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In the VS .NET debugger, you can mark any line of code with a breakpoint. When the application runs in the debugger and reaches a line that has a breakpoint, it causes the execution to pause. When the application has paused like this, it is said to be in break mode. With the application in break mode, you can perform a number of activities. For example, you can examine the values of variables and object properties, and you can even change these values. You can also ask the debugger to continue execution one line at a time, stepping through or over subroutines as you desire. Stepping through the code like this is a particularly useful technique, because it allows you to watch the code, observing the values contained in variables object properties, in slow motion, and use this analysis to spot semantic coding errors. To set a breakpoint in the code, click in the gray margin to the left of the line where you want execution to pause, or position the cursor over the line you re interested in and press F9. You ll notice that a red filled circle appears, giving a clear indication that a breakpoint has been set and is enabled at that point, as shown in Figure 11-13.
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CHAPTER 11 DEBUGGING AND EXCEPTION HANDLING
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Figure 11-13. A breakpoint set in code It s often useful to have a few breakpoints in different places in your code. To allow you to manage your breakpoints, the VS .NET debugger provides the Breakpoints window, shown in Figure 11-14. To view the Breakpoints window, select Debug Windows Breakpoints or press Ctrl+Alt+B. Using this window, you can perform the following tasks: Add and remove breakpoints. Enable and disable breakpoints. (A disabled breakpoint will keep its place in the code, but won t pause execution when execution reaches that line of code.) Set the properties of a breakpoint.
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To remove all your breakpoints in a single step, press Ctrl+Shift+F9. Tip
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Figure 11-14. The VS .NET debugger s Breakpoints window In the next few examples, you will get some hands-on experience using the VS .NET debugger to set breakpoints in the Friends Reunion application. Try It Out: Set a Breakpoint in the VS .NET Debugger You ll start by creating a breakpoint that causes the application to pause whenever a request for an already authenticated user is about to be processed.
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CHAPTER 11 DEBUGGING AND EXCEPTION HANDLING
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1. Open the code-behind file for Global.asax. Find the following line in the Application_AuthenticateRequest() method: con = New SqlConnection( _ ConfigurationSettings.AppSettings("cnFriends.ConnectionString")) 2. Set a breakpoint at this line, by clicking the gray margin to the left of the code (adjacent to that line), or by placing your cursor on the line and pressing F9. You should see the breakpoint indicator, as shown in Figure 11-15.
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Figure 11-15. Setting a breakpoint in the Application_AuthenticateRequest() method 3. Press Ctrl+Alt+B to view the details about this breakpoint in the Breakpoints window.
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4. In the Solution Explorer, right-click Default.aspx and select Set As Start Page. 5. To run the application in the debugger, press F5. A new browser instance will open, pointing to the Friends Reunion Login page. Enter the usual credentials (user name and password apress). 6. Click the Login button to submit the login request to the application. This will cause focus to switch to the VS .NET debugger, and you will see that yellow highlighting is used to show the line with the breakpoint on it. Don t stop the application yet; we ll continue from here in the next exercise. But first, let s see what has happened so far.
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CHAPTER 11 DEBUGGING AND EXCEPTION HANDLING
How It Works Because you chose to run the application in the debugger, execution will pause whenever it reaches any line that has an enabled breakpoint. That s exactly what happens here: when the application receives the login credentials, it begins to process them. As part of that process, the application needs to execute the line on which you placed the breakpoint. When the application reaches that line, it does not execute it; instead, it pauses, as shown in Figure 11-16.
Figure 11-16. Execution paused at a breakpoint The debugger highlights the progress it has made by using yellow highlighting (in the code) and a yellow arrow (in the gray area to the left). At this point, the application is in break mode, and the debugger is awaiting further instructions. Note that you can do a lot with a breakpoint by changing its behavior. To view the properties of a breakpoint, select it in the Breakpoints window and click the Properties button in the Breakpoints window, or right-click the line with the breakpoint set and select Breakpoint Properties. You ll see the Breakpoint Properties dialog box, as shown in Figure 11-17. Of particular interest are the Condition and Hit Count buttons at the bottom of this dialog box. Clicking the Condition button allows you to specify a condition that will be evaluated when the breakpoint is hit, and whose result will determine whether or not the execution is paused at that breakpoint. For example, you may want the Friends Reunion application to halt only when a user belonging to the Admin group is logged in. Clicking the Hit Count button allows you to specify the number of times a breakpoint must be hit before it pauses the application.
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