Assertions in Managed Code


For the latest documentation on Visual Studio 2017 RC, see Visual Studio 2017 RC Documentation.

An assertion, or Assert statement, tests a condition, which you specify as an argument to the Assert statement. If the condition evaluates to true, no action occurs. If the condition evaluates to false, the assertion fails. If you are running with a debug build, your program enters break mode.

Asserts in the System.Diagnostics Namespace

The Debug.Assert method

Side effects of Debug.Assert

Trace and Debug Requirements

Assert arguments

Customizing Assert behavior

Setting assertions in configuration files

In Visual Basic and Visual C#, you can use the Assert method from either Debug or Trace, which are in the System.Diagnostics namespace. Debug class methods are not included in a Release version of your program, so they do not increase the size or reduce the speed of your release code.

C++ does not support the Debug class methods. You can achieve the same effect by using the Trace class with conditional compilation, such as #ifdef DEBUG... #endif.

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Use the Debug.Assert method freely to test conditions that should hold true if your code is correct. For example, suppose you have written an integer divide function. By the rules of mathematics, the divisor can never be zero. You might test this using an assertion:

Function IntegerDivide(ByVal dividend As Integer, ByVal divisor As Integer) As Integer  
    Debug.Assert(divisor <> 0)  
    Return CInt(dividend / divisor)  
End Function  

When you run this code under the debugger, the assertion statement is evaluated, but in the Release version, the comparison is not made, so there is no additional overhead.

Here is another example. You have a class that implements a checking account, as follows:

Dim amount, balance As Double  
balance = savingsAccount.balance  
Debug.Assert(amount <= balance)  

Before you withdraw money from the account, you want to make sure that the account balance is sufficient to cover the amount you are preparing to withdraw. You might write an assertion to check the balance:

Dim amount, balance As Double  
balance = savingsAccount.balance  
Trace.Assert(amount <= balance)  

Note that calls to the Debug.Assert method disappear when you create a Release version of your code. That means that the call that checks the balance disappears in the Release version. To solve this problem, you should replace Debug.Assert with Trace.Assert, which does not disappear in the Release version:

Calls to Trace.Assert add overhead to your Release version, unlike calls to Debug.Assert.

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When you use Debug.Assert, make sure that any code inside Assert does not change the results of the program if Assert is removed. Otherwise, you might accidentally introduce a bug that only shows up in the Release version of your program. Be especially careful about asserts that contain function or procedure calls, such as the following example:

' unsafe code  
Debug.Assert (meas(i) <> 0 )  

This use of Debug.Assert might appear safe at first glance, but suppose the function meas updates a counter each time it is called. When you build the Release version, this call to meas is eliminated, so the counter does not get updated. This is an example of a function with a side effect. Eliminating a call to a function that has side effects could result in a bug that only appears in the Release version. To avoid such problems, do not place function calls in a Debug.Assert statement. Use a temporary variable instead:

temp = meas( i )  
Debug.Assert (temp <> 0)  

Even when you use Trace.Assert, you might still want to avoid placing function calls inside an Assert statement. Such calls should be safe, because Trace.Assert statements are not eliminated in a Release build. However, if you avoid such constructs as a matter of habit, you are less likely to make a mistake when you use Debug.Assert.

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If you create your project using the Visual Studio wizards, the TRACE symbol is defined by default in both Release and Debug configurations. The DEBUG symbol is defined by default only in the Debug build.

Otherwise, for Trace methods to work, your program must have one of the following at the top of the source file:

  • #Const TRACE = True in Visual Basic

  • #define TRACE in Visual C# and C++

Or your program must be built with the TRACE option:

  • /d:TRACE=True in Visual Basic

  • /d:TRACE in Visual C# and C++

If you need to use the Debug methods in a C# or Visual Basic Release build, you must define the DEBUG symbol in your Release configuration.

C++ does not support the Debug class methods. You can achieve the same effect by using the Trace class with conditional compilation, such as #ifdef DEBUG... #endif. You can define these symbols in the <Project> Property Pages dialog box. For more information, see Changing Project Settings for a Visual Basic Debug Configuration or Changing Project Settings for a C or C++ Debug Configuration.

Trace.Assert and Debug.Assert take up to three arguments. The first argument, which is mandatory, is the condition you want to check. If you call Trace.Assert(Boolean) or Debug.Assert(Boolean) with only one argument, the Assert method checks the condition and, if the result is false, outputs the contents of the call stack to the Output window. The following example shows Trace.Assert(Boolean) and Debug.Assert(Boolean):

Debug.Assert(stacksize > 0)  
Trace.Assert(stacksize > 0)  

The second and third arguments, if present, must be strings. If you call Trace.Assert or Debug.Assert with two or three arguments, the first argument is a condition. The method checks the condition and, if the result is false, outputs the second string and third strings. The following example shows Debug.Assert(Boolean, String) and Trace.Assert(Boolean, String) used with two arguments:

Debug.Assert(stacksize > 0, "Out of stack space")  
Trace.Assert(stacksize > 0, "Out of stack space")  

The following example shows Assert and Assert:

Debug.Assert(stacksize > 0, "Out of stack space. Bytes left:" , Format(size, "G"))  
Trace.Assert(stacksize > 0, "Out of stack space. Bytes left:" , Format(size, "G"))  
Trace.Assert(stacksize > 0, "Out of stack space. Bytes left:", "inctemp failed on third call" )  

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If you run your application in user-interface mode, the Assert method displays the Assertion Failed dialog box when the condition fails. The actions that occur when an assertion fails are controlled by the Listeners or Listeners property.

You can customize the output behavior by adding a TraceListener object to the Listeners collection, by removing a TraceListener from the Listeners collection, or by overriding the TraceListener.Fail method of an existing TraceListener to make it behave differently.

For example, you could override the TraceListener.Fail method to write to an event log instead of displaying the Assertion Failed dialog box.

To customize the output in this way, your program must contain a listener, and you must inherit from TraceListener and override its TraceListener.Fail method.

For more Information, see Trace Listeners.

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You can set assertions in your program configuration file as well as in your code. For more information, see Trace.Assert or Debug.Assert.

Debugger Security
Tracing and Instrumenting Applications
How to: Compile Conditionally with Trace and Debug
C#, F#, and Visual Basic Project Types
Debugging Managed Code