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Tip of the Day
Language: VB.NET
Expertise: Intermediate
Jun 24, 2004

Using Multiple Breakpoints on a Single Code Line

With the release of Microsoft Visual Studio.NET, you can now set multiple breakpoints on a single line.
  • Example 1: Breakpoints on expressions in the for loop statement:
    
       for (InitExpression; ConditionExpression; LoopIterratorExpression;)
          Statement
    
  • Example 2: Breakpoints on expressions separated by the sequential-evaluation operator (,) when used in initializers:
    
       Type var1 = Function(), var2 = FunctionOther();
    
  • Example 3: Breakpoints on expression statements on a single line:
    
       Expression1; Expression2; Expression3;  //style ???
    
For these three cases, place the mouse cursor over the desired expression and press F9 or right click and select the "Insert Breakpoint" item on the pop-up menu.

Each breakpoint can be seen in the Breakpoints window on separate rows. To display the Breakpoints window, press Ctrl-Alt-B on a code line or click the Debug\Windows\Breakpoints submenu.

The "Name" column consists of the file name, line number, and character number that is the first position of the expression for file breakpoints or the qualified method name for function breakpoints. You may see two icons on the left side of this column. One, a filled circle, represents the breakpoint itself and the other is a checkbox that represents the enabled\disabled state. The corresponding expression on the code line is colored, and you'll see a breakpoint glyph (circle) on the left side bar. A colored glyph represents an active (enabled) breakpoint. Only one glyph is shown for any number of breakpoints on a single line. Clicking on this glyph removes all breakpoints on this line. Right clicking on any breakpoint in the Breakpoints window and selecting the "Delete" item on the pop-up menu deletes this breakpoint. If a breakpoint is disabled, the corresponded expression is boxed instead of colored.

Unfortunately, I could not find how to set a breakpoint on the expressions connected with logical AND/OR operators, expressions in the ternary (?:) operators and function calls used in the parameter lists.

  • Example 4: Breakpoints on the same expression or line but with different conditions or count number: More than one breakpoint can be set on the same expression in VS.NET as well as in VS 6.0 (not managed code). These breakpoints will have different conditions and/or count numbers. To do this in VS.NET, place the mouse cursor over the desired expression, right click and select the "New Breakpoint" item on the pop-up menu. You'll see the "New Breakpoint" dialog box. Click the "File" tab to set a file breakpoint. The "File," "Line" and "Character" fields are prefilled. Click on the "Condition" button and provide an expression that should be satisfied ("is true" or "has changed") in order to hit the breakpoint. The default is "no condition"—that means "always satisfied." Also, you can click the "Hit count" button to set a condition based on the hit count value, which will break code execution. The default is "break always." The following is an example of an "is true" condition:
    
          ClassName.MethodReturnBool() && (variableName == aNumber).
    
You should be very careful when using functions in the condition expressions because their execution may have side effects on the logic of your program.

If the specified condition cannot be evaluated to a boolean value, the following error is displayed after starting the debugger (pressing F5):


 "The following breakpoint cannot be set
At YourFileName.cs, line YourFileLineNumber character YourCharNumber
(YourNameSpace.YourClassName.YourMethod(object sender, System EeventArgs e), line
YourMethodLineNumber), when YourAnotherClassName.YourConditionMethod(); is true
Error: managed EE does not understand expressions syntax.
EE here stands for Expression Evaluator, the debugger component that displays variable values. After clicking OK, the second error is displayed:

Execution has stopped at the beginning of the program because one or more breakpoints could not be set.
In this example, the semicolon in the condition expression causes the error in this example. If the conditional expression cannot be executed when it is reached, the following error is displayed:

The following breakpoint cannot be set
At YourFileName.cs, line YourFileLineNumber character YourCharNumber
(YourNameSpace.YourClassName.YourMethod(object sender, System EeventArgs e), line
YourMethodLineNumber), when YourAnotherClassName.NotExistingMethod() is true.
The condition is invalid.
This message is generated when a variable in the condition expression has not come into scope.

If a function breakpoint set for the Function tab is not set on the first line, the following error is displayed after starting the debugger:


The following breakpoint cannot be set
At YourNameSpace.YourClassName.YourMethodName(), line
YourMethodLineNumber (YourFileName.cs, line YourFileLineNumber)
Function breakpoints are only supported on the first line. An exclamation point, representing an error, is displayed in the breakpoint glyph, after a breakpoint related error message is displayed.

A question mark, that represents a warning, is displayed in the breakpoint glyph, when the debugger thinks that code at this location is not loaded. This breakpoint becomes disabled. For example, this glyph is displayed for a function breakpoint that is set on an event handler delegate buttonName_Click() after the debugger has started.

A breakpoint, that stopped the execution, is shown in the bold font style in the Breakpoints window.

Boris Eligulashvili
 
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