I received a question about this recently, so i figured i'd elaborate here with a little example...

Let's assume we have the following three dlls:

   anycpu.dll      -- compiled "any cpu"

   x86.dll           -- compiled "x86"

   x64.dll           -- compiled "x64"

And the following three exes:

   anycpu.exe     -- compiled "any cpu"

   x86.exe          -- compiled "x86"

   x64.exe          -- compiled "x64"

What happens if you try to use these exes and dlls together? We have to consider two possible scenarios, running on a 32-bit machine and running on a 64-bit machine...

On a 32-bit x86 machine:

anycpu.exe -- runs as a 32-bit process, can load anycpu.dll and x86.dll, will get BadImageFormatException if it tries to load x64.dll

x86.exe -- runs as a 32-bit process, can load anycpu.dll and x86.dll, will get BadImageFormatException if it tries to load x64.dll

x64.exe -- will get BadImageFormatException when it tries to run

On a 64-bit x64 machine:

anycpu.exe -- runs as a 64-bit process, can load anycpu.dll and x64.dll, will get BadImageFormatException if it tries to load x86.dll

x86.exe -- runs as a 32-bit process, can load anycpu.dll and x86.dll, will get BadImageFormatException if it tries to load x64.dll

x64.exe -- runs as a 64-bit process, can load anycpu.dll and x64.dll, will get BadImageFormatException if it tries to load x86.dll