Implementing .NET Interface in Delphi

It will be quite difficult (I am not even sure if it's possible) to implement the .NET interface directly in Delphi. There are, however, a few options available to you depending on how dirty you would like to get your hands.

1) You could create a ActiveX DLL in Delphi and then import this library into .NET. You could then create a proxy class in C# that implements the interface and just route the calls onto the Delphi ActiveX DLL.

2) You could create a native windows DLL in Delphi and then code the interface in .NET. You could then create a proxy class in C# that implements the interface and just route the calls onto the Delphi DLL.

3) You could use a plugin framework like Hydra (http://www.remobjects.com/hydra/) that supports both .NET and Delphi.

In all of these cases you should be careful to handle the 32/64 bit situations as .NET code is capable of running in both but the Delphi code would need to be compiled for each environment independently.

Another option depending on your situation might be to write the Delphi code in a form of object pascal that will produce .NET byte code called Oxygene (http://www.remobjects.com/oxygene/).

Edit: If you are trying to create Delphi plug-ins for use in your .NET plugin framework it would be best to do something like this:

Create a new interfaces in Delphi that match your IPlugIn and IPlugInHost interfaces. You could then build a .NET Delphi plugin wrapper that will have some configuration information to specify which Delphi plugin to load. This .NET wrapper would then load the Delphi DLL and call the DLL methods as required. Visual stuff would need some sort of ActiveX wrapper but you should be able to use the non visual stuff fine.