Understanding COM Objects and how to declare them

Question

Say I want to create the interface for IMMDeviceEnumerator.

I see examples online showing the definition:

[ComImport]
[Guid("A95664D2-9614-4F35-A746-DE8DB63617E6")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
public interface IMMDeviceEnumerator
{

}

What I understand (maybe): The [ComImport] Attribute specifies that it's from a dll. The [Guid] attribute is the interface identifier.

What I don't understand: How was this GUID value obtained? What does the [InterfaceType] attribute do? How do I fill in the rest of the functions?

I'm so lost trying to figure this stuff out, all the documentation is super opaque.

Answer 1

How was this GUID value obtained?

The GUID is created as part of the COM interface definition; since you're trying to call someone else's object - you need to use their GUID. You can find it in the mmdeviceapi.h the MMDevice docs point to.

Header file Mmdeviceapi.h defines the interfaces in the MMDevice API.

 MIDL_INTERFACE("A95664D2-9614-4F35-A746-DE8DB63617E6")
    IMMDeviceEnumerator : public IUnknown

The normal way to do this is to add a reference to the COM dll or run tlbimp.exe which will generate a COM Class Wrapper for you with all the magic goo.

If a COM type library isn't available, though - then you basically have to do it yourself by going through the IDL file.

Like p/invoke signatures, this can get pretty painful - so best to use an existing one if you can.

For the larger questions of COM interop, it basically requires learning a little bit of COM and being familiar with C#. The general docs are out there, but usually if you're just trying to use a well known COM component you're best off using a library if you can.