With Visual Studio 2010 came the release of Visual Studio .NET 4.0. .NET 4.0 adds a lot of great features and new capabilities; however with .NET 4.0 came a new 4.0 debug engine in Visual Studio 2010 (so there are now two managed debug engines, 2.0/3.0/3.5 and 4.0). Ordinarily if you are developing/debugging your managed application inside of Visual Studio and begin debugging (F5) Visual Studio knows which engine to use. Likewise if you are attaching to a process, Visual Studio is able to tell which version of the .NET Runtime you are running on and attaches with the appropriate managed engine.
There are two scenarios however that people have been encountering since Visual Studio 2010 released where Visual Studio cannot correctly determine which engine to attach with, and therefore uses the default of 4.0 leaving 2.0/3.0/3.5 applications “un-debuggable”; (1) developing a class library plug-in for a third party application (Excel, Work, AutoCAD, etc), and (2) interop debugging when launching a native .exe project. In both cases the symptoms will be the same, you will be unable to hit breakpoints in your managed code (they will be hollow with a message that “No symbols have been loaded for this document”) as well as being unable to step into the managed code.
As I explained above, the cause of this is the managed application is 3.5 or previous, and the debugger is attached with the 4.0 engine. The best workaround is to add a <supportedRuntime> tag to your application’s *.exe.config file. For example, if I am debugging the application foo.exe I would edit (or create if necessary) the foo.exe.config file in the same directory as foo.exe. I would then add the <supportedRuntime> tag with the correct version of the .NET runtime in the <startup> section as shown in the example below. This will let Visual Studio know which version of the .NET runtime you will be running (so if you are developing a class library plug-in for Excel, you would add the <supportedRuntime> tag to the excel.exe.config file).
<!-- sample foo.exe.config -->
<?xml version ="1.0"?>
<supportedRuntime version="v2.0.[version on your machine]" />
To determine the correct [version on your machine] look in the “C:\Windows\Microsoft.NET\Framework” directory for the most recent “v2.0.xxxxx” folder.