Debugging Exceptions

If an exception is thrown while debugging and 0:00 nothing handles it., the debugger breaks and alerts us to the condition. 0:02 Let's take a deeper look at how Visual Studio helps us to debug exceptions. 0:07 Let's say for example, that we forgot to initialize the song variable up here. 0:12 Now let's start debugging. 0:19 I'll enter option 2 here. 0:23 When the program gets here, it throws an exception 0:24 because songs is null, and we can't call the for each method on a null variable. 0:30 When the debugger pauses as a result of an exception, 0:36 we get this pop up that contains a bunch of additional information. 0:39 Here we can see the exception name, and here, 0:43 right beside additional information, we can see the exception's message. 0:46 It says, object reference not set to an instance of an object. 0:50 Visual Studio provides some debugging tips for 0:54 many of the most common exception types. 0:56 We can see even more information about the exception 0:59 by clicking on View Details down here. 1:01 Here we see the contents of the exception object. 1:05 This often has a lot of really useful information. 1:08 Here we can see if there was an inner exception or 1:11 an exception that caused this exception. 1:14 We can also see the stack trace up to the point where the exception was thrown. 1:17 This is often different than the stack trace that is shown 1:22 in the call stack window. 1:25 Because the exception could have been thrown from external code, and 1:26 where the debugger has paused is just the first place where it is propagated back 1:31 to our code. 1:34 Visual Studio will pause the debugger for all unhandled exceptions. 1:36 But what about exceptions we have handlers for? 1:40 Sometimes we still want to break at the point where these 1:44 exceptions are thrown too. 1:47 Let's wrap this line by adding a try catch around it. 1:48 We'll go ahead and handle all exception types by catching System.exception. 2:00 And I'll just do nothing here. 2:09 I should mention that catching all exception types and 2:13 doing nothing is a really bad programming practice. 2:16 But again, this is just for demonstration purposes. 2:19 Now let's start the debugger and see what happens. 2:22 Again, I'll hit option two here. 2:28 Because we are now handling the exception, 2:31 the debugger no longer breaks when the exception is thrown. 2:33 We can tell it to continue breaking by going into the exception settings window. 2:36 If the exception settings window isn't already open, 2:41 you can open it from the debug menu under Windows. 2:43 You can also often find it down here. 2:46 Here we see many categories of exceptions. 2:48 Because this is a C Sharp program, 2:52 we're mostly interested in the Common Language Runtime exceptions. 2:54 These other exception types don't apply to us right now. 2:58 We can scroll down here and find the null reference exception, 3:01 or we can just type system.null, and it will find it for us quickly. 3:06 By clicking on this checkbox, we're telling Visual Studio to break on this 3:13 exception, even if it is handled at some point in the code. 3:17 Now if we restart the debugger and enter option 2 again, 3:20 we'll see that it breaks on line 42 of program that CS. 3:24 Notice that this list only contains exceptions provided by the .Net framework. 3:32 We can add our own custom exceptions by right clicking 3:36 on Common Language Runtime Exceptions, and clicking Add Exception. 3:40 Now we can add our own exception type with its full namespace name. 3:43 We don't have any custom exceptions in our program so far, but 3:47 this is a good thing to be aware of in the future. 3:50 There are a keyboard shortcuts for just about everything we've talked about here. 3:54 Often times using a keyboard can make coding debugging and 3:58 navigating around the code much more efficient. 4:02 Check out the link in the teacher's notes for a list of the fall short cuts. 4:05 I hope you feel much more comfortable debugging in Visual Studio. 4:08 I think you'll find having a good debugger at your fingertips can 4:12 make it a lot easier to track down pesky bugs the crop up from time to time. 4:15 Until next time, happy coding. 4:19