A reference to the dll could not be added

You can add a DLL (or EXE) to a project only if it is a .NET assembly. If it's not you will see this error message.

regsvr32 also makes certain assumptions about the structure and exported function in the DLL. It has been a while since I used it but it has to do with registering COM servers so certain entry points need to be available. If regsvr32 fails the DLL doesn't provide those entry points and the DLL does not contain a COM component.

You only chance for using the DLL is to import it like any other non-.NET binary, e.g. when you use certain Win32 APIs. There is an old MSDN Magazine Article that might be helpful. See the following update for info where to get the article.

Update 12 March 2018: The link to the MSDN Magazine no longer works as it used to in August 2010. The article by Jason Clark is titled ".NET Column: Calling Win32 DLLs in C# with P/Invoke". It was published in the July 2010 issue of MSDN Magazine. The "Wayback Machine" has the article here at the moment (formatting is limited). The entire MSDN Magazine issue July 2010 is available here (HCM format only, instructions for how to use HCM files here).

The following worked for me:

Short answer

Run the following via command line (cmd):

TlbImp.exe cvextern.dll        //where cvextern.dll is your dll you want to fix.

And a valid dll will be created for you.

Longer answer

• Open cmd

• Find TlbImp.exe. Probably located in C:\Program Files (x86)\Microsoft SDKs\Windows\v7.0A\Bin. If you can't find it go to your root folder (C:\ or D:) and run:

  dir tlbimp.exe /s              //this will locate the file.
  

• Run tlbimp.exe and put your dll behind it. Example: If your dll is cvextern.dll. You can run:

  TlbImp.exe cvextern.dll
  

• A new dll has been created in the same folder of tlbimp.exe. You can use that as reference in your project.

I used dependency walker to check out the internal references the dll was having. Turns out it was in need of the VB runtime msvbvm60.dll and since my dev box doesnt have that installed I was unable to register it using regsvr32

That seems to be the answer to my original question for now.