Visual Studio's breakpoints appear pretty trivial at first, but you can create some that have quite a bit of intelligence.
Visual Studio offers a powerful debugger to aid with testing and troubleshooting your applications. One of the most common uses of a debugger is to set breakpoints, which are positions in the code that, when reached, cause the program execution to pause, allowing the developer to inspect the code and state of the program. When a breakpoint is reached and the application suspended, the application is in break mode. In break mode, you, the developer, can examine and change the values of the program variables. With Visual Studio 2005, you can even change the program's code during this time.
In its simplest form, a breakpoint interrupts program execution whenever it is reached. However, Visual Studio allows for more intelligent breakpoints, ones that suspend the program only if a certain condition has been met or if the breakpoint has already been passed over a certain number of times.
Developers often want to suspend their program's execution and enter break mode when a specific line of code is reached, such as the first line of code for a particular method or a line of code inside a conditional statement. The simplest way to add a breakpoint to a particular line of code is to click in the margin for that line of code. As Figure 5-1 shows, this will add a red circle in the margin, indicating that a breakpoint has been set
Clicking in the margin toggles the breakpoint for that line of code, so to remove a breakpoint, click the red circle. The F9 key (Debug.ToggleBreakpoint) will also toggle the breakpoint for the line of code on which the cursor is located.
Breakpoints can also be added through the New Breakpoint dialog. To launch the New Breakpoint dialog, go to the Debug menu and choose the New Breakpoint menu option or press Ctrl-B (Debug.NewBreakpoint). This will display the New Breakpoint dialog shown in Figure 5-2.
The New Breakpoint dialog has four tabs:
Used to add a breakpoint to a specific line of a specific function.
The tab shown in Figure 5-2; adds a breakpoint to a specific line in a specific file in the solution.
Adds a breakpoint to a specific memory address. Commonly used when debugging unmanaged code.
Used to add a breakpoint on a variable. When the variable changes, the breakpoint will be hit. Data breakpoints can be used only when debugging unmanaged code.
To add a breakpoint to a specific line of code, use the File tab. As Figure 5-2 shows, the File tab prompts you for the file and the line number for the breakpoint.
In addition to being able to break on a particular line of code, Visual Studio makes it easy to break when a particular function is invoked. From the Function tab of the New Breakpoint dialogshown in Figure 5-3you can enter the name of the function where the breakpoint should be added.
In order to set the breakpoint, the function name needs to be fully qualified. That is, you need to specify the function name as: namespace.class.methodName. For example, suppose you have a C# Solution named skmDataStructures, which contains, among others, a class named BST with a method named Add. To place a breakpoint at this method, simply type in the function name as skmDataStructures.BST.Add. However, if you type in just the method name or the class name and method name, Visual Studio is smart enough to determine the fully qualified name. In fact, if there are any conflicts, you will be prompted to select to what method(s) to add breakpoints, as shown in Figure 5-4.
You can also configure the debugger to break on a particular function call by using the Call Stack window. When in debug mode, display the Call Stack window by going to the Debug menu's Window submenu and choosing the Call Stack menu option. The Call Stack, as its name implies, lists the functions on the call stack, along with the parameter types and values. To add a breakpoint through the Call Stack window, right-click on the function name and choose the Insert Breakpoint option from the context menu, as shown in Figure 5-5.
Adding breakpoints through the Call Stack window is a handy technique when debugging recursive functions.
The Breakpoints window provides a list of the current breakpoints and allows you to enable or disable breakpoints, delete breakpoints, add new breakpoints, and edit the properties of existing breakpoints. To display the Breakpoints window, go to the Debug menu's Windows submenu and select the Breakpoints option. You can also display this window by pressing Ctrl-Alt-B (Debug.Breakpoints). Figure 5-6 shows the Breakpoints window.
To add a new breakpoint, click the New icon in the upper-left corner, which will display the New Breakpoint dialog. To delete a breakpoint, select the breakpoint and click the Delete icon or delete all breakpoints in one go by clicking the Clear All Breakpoints icon or pressing Ctrl-Shift-F9 (Debug.ClearAllBeakpoints). From the Breakpoints window, you can disable breakpoints without deleting them, either by unchecking a particular breakpoint or by clicking the Disable All Breakpoints icon. To edit a breakpoint, select the breakpoint and click the Properties icon, or right-click on the breakpoint from this window and choose the Properties option. This will display the Breakpoint Properties dialog, which is analogous to the New Breakpoint dialog, but with the selected breakpoint's values shown.
Breakpoints in Visual Studio can be configured to cause the program to enter break mode only when a particular condition holds. To add a condition to a breakpoint, view the breakpoint's properties by selecting the breakpoint from the Breakpoints window and clicking on the Properties icon. From any of the tabs, you will find a button titled Condition. Clicking on this will display the Breakpoint Condition dialog, where you can specify the condition to be watched. Figure 5-7 shows the Breakpoint Condition dialog.
If you specify a break condition, whenever the debugger reaches the breakpoint, it evaluates the condition, and if the condition is met, the program enters break mode. If, however, the condition is not met, then the program continues execution. The break condition specified can be any valid debugger expression, which may include the relational operators <, >, = =, !=, and so on, along with many nonrelational operators, including +, -, &&, ||, and so on. Variables as well as object properties can be used in expressions. For a complete list of valid debugger expressions, refer to:
Finally, the breakpoint condition can be configured so that the program enters break mode when the condition is true or when the condition has changed.
Breakpoints, by default, cause the program to enter break mode whenever they are hit and their condition, if any, is met. However, you can configure a breakpoint to enter break mode based on the breakpoint's hit count. The hit count of a breakpoint is the number of times the breakpoint has been reached and the condition, if specified, has been met. Through the Breakpoint Properties dialog box, you can indicate when a breakpoint should cause the program to enter break mode based on its hit count value.
To configure this information, open up the properties for a breakpoint and click the Hit Count button Clicking on this button will display the Breakpoint Hit Count dialog, shown in Figure 5-8.
From the Breakpoint Hit Count dialog, you can specify how often a breakpoint should cause the program to break. As you can see in Figure 5-8, the default is to break always. You can, however, specify it to break when the hit count equals, is a multiple of, or is greater than or equal to a specified value, as shown in Figure 5-9.
With this setting, the breakpoint won't cause the program to enter break mode until it has been reached five times. The hit count technique is especially useful when you want to place a breakpoint within a loop, but are interested in breaking only in specific cycles of the loop. For example, if you wanted to break only after the loop had iterated a dozen times, you could set the breakpoint to break only when its hit count was greater than or equal to 12. You could also configure the debugger to stop on each tenth iteration of the loop by configuring the breakpoint to break only when its hit count is a multiple of 10.
A large part of the life cycle of a computer program is testing and debugging. Visual Studio's powerful debugger enables developers to rapidly debug applications. One of the most common tools used in debugging are breakpoints. Breakpoints in Visual Studio can be added to specific lines of code or particular functions and can be given conditions on when to break and how often, based on a hit count.
Scott Mitchell