Visual Studio 2005 provides you with a visual indication of what has changed during the editing session. It sets apart the changes you have made that have not yet been saved and changes you have made that have been saved, during the current session.
Edit and Continue feature allows you to make changes to your application while it is running under the debugger without stopping and restarting the application. The changes you make during break mode are applied immediately when you continue execution of your application.
To access the Edit and Continue page, click the Tools menu and select Options. In the Options dialog box, expand the Debugging folder and select Edit and Continue checkbox to enable it. You can use Edit and Continue to edit your code in Break mode, and then continue without stopping and restarting execution.
To edit code in Break mode: Enter Break mode by doing the following: Select Break All from the Debug menu, and set a breakpoint in your code and execute that code. Whenever the code is edited in the current session that is not yet saved or executed then a vertical yellow edit mark appears besides the changed code.
Now if this current session that is edited, is saved or executed then the yellow edit marks are changed to green edit marks with the changes incorporated in the code. If the current session is saved and/or closed and then reopened, then no edit marks are visible.
If you attempt to make a code change that is not allowed by Edit and Continue, your edit will be underlined by a purple wavy underline and a task will appear in the Task List. You will not be able to continue code execution unless you undo the illegal code change.
On the Debug menu, click Continue to resume execution. If you want to retain the results of such an illegal edit, you must stop debugging and restart your application. Your code now executes with your applied edits incorporated into the project.
From: Microsoft