If you are working on ASP.NET MVC web sites in Visual Studio 2013 (VS2013), you need to make one registry change if you want to run IIS Express as a 64-bit process by default.  Use one of the methods, below.

Command-Line:

reg add HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\12.0\WebProjects /v Use64BitIISExpress /t REG_DWORD /d 1

Regedit:

  1. Navigate to:

    HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\12.0\WebProjects
  2. Make sure there is a REG_DWORD value named “Use64BitIISExpress”.  If not, create it.
  3. Set its value from 1 (default = 0).

That’s it.  Now you can debug your 64-bit web sites.

If you don't do this, you will likely get a message similar to:

"Could not load file or assembly or one of its dependencies. An attempt was made to load a program with an incorrect format."

I found this solution for Visual Studio 2012 (VS2012) on Stack Overflow here.

Rob