.net – Identifying COM components in a .NET application

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I've inherited a .NET application that pulls together about 100 dlls built by two teams or purchased from vendors. I would like to quickly identify whether a given dll is a .NET assembly or a COM component. I realize that I could just invoke ildasm on each dll individually and make a note if the dll does not have a valid CLR header, but this approach seems clumsy and difficult to automate.

Best Answer

If you want to approach from the COM side, testing for COM objects in a DLL boils down to looking for an export named "DllGetClassObject". This is because an in-proc COM object is accessed by the COM runtime by calling DllGetClassObject() on that DLL.

You could do this from a batch file using DUMPBIN.EXE which comes with Visual Studio as follows:

dumpbin unknown.dll /exports | find "DllGetClassObject"

The above command line will produce one line of text if it is an unmanaged DLL that contains COM objects, or zero bytes of output otherwise.

You could do this programmatically by loading each DLL and try to do a GetProcAddress() on that entry point. Here is a tested and working C# command line program that uses this technique:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;

static class NativeStuff
{
    [DllImport("kernel32.dll")]
    public static extern IntPtr LoadLibrary(string dllToLoad);

    [DllImport("kernel32.dll")]
    public static extern IntPtr GetProcAddress(IntPtr hModule, string procedureName);

    [DllImport("kernel32.dll")]
    public static extern bool FreeLibrary(IntPtr hModule);
}

namespace IsComDLL
{
    class Program
    {
        static void Main(string[] args)
        {
            if ( (args.Length == 0 ) || String.IsNullOrEmpty( args[0] ) )
            {
                Console.WriteLine( "Give DLL name on command line" );
                Environment.Exit(255);
            }

            IntPtr pDll = NativeStuff.LoadLibrary(args[0]);
            if ( pDll == IntPtr.Zero )
            {
                Console.WriteLine( "DLL file {0} not found", args[0] );
                Environment.Exit(256);
            }

            IntPtr pFunction = NativeStuff.GetProcAddress(pDll, "DllGetClassObject");
            int exitValue = 0;
            if (pFunction == IntPtr.Zero)
            {
                Console.WriteLine("DLL file {0} does NOT contain COM objects", args[0]);
            }
            else
            {
                Console.WriteLine("DLL file {0} does contain COM objects", args[0]);
                exitValue = 1;
            }

            NativeStuff.FreeLibrary(pDll);

            Environment.Exit(exitValue);
        }
    }
}
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