Search notes:

COM related methods in the System.Runtime.InteropServices.Marshal class

Some(!) COM related methods in System.Runtime.InteropServices.Marshal include

AddRef and its counterpart Release
GetComInterfaceForObject (returns IntPtr)
GetIUnknownForObject (returns IntPtr)
GetIDispatchForObject (returns IntPtr)
QueryInterface
AllocCoTaskMem
GetEndComSlot
IsComObject
GetObjectForIUnknown (returns System.Object)

Some GetXXX methods return an System.IntPtr that represents an IUnknown interface.

The AddRef and Release methods require such a pointer in their argument.

See also

Interoperability between managed and unmanaged code