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

Question

I\'ve got an arbitrary list of .NET assemblies.

I need to programmatically check if each DLL was built for x86 (as opposed to x64 or Any CPU). Is this possible?

Answer 1

More generic way - use file structure to determine bitness and image type:

public static CompilationMode GetCompilationMode(this FileInfo info)
{
    if (!info.Exists) throw new ArgumentException($"{info.FullName} does not exist");

    var intPtr = IntPtr.Zero;
    try
    {
        uint unmanagedBufferSize = 4096;
        intPtr = Marshal.AllocHGlobal((int)unmanagedBufferSize);

        using (var stream = File.Open(info.FullName, FileMode.Open, FileAccess.Read))
        {
            var bytes = new byte[unmanagedBufferSize];
            stream.Read(bytes, 0, bytes.Length);
            Marshal.Copy(bytes, 0, intPtr, bytes.Length);
        }

        //Check DOS header magic number
        if (Marshal.ReadInt16(intPtr) != 0x5a4d) return CompilationMode.Invalid;

        // This will get the address for the WinNT header  
        var ntHeaderAddressOffset = Marshal.ReadInt32(intPtr + 60);

        // Check WinNT header signature
        var signature = Marshal.ReadInt32(intPtr + ntHeaderAddressOffset);
        if (signature != 0x4550) return CompilationMode.Invalid;

        //Determine file bitness by reading magic from IMAGE_OPTIONAL_HEADER
        var magic = Marshal.ReadInt16(intPtr + ntHeaderAddressOffset + 24);

        var result = CompilationMode.Invalid;
        uint clrHeaderSize;
        if (magic == 0x10b)
        {
            clrHeaderSize = (uint)Marshal.ReadInt32(intPtr + ntHeaderAddressOffset + 24 + 208 + 4);
            result |= CompilationMode.Bit32;
        }
        else if (magic == 0x20b)
        {
            clrHeaderSize = (uint)Marshal.ReadInt32(intPtr + ntHeaderAddressOffset + 24 + 224 + 4);
            result |= CompilationMode.Bit64;
        }
        else return CompilationMode.Invalid;

        result |= clrHeaderSize != 0
            ? CompilationMode.CLR
            : CompilationMode.Native;

        return result;
    }
    finally
    {
        if (intPtr != IntPtr.Zero) Marshal.FreeHGlobal(intPtr);
    }
}

Compilation mode enumeration

[Flags]
public enum CompilationMode
{
    Invalid = 0,
    Native = 0x1,
    CLR = Native << 1,
    Bit32 = CLR << 1,
    Bit64 = Bit32 << 1
}

Source code with explanation at GitHub