Background
In C#, an interface is by definition empty and has many possible implementations. In COM, in general, an interface will have one implementation and defines a calling contract, not an implementation contract (like with web services or CORBA). In C#, the implementation of an interface is .NET specific. In COM, the implementation of an interface is a language-neutral, but a binary implementation (as opposed to SOAP messages, which is textual/XML). This binary definition has always been food for criticasters of COM and the slow (if at all) adoption of COM on non-Windows systems (again, as opposed to web services).
For practically all IDL commands, there's an equivalent possibility in C#, albeit not always within just in interface. The base interface of COM is always IUknown, which is used for object reference counting, which must be included by all COM objects.
In speech, when talking about a COM interface, you usually talk about an implementation. In c#, you usually talk about the empty contracts that callers and implementers understand.
Translate IDL
The IDL above is for DOM, as you mention. The DOM is implemented in C# and is much more powerful than its COM cousins (and many versions). If you really want to create a C# class wrapper that can call the COM DOM interfaces:
- use MIDL (comes with Visual Studio) to create a TLB,
- then run tlbimp.exe (comes with .NET) to create a .NET COM wrapper, which you can include in your project.
You can also run tlbimp.exe directly on the COM dlls (in process) or executables (out of process) to create a .NET COM Wrapper.
More information
A basic, yet extremely brilliant and thorough introduction to COM is Don Box's famous book Essential COM, about IDL I suggest Essential IDL by Gudgin.
Absolutely everything there is to know about COM and .NET is written down in the comprehensive, yet slightly bloated, .NET and .COM The Complete Interoperability Guide by Nathan. Not a book you will read from cover to cover, but it makes excellent reference material.