Any CPU vs x64 vs x86

When we compile .net applications/libraries we get IL's, not Machine readable codes. So when we say 'Any CPU'(in a way architecture-neutral PE files) we create neutral IL's, and as part of JIT compilation, output will be either x64 or x86 PE's based on executing machine or calling process.

On a 32-bit machine:

• Any CPU: runs as a 32-bit process, can load Any CPU and x86 assemblies, will get BadImageFormatException if it tries to load an x64 assembly.
• x86: same as Any CPU.
• x64: BadImageFormatException always.

On a 64-bit machine:

• Any CPU: runs as a 64-bit process, can load Any CPU and x64 assemblies, will get BadImageFormatException if it tries to load an x86 assembly.
• x86: runs as a 32-bit process, can load Any CPU and x86 assemblies, will get BadImageFormatException if it tries to load an x64 assembly.
• x64: same as Any CPU.

It is the JIT compiler that generates an assembly code that's compatible with the requested target based on this flag.

Also CorFlags.exe assembly.dll|assembly.exe will tell us whether it is targeted x86 or Any CPU

Just for clarification, CorFlags.exe is part of the .NET Framework SDK. I have the development tools on my machine, and the simplest way for me determine whether a DLL is 32-bit only is to:

1. Open the Visual Studio Command Prompt (In Windows: menu Start/Programs/Microsoft Visual Studio/Visual Studio Tools/Visual Studio 2008 Command Prompt)
2. CD to the directory containing the DLL in question
3. Run corflags like this: corflags MyAssembly.dll

You will get output something like this:

    Microsoft (R) .NET Framework CorFlags Conversion Tool.  Version  3.5.21022.8
Copyright (c) Microsoft Corporation.  All rights reserved.
Version   : v2.0.50727
CLR Header: 2.5
PE        : PE32CorFlags  : 3
ILONLY    : 1
32BIT     : 1
Signed    : 0

The key is the "32BIT" flag as documented above: 1 = x86; 0 = Any CPU.

·        Any CPU: PE = PE32    and  32BIT = 0

·         x86:         PE = PE32    and  32BIT = 1

      ·         x64:         PE = PE32+  and  32BIT = 0

Compilation and Deployment in ASP.NET 2.0

Just for my reference, good article from Rick Strahl

500.0 Internal Server Error - aspnet_isapi.dll failed in IIS 7.5

If you experience '500.0 Internal Server Error - aspnet_isapi.dll failed' even after trying all possibilities like 'aspnet_regiis -i' etc..., just uninstall IIS and Re-install again, it just works. It is Microsoft:)

I faced this when i installed Framework 4.0, so uninstalled IIS 7.5/Framework4.0 and re-installed both.