R – Load a .NET assembly from a COM ProgID without creating a COM object

com-interoplegacynet

A bit of an odd question and probably backwards from what most people want to do, but I'm trying to work around a legacy COM issue.

I have two components, both of which are actually .NET assemblies, but for historical reasons one is loading the other as a COM object (the assembly is registered for COM Interop). It's a plug-in architecture where the plug-in is identified by its COM ProgID, so that's the only piece of information I get to load the plug-in assembly.

One technique I've tried is: 

var objType = Type.GetTypeFromProgID("My.ProgId");
var objLateBound = Activator.CreateInstance(objType);
IMyInterface netAssembly;
try
    {
    netAssembly = (IMyAssembly)objLateBound;
    }
catch (Exception)
    {
    netAssembly = null;
    }

If the cast to a .NET interface succeeds, the I know I have a .NET assembly and can access it through the interface. However, the technique is a bit clumsy and I'm getting problems with the COM side of things especially on 64-bit systems. I'd rather just eliminate loading the COM object and load the plug-in directly as a .NET assembly, if possible.

But the only piece of information I've got to go on is the plug-in's COM ProgID.

So, how can I go from a COM ProgID to loading a .NET Assembly, without creating any COM objects?

Best Answer

Look up in the registry to find the DLL associated with the ProgID you have. Once you have its full path, load it as a normal .NET assembly:

var type = Type.GetTypeFromProgID("My.ProgId", true);
var regPath = string.Format(@"{0}\CLSID\{1:B}\InProcServer32", 
    Registry.ClassesRoot, type.GUID);
var assemblyPath = Registry.GetValue(regPath, "", null);
if (!string.IsNullOrEmpty(assemblyPath))
{
    var assembly = Assembly.LoadFrom(assemblyPath);
    // Use it as a normal .NET assembly
}
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