How to disable optimizations when debugging Reference Source

When you debug code in the .NET Framework using the newly available Reference Source functionality in VS 2008, you may notice that many variables are not available for inspection.


This is because you're debugging against retail-optimized code.  In many cases, since you can still step through, this is something that's manageable.

But what if you really need to get a better idea of what's going on?  Fortunately, there is a way.

What you need to do is to tell the CLR not to load the pre-JIT (aka NGEN) images.  Here is how to do it. 

First, create a CMD file that sets an environment variable, then launches Visual Studio.  I called mine "NoOptDevEnv.cmd", and it's contents are as follows:

set COMPLUS_ZapDisable=1
cd /d "%ProgramFiles%\Microsoft Visual Studio 9.0\Common7\ide\"
start devenv.exe

I put this cmd file on my desktop.   When I want to disable optimizations, and only when I want to do this, I launch VS from this command. 

Once in my Visual Studio project, do the following steps:

1) Right click on your project file and choose "Properties"

2) Choose the "Debug" tab and uncheck "Enable the Visual Studio Hosting Process"

3) Launch your application in the debugger.

Now, you'll see full local and member variable information:


Finally, you may be asking yourself "What is the Visual Studio Hosting Process (aka VSHost), and what happens if I turn it off?"  And this is a good question.

For the most part, disabling VSHost won't have any major impacts, but it will disable two features that you may be using.

First, you will not be able to do "Debug In Zone", which allows you to debug a process in the context of a security zone such as "Internet" or "Intranet".  That won't work without VSHost.

Second, Design Time Expression Evaluation for class libraries will also not work.  What that means, for example, is that if you are developing a Class Library, you won't be able to execute code from it in the Immediate Window while under the debugger.

In general, I recommend re-enabling VSHost (undo step 2 above) when you are finished with your debugging.

Hope that helps!