.net programs on 32 and/or 64 bit machines

Generally, a .net program can run on a 32 bit machine as well as on a 64 bit machine. But sometimes it is necessary to run the program also on a 64 bit machine in 32 bit mode, the so-called WoW64.

WoW64 stands for "Windows on 64-bit Windows", and it contains all the 32-bit binary files required for compatibility, which run on top of the 64 bit Windows. So, yeah, it looks like a double copy of everything in System32 (which despite the directory name, are actually 64-bit binaries).

You will need WoW64 par example, if you want to call 32 bit ActiveX components. Visual Studio provides for this purpose the so-called platform target:

• x86
  32 bit application, runs either on Win32 or on Win64 in WoW64
• x64
  64 bit application, runs only on Win64 (not in WoW64)
• Any CPU
  runs on Win32 as 32 bit application and on Win64 as 64 bit application

This info will be stored in the PE header. At application startup, Windows checks the settings and starts the application in the appropriate mode (or not). If you want to check later, for which platform the application was built, you can use the corflags tool in Visual Studio Command Prompt:

> corflags MyApp.exe
Microsoft (R) .NET Framework CorFlags Conversion Tool.  Version  4.0.30319.1
Copyright (c) Microsoft Corporation.  All rights reserved.

Version   : v4.0.30319
CLR Header: 2.5
PE        : PE32
CorFlags  : 11
ILONLY    : 1
32BIT     : 1
Signed    : 1

The interesting parts are PE and 32BIT. The values are a little bit strange and hard to remember:

Platform target	PE	32BIT
x86	PE32	1
x64	PE32+	0
Any CPU	PE32	0

References

• CorFlags.exe (CorFlags Conversion Tool)
• Sorting Out the Confusion: 32- vs. 64-Bit, CLR vs. Native, C# vs. C++