Join Walt Ritscher for an in-depth discussion in this video Debugging code, part of Visual Studio 2010 Essential Training.
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Nobody likes to write code that contains bugs, but it is a fact of the programmer's life. So in this movie, I would like to show you the debugging tools that are available inside Visual Studio. I'm inside Visual Studio, and I've opened a solution called DebuggingCode, which contains one project called ExploringBreakpoints. Let me show you the main UI for this application. I'm going to double-click on MainWindow.xaml. It's a simple form that contains seven buttons. Each of these buttons contains one of the demos I'm going to show today. Before I continue, let's make sure we have one setting enabled in Tools > Options.
I'll go to Tools > Options, and then you want to go down to the Debugging section, and go to Edit and Continue. Make sure there is a check mark in Enable Edit and Continue. This allows us to edit the code while we're sitting at a breakpoint. I don't believe this works on 64- bit operating systems, however. So now I'm ready to add a breakpoint. I'm going to switch to my Code view by double-clicking on the Simple Breakpoint button. Here is the code that I want to examine under a debugger.
So I can put my cursor on line 37, and then I can press the F9 key. Notice that the line turns red, and on the left margin is a red circle. That signifies that there is a breakpoint waiting at this line. To turn off the breakpoint, I can press F9 again. Another way of adding a breakpoint is to click on the margin. Now that I have this breakpoint, I need to run the application under a debugger. The easiest way to do that is to click the Start Debugging or press F5.
Now when I click on the button, we switch over to Visual Studio, and we're sitting here looking at the breakpoint. This line of code has not run yet. To run this line of code, I need to click on this Step Into button. Now you can see the breakpoint is still on line 38, but the next line of code that's going to execute is line 39. Another way of putting a breakpoint in your code is to write a little bit of code. I can come down to this section and write a System.Diagnostics.Debugger.Break().
That means that when the debugger encounters this line, it'll automatically stop. Now, you probably don't want to leave this in your production code, so what you can do is wrap it in an if statement, like I'm doing here. I'm going to uncomment this by doing a Ctrl+K, Ctrl+U. Now, when I run this breakButton, I'll switch back to my UI, I'll press F5 to continue running the application, and then I can click on this button, and you see again I have a yellow marker sitting on that line.
Now, there's no red indicator on this one because that you wrote the code to stop instead of adding a breakpoint. And then I'll press F5. If you have an exception in your code, and you don't have an exception handler, which is called an untrapped exception, then you'll get an opportunity to debug that if you're running under a debugger. Let me click on this button here. So I wrote some code that was attempting to open a file stream, and I passed it an invalid text file name.
So when .NET went out to open this file, it couldn't find the file, which is what this exception error is I'm getting here, "Could not find file 'C:\bogus.txt'." This is a nice little window that shows me there's an error at this line. It switched over to a break, so that I can look at the code and see if I can figure out what's wrong. I can click on this View Detail button to learn more about the exception, and I can dismiss that dialog and then make any changes necessary and then try to run this application again.
I'm going to stop the application for a second and rerun the application. Now, we're going to look at how to step through our code. I'm going to click on this button. I do believe I need a breakpoint on this line. So, I switched over to my code. This is the stepping code. I'm going to add a breakpoint right here on line 75, F9, Alt+Tab back to my Running application and then click on that button again.
Now, I should say something about breakpoints. When you save your application, it's going to store your breakpoints in a special user-specific file. So if I open this file again tomorrow, my breakpoints will still be here. If you were to open this file--say I sent you a copy of my solution--you wouldn't have my breakpoints, because you get your own user-specific Solution file. I'm ready to run this line of code. Now, I've showed you earlier, you can click on this button; you can also press F11 to do a single step. Watch what happens if I do a single step by pressing F11.
There is a function over here called CalculateMonthlyPayment, and now I've entered that function. Now, I can press F11 to walk through my code over here. Then I'll return back to the previous code. Another way of walking through your code is to click on this Step Over. What that does is it looks at this and says, "We'll there are six or seven lines of code over there, in that method. I'm going to skip over those. I'm just going to run this as if it was a single line of code." So, by pressing Step Over, I don't have to walk through that method that I already have debugged.
See, when I clicked on that button, it immediately went to the next one. There is another setting. Let me click on the Step Into. Now I'm back inside that CalculateMonthlyPayment. Let's say that I've run a couple of lines of code in here, and I don't want to step through the rest of the code. I can use this Step Out button to finish running all the code in here and then stop on the first line of code back in the calling function. So let me show you how that one works. As you can see, it returned back to the calling method. You remember that Edit and Continue feature I turned on earlier? I can now rerun some of these lines of code by dragging this yellow marker up and rerunning that line of code by pressing F10.
Another thing that I can do is I can now edit this code while I'm sitting at a breakpoint. Let's say I don't like the name of this variable. I'm going to have to rename it in several places. But now that I've renamed it successfully, I can press F10 to successfully run that code. So I'm able to edit the code while I was sitting at a breakpoint. Let me press F5 and click on the next button. Now, I've got three more examples I want to show you.
So let me shut this application down and add some breakpoints before I continue. I'll press F9 while I'm on this DataTips. I'll press F9 while I'm down here on the visualizerButton, and then I'm going to add a special better breakpoint down in this area. I'll come back and show you that in just a second. Actually, I think I'll show it to you now. I'm going to add a special breakpoint here. One of the new things in Visual Studio is that you can add breakpoints that have special counts. So, I'm going to add a breakpoint here, and then I'm going to right-click on it and set Hit Count. Then I'm going to choose when the hit count is a multiple of 60.
Notice that there's a special Plus sign now on the breakpoint? Let's run these last three demos. Again, I'm going to go up and choose Debug > Start Debugging. There is a special tooltip in Visual Studio called a datatip. I'm going to press F10 a couple of times to move down into my code, and then I'm going to hover over this variable. You can see the datatip. It tells me that the current value of that variable is 0.0.
And if I come down here and click on this birthDate, it shows that it's 5/5/1992. Notice there's also a Plus symbol here, where I can see more specific details about that item. There is also a little stickpin over here. Click on this and then it adds the tip to my coding window. So now that you notice as I move to my code, that little datatip is now stuck there for me to keep watch on it. Let me press F10 a few times here. We'll run through this code. This is another feature, I can choose Run To Cursor.
I'm going to click down here and right-click and choose Run To Cursor. Now it'll run down to the line of code where my cursor is sitting, and then I can hover over this to see that I have five items inside that SortedDictionary. Now, I'm going to press F5 to return back to the UI, and I'll show you this other cool thing called the Visualizer. I'll press F10 twice to run down to this curly brace. I've just loaded an XML file into this variable. If you hover over it, you can see that there is a lot of raw text in there, but there's this thing called the data visualizer, where I have the magnifying glass.
I can click on this magnifying glass and then choose a different representation of the data. I like this XML Visualizer better than the raw Text view. That's what's currently stored in that variable. I'll press F5 one more time, and you probably remember just a few minutes ago, I added that special breakpoint. Now, I'm going to execute that breakpoint by clicking here, and I have just run through this loop counter and if you hover over the counter, you'll see that I'm at 60, because that was my condition I set up for the counter.
If I press F5 again, I'll now have run through another 60 iterations. So rather than having me step through this over and over again, I can just skip forward 60 steps in this loop. These debugger tools that I'm showing you are just the ticket when you have an unexplained problem in your code. In the next movie, I'll show you how to use the Watch and other debug windows.
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