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Assertions in Managed Code

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

In Visual Basic and Visual C#, you can use the Assert method from either the Debug class or the Trace class (part of 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. For more information, see Debug Class and Trace Class.

Managed Extensions for C++ does not support the Debug class methods. You can achieve the same effect by using the Trace class with conditional compilation (#ifdef DEBUG... #endif).

The Debug.Assert Method

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:

[Visual Basic .NET]
Function IntegerDivide(ByVal dividend As Integer, ByVal divisor As Integer) As Integer
  Debug.Assert(divisor <> 0)
  Return CInt(dividend / divisor)
End Function
[C#]
int IntegerDivide ( int dividend , int divisor )
  { Debug.Assert ( divisor != 0 );
    return ( dividend / divisor ); }

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

Let's look at another example. Suppose you have a class that implements a checking account. 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:

[Visual Basic .NET]
Dim amount, balance As Double
balance = savingsAccount.balance
Debug.Assert(amount <= balance)
SavingsAccount.Withdraw(amount)
[C#]
float balance = savingsAccount.Balance;
Debug.Assert ( amount <= balance );
savingsAccount.Withdraw ( amount );

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 will disappear in the Release version. To solve this problem, you should replace Debug.Assert with Trace.Assert, which does not disappear in the Release version:

[Visual Basic .NET]
Dim amount, balance As Double
balance = savingsAccount.balance
Trace.Assert(amount <= balance)
SavingsAccount.Withdraw(amount)
[C#]
float balance = savingsAccount.Balance;
Trace.Assert ( amount <= balance );
savingsAccount.Withdraw ( amount );

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

Side Effects of Debug.Assert

When you use Debug.Assert, make sure that any code inside the Assert does not change the results of the program if the Assert is removed. Otherwise, you may 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:

[Visual Basic .NET]
' unsafe code
Debug.Assert (meas(i) <> 0 )
[C#]
// unsafe code
Debug.Assert (meas(i) != 0 );

This use of Debug.Assert may 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 will 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 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:

[Visual Basic .NET]
temp = meas( i )
Debug.Assert (temp <> 0)
[C#]
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.

Trace and Debug Requirements

For Trace methods to work, your program must have on of the following at the top of the source file:

  • #Const TRACE = True in Visual Basic
  • #define TRACE in Visual C# and Managed Extensions for C++

Or your program must be built with the TRACE option:

  • /d:TRACE=True in Visual Basic
  • /d:TRACE in Visual C# and Managed Extensions for C++

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

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.

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

Assert Arguments

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 or Debug.Assert 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 examples show Debug.Assert and Trace.Assert used with one argument.

[Visual Basic .NET]
Debug.Assert(stacksize > 0)
Trace.Assert(stacksize > 0)
[C#]
Debug.Assert ( stacksize > 0 );
Trace.Assert ( stacksize > 0 ); 

For more information, see Debug.Assert Method (Boolean) or Trace.Assert Method (Boolean).

The second and third arguments, if present, must be strings. If you call Trace.Assert or Debug.Assert with two or three arguments (one or two strings), the Assert method checks the condition and, if the result is false, outputs the string or strings. The following examples show Debug.Assert and Trace.Assert used with two or three arguments:

[Visual Basic .NET]
Debug.Assert(stacksize > 0, "Out of stack space")
Trace.Assert(stacksize > 0, "Out of stack space")
[C#]
Debug.Assert ( stacksize > 0, "Out of stack space" );
Trace.Assert ( stacksize > 0, "Out of stack space" ); 

For more information, see Debug.Assert Method (Boolean, String) or Trace.Assert Method (Boolean, String).

[Visual Basic .NET]
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" )
[C#]
Debug.Assert ( stacksize > 100, "Out of stack space" , "Failed in inctemp" );
Trace.Assert ( stacksize > 0, "Out of stack space", "Failed in inctemp" ); 

For more information, see Debug.Assert Method (Boolean, String, String) or Trace.Assert Method (Boolean, String, String).

Customizing Assert Behavior

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 Debug.Listeners or Trace.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 Fail method of an existing TraceListener to make it behave differently. For example, you could override the Fail method to write to an event log instead of displaying the Assertion Failed dialog box. For more information, see TraceListener Class. To customize the output in this way, your program must contain a listener, and you must inherit from the TraceListener and override its Fail method. For more information, see Debug.Fail or Trace.Fail.

Setting Assertions in Configuration Files

If you prefer, you can set an assertion in your program configuration file instead of in your code. For details, see Debug.Assert Method (Boolean) or Trace.Assert Method (Boolean).

See Also

Instrumentation Techniques (Debug and Trace) for the .NET Framework | Conditional Compiling with Trace and Debug | Debug.Assert | Trace.Assert | Debugging Visual C# and Visual Basic Projects | Debugging Managed Code

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