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Another tool that you can use when you are programming in either Visual Basic or C# and you want to see what's going on inside the code is to set breakpoint. A breakpoint is a way of marking the bit of code, so that when you hit the break point the application is suspended for a moment. As long as the breakpoint is active, you can then inspect the values of variables and other aspects of your application state. For this demonstration I'll start with the file LoopingCSharp. aspx. This file already has the For and the While loops inside the Run Button Click method.
I will save this file under a new name, naming the new version Breakpoints.aspx. You can set a breakpoint on any line of code. You can do this in a couple of ways. One approach to creating breakpoint is to right click on the line and then select Breakpoint, Insert Breakpoint. You will see that that results in selecting the code and highlighting it and you will also see a really large graphic on the left side of the screen in what's called the gutter area, right next to the line numbers.
You can also set and clear breakpoints just by double clicking on that area. So I have double-clicked and you will see that the breakpoint is gone. Then I'll double-click again and you will see that the breakpoint comes back. Now when you set a breakpoint you then have to turn on debugging. By default, in ASP.NET website that's created in Visual Web Developer has debugging turned off. But in order to use the breakpoint and suspend the page as it's running you have to turn debugging on. You can do this through the web.config file explicitly or you can get Visual Web Developer to do it for you just by telling Visual Web Developer that you want to debug the page.
Here is how you do it. I have already set a breakpoint on the For loop. I'll set one on this line with the While loop as well. So now I'll be hitting the breakpoint right before I output the value of the counter variable in both loops. Now I'll go to the menu and select Debug and now for the first time, I'll select Start Debugging. Visual Web Developer detects that this website is not configured to run in Debug mode. So it asks me whether I would like to modify the web.config file to allow debugging. There is also an important note that before you deploy the website to a production environment you should turn debugging off.
I will use the default selection to modify the web.config file and click OK. Visual Web Developer also detects whether Script Debugging is turned on or off. If you want to debug your client side code, you should turn the Script Debugging on. Here though, I'm only debugging server-side code. So I'll say Yes, I want to continue debugging. It takes a moment for the page to be rebuilt and then appear in the web browser. The first time you go into debugging in this context, you may see that the web browser doesn't automatically take focus. If that happens, just click the browser icon down on the task bar.
Now I'll click the Run Code button you will see that I hit the breakpoint in the For loop. The current line that is being suspended is highlighted in yellow and you will also see the breakpoint listed over on the left side. Down at the bottom in the Locals panel, you will see a listing of all available arguments. The counter variable, which was declared inside the function, is listed there with its initial value of 1. Now to continue executing the code, I'll go to the Toolbar and click the Continue button which is a little arrow pointing to the right. Each time I click it executes the code until it gets to another breakpoint and keep your eye down here on the counter variable and you will see that each time I click the Continue button, the counter variable is updating. I'll Continue again and now I'm hitting the breakpoint in the While loop and I'll click it again. One more time and this time because I have gotten out of all the loops, I'll see that the browser comes back and shows me the result of the code.
Now I'm going to execute the breakpoint one more time to show you one very important thing. I'll clear the console and then run the code again. I'll continue a couple of times and you will see that I have hit the output function twice. I'll press Alt+Tab and switch back to the browser and show you that the output doesn't appear in the browser because until you have completed executing the entire page, the content doesn't get sent back to the browser. I will press Alt+Tab again and switch back to Visual Web Developer. I'll click Continue, Continue again and one more time and now I come back to the webpage in the browser because its code has been completed and all of the response has been sent back to the browser and as being displayed appropriately.
So that's basically how you use breakpoints. You place the breakpoint on the line that you want to suspend on while the page is running and then when you hit the breakpoint you will come back to Visual Web Developer and you will be able to use the Locals panel to inspect the data. There are other debugging panels available as well. The Watch panel allows you to put an arbitrary expressions up to a certain point and see what their values are and you can also use the Call Stack, the Immediate Window and the Output panel which you see regularly but which has more detailed information when you are in Debug mode.
When you are done debugging just close the browser and when you come back to Visual Web Developer, it automatically removes those additional panels and shows you the original window layout.
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