How can I determine if a .NET assembly was built for x86 or x64?

Look at System.Reflection.AssemblyName.GetAssemblyName(string assemblyFile)

You can examine assembly metadata from the returned AssemblyName instance:

Using PowerShell:

[36] C:\> [reflection.assemblyname]::GetAssemblyName("${pwd}\Microsoft.GLEE.dll") | fl

Name                  : Microsoft.GLEE
Version               : 1.0.0.0
CultureInfo           :
CodeBase              : file:///C:/projects/powershell/BuildAnalyzer/...
EscapedCodeBase       : file:///C:/projects/powershell/BuildAnalyzer/...
ProcessorArchitecture : MSIL
Flags                 : PublicKey
HashAlgorithm         : SHA1
VersionCompatibility  : SameMachine
KeyPair               :
FullName              : Microsoft.GLEE, Version=1.0.0.0, Culture=neut... 

Here, ProcessorArchitecture identifies target platform.

• Amd64: A 64-bit processor based on the x64 architecture.
• Arm: An ARM processor.
• IA64: A 64-bit Intel Itanium processor only.
• MSIL: Neutral with respect to processor and bits-per-word.
• X86: A 32-bit Intel processor, either native or in the Windows on Windows environment on a 64-bit platform (WOW64).
• None: An unknown or unspecified combination of processor and bits-per-word.

I'm using PowerShell in this example to call the method.

You can use the CorFlags CLI tool (for instance, C:\Program Files\Microsoft SDKs\Windows\v7.0\Bin\CorFlags.exe) to determine the status of an assembly, based on its output and opening an assembly as a binary asset you should be able to determine where you need to seek to determine if the 32BIT flag is set to 1 (x86) or 0 (Any CPU or x64, depending on PE):

Option    | PE    | 32BIT
----------|-------|---------
x86       | PE32  | 1
Any CPU   | PE32  | 0
x64       | PE32+ | 0

The blog post x64 Development with .NET has some information about corflags.

Even better, you can use Module.GetPEKind to determine whether an assembly is PortableExecutableKinds value PE32Plus (64-bit), Required32Bit (32-bit and WOW), or ILOnly (any CPU) along with other attributes.

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        : PE32
CorFlags  : 3
ILONLY    : 1
32BIT     : 1
Signed    : 0

As per comments the flags above are to be read as following:

• Any CPU: PE = PE32 and 32BIT = 0
• x86: PE = PE32 and 32BIT = 1
• 64-bit: PE = PE32+ and 32BIT = 0

How about you just write you own? The core of the PE architecture hasn't been seriously changed since its implementation in Windows 95. Here's a C# example:

    public static ushort GetPEArchitecture(string pFilePath)
    {
        ushort architecture = 0;
        try
        {
            using (System.IO.FileStream fStream = new System.IO.FileStream(pFilePath, System.IO.FileMode.Open, System.IO.FileAccess.Read))
            {
                using (System.IO.BinaryReader bReader = new System.IO.BinaryReader(fStream))
                {
                    if (bReader.ReadUInt16() == 23117) //check the MZ signature
                    {
                        fStream.Seek(0x3A, System.IO.SeekOrigin.Current); //seek to e_lfanew.
                        fStream.Seek(bReader.ReadUInt32(), System.IO.SeekOrigin.Begin); //seek to the start of the NT header.
                        if (bReader.ReadUInt32() == 17744) //check the PE\0\0 signature.
                        {
                            fStream.Seek(20, System.IO.SeekOrigin.Current); //seek past the file header,
                            architecture = bReader.ReadUInt16(); //read the magic number of the optional header.
                        }
                    }
                }
            }
        }
        catch (Exception) { /* TODO: Any exception handling you want to do, personally I just take 0 as a sign of failure */}
        //if architecture returns 0, there has been an error.
        return architecture;
    }
}

Now the current constants are:

0x10B - PE32  format.
0x20B - PE32+ format.

But with this method it allows for the possibilities of new constants, just validate the return as you see fit.