Differences between 32 and 64Bit .NET (4) applications

Question

Hi there,

what are the differences between 32 and 64Bit .NET (4) applications? Often 32Bit applications have problems running on 64bit machines and conversely. I know I can declare an integer as int32 and int64(certainly int64 on 32Bit systems make problems). Are there other differences between programming an 32 OR 64Bit or a both 32 AND 64Bit compatible application?

Thanks

Answer 1

Some differences:

1. 32-bit and 64-bit applications can only load DLLs of the same bitness. This can be an issue for managed projects if your platform target is "Any CPU" and you reference or P/Invoke 32-bit native DLLs. The issue arises when your "Any CPU" program runs on a 64-bit machine, since your application runs as a 64-bit process. When it tries to load the 32-bit native DLL dependency, it will throw an exception (BadImageFormatException) and likely crash.

2. There are also filesystem and registry issues. A WOW64 process that tries to read from C:\Program Files will end up getting redirected to C:\Program Files (x86) unless it first disables Windows filesystem redirection (see Wow64DisableWow64FsRedirection). For versions of Windows before Windows 7, there were also registry reflection issues that were similar to the filesystem redirection issues mentioned above. This MSDN article explains it well.

3. Platform-specific types like IntPtr will have different sizes. This could be an issue in code that assumes a fixed size (serialization, marshaling).

4. There are separate physical directories for the 32- and 64-bit files in the GAC. For my system, they are at C:\Windows\Microsoft.NET\assembly\GAC_32 and C:\Windows\Microsoft.NET\assembly\GAC_64.

5. The virtual address space size of 32- and 64-bit applications is different. For 32-bit apps, the size is either 2GB (default) or 3GB (with 4GT enabled). For 64-bit apps, the size is 8TB. The 32-bit address space can be a limitation for very large applications.

6. A little more obscure, but a lot of interprocess Win32 calls won't work between a 32- and 64-bit process. For example, a 32-bit process can fail when trying to call ReadProcessMemory on a 64-bit process. The same goes for WriteProcessMemory, EnumProcessModules, and a lot of similar methods. This can be seen in C# applications if you try to enumerate the modules of a 64-bit app from a 32-bit app using the System.Diagnostics.Process.Modules API.

Answer 2

I think with managed code in general you must not have any problems. Potential problems can come from unmanaged code for example because of variable sizes are different in 32bit and 64bit systems, pointers are different and etc... For example size of the int variable in C/C++ depends from the system. As for managed code as already mentioned WOW can handle that.