Accessing .NET components from COM clients and vice versa

In this article I am going to explain how to access a .NET component from a COM client and, vice versa, how to access COM components from .NET clients.

COM is all about how little pieces of code find other little pieces of code and how they communicate with each other. COM precisely defines how the location and communication takes place.

In the .NET world, little pieces of code still find and talk with little pieces of code, but they don't use COM. Hence, if you want to access a .NET component from a COM client, you cannot do it directly. The solution to this is CCW (COM callable wrapper) which acts as a proxy for the .NET object. Similarly, if you want to access a COM component from a .NET client you will have to create a RCW (runtime callable wrapper).

Accessing .NET components from COM clients

This example demonstrates how a VB6 COM client using CCW accesses a component developed in VB.NET.

VB.NET component (testCCW.vb)

  imports system
  namespace CCWComponent
  public class CCWClass
    public function PassStr as string
      PassStr = "Hi From .NET Component"
    end function
  end class
  end namespace  

Save this code to a text file with the name testCCW.vb. Now compile this file from the command line with the statement:

Vbc /t:library testCCW.vb

The VBC compiler will create a testCCW.dll file for you -- this is the .NET assembly.

Now, the next step is to create a COM Callable Wrapper proxy for the component testCCW.dll file. The regasm utility can register the .NET component and also create a .TLB file, which can be referenced from any COM client.

If you want to just register the .NET component with the registry, use Regasm testCCW.dll. It will create the required registry entries. It is useful when you want to create late binded clients. If you want to register and generate the Type library, i.e .TLB file, from the .NET component, use:

Regasm testCCW.dll /tlb:testCCW.tlb

It will create a file testCCW.tlb, which can be reference by COM clients It is useful when you want to create COM clients that want to use early binding to the .NET component.

COM client (VB6) (Late binded)

Private Sub Command1_Click()
  dim o 
  set o = createobject("CCWComponent.CCWClass")
  msgbox o.PassStr
end sub  

The same client can also early bind to the CCW of the .NET component if the CCW of the .NET component was exported to .tlb file using regasm /tlb switch.

Most important

In order to find the assembly of the .NET component, the COM client executable has to be in the same directory as the .NET component, or the .NET component has to be in the Global Assembly cache.

Using RCW (runtime callable client)

If you want to access the COM component from the .NET client you will need to create a wrapper for the COM component using the TLBImp utility.

COM Server ComSrv.dll (MyCom.ComComponent)

Add the following code in a ActiveX DLL:

Public Function SayHi() As String
   SayHi = "Hi From COM Component"
End Function 

After compiling the component, create a wrapper using tlbimp utility.

Tlbimp ComSvr.dll /out c:<Path>

It is recommended to use the /out switch to prevent accidentally overwriting the COM DLL. The .NET proxy for the COM server will be created in the path specified in the /out switch.

VB.NET client (Ntest.vb)

imports system
imports microsoft.visualbasic
imports MyCom

class NTest
  shared sub main
    dim o as new MyCom.ComComponent
    msgbox (o.sayHi)
  end sub
end class 

Save the file with the name Ntest.VB. Compile the VB.NET client and run it using: vbc /r:ComSvr.dll Ntest.vb.