection container that enables ASP.NET server controls to maintain a list of their child controls.
Err Object
Contains information about run-time errors.
Character Data Types
Describes the Char and String types.
Format Function
Returns a string formatted according to instructions contained in a format style expression.
Asc, AscW Functions
Returns an Integer value representing the character code corresponding to a character.
Chr, ChrW Functions
Returns the character associated with the specified character code.

Procedures

For...Next Statements (Conceptual)
Describes how to execute a block of statements a specified number of times.
For Each...Next Statements (Conceptual)
Describes how to execute a block of statements for each element in a collection.
IEnumerable.GetEnumerator Method
Returns an enumerator for an array to allow reading its data.
Add Method
Adds a member to a Visual Basic .NET Collection object.
Item Property
Returns a specific member of a Visual Basic .NET Collection object either by position or by key.
ComboBox.ObjectCollection Class
Manages the collection of items in a ComboBox, including strings, images, and custom business objects.
AppDomain Class
Implements an application domain, which is an isolated environment where applications execute.
Thread Class
Creates and controls a thread, sets its priority, and gets its status.
System.Reflection Namespace
Contains classes and interfaces that provide a managed view of loaded types and their members, with the ability to dynamically create and invoke types.
TypeAttributes Enumeration
Specifies Visual Basic .NET type attributes that match the CorTypeAttr enumerator as defined in the corhdr.h file.
Declare Statement
Declares references to external procedures in a dynamic-link library (DLL).

Other Optimizations

Optimizations, Configuration Properties, <Projectname> Property Pages Dialog Box
Specifies global settings for your project. These settings act as defaults, controlling the appearance and behavior of the items in your project throughout its life cycle.
Property Pages Build Tab
Allows you to set options such as Option Strict, as well as custom constraints that are in place when your project is initialized.
Property Pages General Tab
Allows you to access and change the name of your project, as well as the root namespace.
MousePointer Property (ActiveX Controls)
Returns or sets a value indicating the type of mouse pointer displayed when the mouse is over a particular part of an object at run time.
ControlStyles Enumeration
Specifies the style and behavior of a control.
Label Class (System.Web.UI.WebControls)
Implements a label control, which displays text on a Web page.
Label Class (System.Windows.Forms)
Implements a standard Windows label, typically used to provide descriptive text for a control.
TextBox Class (System.Web.UI.WebControls)
Constructs a text box, which lets the user enter text, and defines its properties.
TextBox Class (System.Windows.Forms)
Represents a Windows text box control, which allows the user to enter text in an application.

Performance Papers

Performance Considerations for Run-Time Technologies in the .NET Framework
Surveys various technologies at work in the managed world and explains how they impact performance. Coverage includes garbage collection, the JIT compiler, remoting, value types, and security.
Performance Tips and Tricks in .NET Applications
Describes how to fine-tune your managed applications for optimal performance. Includes sample code, design guidelines, and language-specific tips.
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