How to determine whether a DLL is a managed assembly or native (prevent loading a native dll)?

Question

Original title: How can I prevent loading a native dll from a .NET app?

Background:

My C# application includes a plugin framework and generic plugin loader.

The plugin loader enumerates the application directory in order to identify plugin dlls (essentially it searches for *.dll at this time).

Within the same application directory is a native (Windows, non-.net) dll, which, indirectly, one of the plugin dlls depends upon.

The plugin loader blindly assumes that the native.dll is a .NET Assembly dll, simply because it only checks the file extension. When it attempts to load the native dll, an exception is thrown:

"Could not load file or assembly 'native.dll' or one of its dependencies. The module was expected to contain an assembly manifest."

I basically create a diagnostic report if plugin loading fails, so I'm trying to avoid having this log filled up with messages about not being able to load the native dll (which I don't even want to attempt).

The question:

Is there some .NET API call that I can use to determine whether a binary happens to be a .NET assembly so that I don't attempt to load the native dll at all?

Perhaps longer term I will move my plugins to a subdirectory, but for now, I just want a work around that doesn't involve hard-coding the "native.dll" name inside my plugin loader.

I guess I'm looking for some kind of static Assembly.IsManaged() API call that I've overlooked.... presumably no such API exists?

Answer 1

Answer quoted by lubos hasko is good but it doesn't work for 64-bit assemblies. Here's a corrected version (inspired by http://apichange.codeplex.com/SourceControl/changeset/view/76c98b8c7311#ApiChange.Api/src/Introspection/CorFlagsReader.cs)

public static bool IsManagedAssembly(string fileName)
{
    using (Stream fileStream = new FileStream(fileName, FileMode.Open, FileAccess.Read))
    using (BinaryReader binaryReader = new BinaryReader(fileStream))
    {
        if (fileStream.Length < 64)
        {
            return false;
        }

        //PE Header starts @ 0x3C (60). Its a 4 byte header.
        fileStream.Position = 0x3C;
        uint peHeaderPointer = binaryReader.ReadUInt32();
        if (peHeaderPointer == 0)
        {
            peHeaderPointer = 0x80;
        }

        // Ensure there is at least enough room for the following structures:
        //     24 byte PE Signature & Header
        //     28 byte Standard Fields         (24 bytes for PE32+)
        //     68 byte NT Fields               (88 bytes for PE32+)
        // >= 128 byte Data Dictionary Table
        if (peHeaderPointer > fileStream.Length - 256)
        {
            return false;
        }

        // Check the PE signature.  Should equal 'PE\0\0'.
        fileStream.Position = peHeaderPointer;
        uint peHeaderSignature = binaryReader.ReadUInt32();
        if (peHeaderSignature != 0x00004550)
        {
            return false;
        }

        // skip over the PEHeader fields
        fileStream.Position += 20;

        const ushort PE32 = 0x10b;
        const ushort PE32Plus = 0x20b;

        // Read PE magic number from Standard Fields to determine format.
        var peFormat = binaryReader.ReadUInt16();
        if (peFormat != PE32 && peFormat != PE32Plus)
        {
            return false;
        }

        // Read the 15th Data Dictionary RVA field which contains the CLI header RVA.
        // When this is non-zero then the file contains CLI data otherwise not.
        ushort dataDictionaryStart = (ushort)(peHeaderPointer + (peFormat == PE32 ? 232 : 248));
        fileStream.Position = dataDictionaryStart;

        uint cliHeaderRva = binaryReader.ReadUInt32();
        if (cliHeaderRva == 0)
        {
            return false;
        }

        return true;
    }
}

The missing piece was to offset to the data dictionary start differently depending on whether we are PE32 or PE32Plus:

    // Read PE magic number from Standard Fields to determine format.
    var peFormat = binaryReader.ReadUInt16();
    if (peFormat != PE32 && peFormat != PE32Plus)
    {
        return false;
    }

    // Read the 15th Data Dictionary RVA field which contains the CLI header RVA.
    // When this is non-zero then the file contains CLI data otherwise not.
    ushort dataDictionaryStart = (ushort)(peHeaderPointer + (peFormat == PE32 ? 232 : 248));