Create a DLL from a TLB Assembly

Question

I am having some performance problems when calling a COM TLB methods from C# using the dynamic keyword (more information here). Scince I have not had any luck trying to optimise such call I am now looking at a way to convert my TLB library to a native wrapper DLL which I can use directly in my C# project (at this stage I am not even sure that this will help, but instead abstract the performance problems down a layer).

I have used tlbimp.exe to create a .dll from my COM .tlb file (which is registered) using the VS2013 command prompt and the command

tlbimp F:\SomeDir\GrpSvr.tlb /out:F\SomeDir\GrpSvr.dll 

This has produced a managed wrapper C# .dll that I can inspect this using dotPeek. It contains the expected namespace

namespace GRPSVR
{
    [TypeLibType(TypeLibTypeFlags.FCanCreate)]
    [Guid("FFB54BC4-B15E-11D1-99BC-0000E803C444")]
    [ClassInterface(ClassInterfaceType.None)]
    [ComImport]
    public class GrpCallClass : IGrpCall, GrpCall { /*Expected Methods et al.*/ }
}

and the two interfaces IGrpCall containing the full template for the generated class and and empty interface GrpCall. I register this .dll using regasm.exe and

regasm F:\SomeDir\GrpSvr.dll 

Now, I include a reference to this .dll in my project and attempt to instantiate the GrpCallClass class via

private GRPSVR.GrpCallClass grpSvr = new GRPSVR.GrpCallClass();

But this give the compile time error:

Interop type 'GRPSVR.GrpCallClass' cannot be embedded. Use the applicable interface instead.

So then I try

private GRPSVR.IGrpCall grpSvr = new GRPSVR.GrpCall();

and this works at compile time but at run time I get a

Additional information: Retrieving the COM class factory for component with CLSID {FFB54BC4-B15E-11D1-99BC-0000E803C444} failed due to the following error: 80040154 Class not registered (Exception from HRESULT: 0x80040154 (REGDB_E_CLASSNOTREG)).

But I have registered the .dll.

1. Will this approach help increase the performance of calling method in the initial COM TLB assembly?

2. Can someone explain what I am doing wrong and how I can convert my COM .tlb library into a native wrapper .dll?

Thanks for your time.

Answer 1

I register this .dll using regasm.exe

That was a fatal mistake. That overwrote the registry keys for the native COM server. Regasm should only ever be used when the server was written in a .NET language. You will have to re-install the server to repair the damage. Be sure to run Regasm /unregister first to clean up the registry.

and empty interface GrpCall

That you see an empty interface is the reason you have to use the dynamic keyword in your C# code. The author of the COM server only permits you to use it late-bound. The equivalent of [InterfaceType(ComInterfaceType.InterfaceIsIDispatch)] in .NET. This is not uncommon, it avoids a lot of DLL Hell misery. But sure, late-binding isn't very fast since an extra call is needed at runtime to find the dispid of the method you are calling. Setting up the stack frame to make the call is slow as well, every argument has to be converted to a VARIANT. A x10 slowdown compared to a early-bound call is pretty normal, could be several orders of magnitude for a simple property.

There isn't anything you can do to avoid this, you'll have to work with the server author to get ahead. A pretty-please-with-cheese-on-top is the way to go about it, ask for a dual interface. Expect a "no", you may get a "yes".