Wednesday, March 25, 2009

Where should I put my .Net DLL for COM Interop?

I have a .Net DLL which I want to use from VB6 via COM interop. Where should I put it when I deploy it? (look here to see how to make a .Net DLL useable via VB6)

The key thing is that in order to use your .Net dll from VB6 you do need to register the .Net DLL. You don't need to to do anything special in the VB6 program - it doesn't matter how you reference the .Net DLL in your VB6 program. The command to register a .Net DLL is "regasm". This lives under "C:\Windows\Microsoft.Net\Framework\v2.0.50727" (or whatever .Net framework version you have).

There are three options for where to put your .Net DLL:

1. Put it in the GAC. If you want to share the same .Net DLL with more than one VB6 program then you can put it in the GAC (Global Assembly Cache). To do this, you can use an MSI file, or manually, you can copy it to c:\windows\assembly, and then register it via "regasm c:\winnt\assembly\mydotnet.dll".

2. Put it in the EXE folder. If you want the .Net DLL to live in the same place as your VB6 EXE then you can copy it to that folder, and then use "regasm c:\vb6\exe\folder\mydotnet.dll". Note that the .Net DLL has to be in the same folder as the VB6 EXE. So if you have VB6 EXE calling a VB6 DLL which calls your .Net dll then it must go in the EXEs folder - it won't find it in VB6 DLLs folder.

3. Put it in any folder via codebase. If you want the .Net DLL to live anywhere else, you can use use "regasm c:\program files\vanwills\mydotnet.dll /codebase". This will register the .Net DLL to the path you provide. This is a bit like the old VB6 regsvr32 command. So using this we can have the .Net DLL sitting in the "c:\vb6\dll\folder" folder.

The only other point to note is that .Net will always check the GAC first before other locations. So if we have a .Net DLL in the GAC, then even if we register the same DLL somewhere else via the "/codebase" option, it will always use the GAC DLL. You can get around this by changing the version number on the DLL - it will always load the version with the highest number first.