How to get MemberInfo of members declared in base interface?

I attempted to get all the members in an interface with the following code in VB

Imports System.Reflection

Module Module1

    Sub Main()
        Dim T = GetType(IDerived)
        'List all the members available in IDerived
        For Each member In T.GetMembers(BindingFlags.Public Or BindingFlags.Instance)
            Console.WriteLine(member)
        Next
    End Sub

End Module

Interface IBase
    Sub BaseMethod()
End Interface

Interface IDerived
    Inherits IBase

    Sub DerivedMethod()
End Interface

And expcected the output includes both BaseMethod and DerivedMethod.

However, I got only

Void DerivedMethod()

This is different from the behavior of GetMembers that applied to a class Type, which will return all the derived members.

1. I'm curious why it is the case. I suspect it may be designed to avoid name collision or something.

2. And the most important, how to get the members decalared in base interfaces? I have a solution, that I can enumerate all the interfaces it derived from, and then invoke GetMembers one by one, but it seems rather tedious.