You might get the following error if you attempt to create a 32 bit application that references the 64 bit SharePoint dlls such as Microsoft.SharePoint.dll etc
Microsoft SharePoint is not supported in 32-bit process. Please verify that you are running in a 64-bit executable.
In a console or Windows application the fix to this is fairly straight forward – just make sure your Visual Studio project targets Any CPU or force it to have a Platform Target of 64 bit.
Resolving this during Visual Studio development with IIS Express is a little more challenging. As far as I can tell IIS express runs as 32 bit process by default. This means it really won’t like them there 64 bit SharePoint DLLs.
The fix is to force IIS Express to run as a 64 bit process by making sure the following registry key is set to 1:
I hope that saves someone the 30 mins I just spent!