Interface and inheritance in C# [on hold]

i'm trying to figure a way on how to use interface inheritance and how i can work around a situation where the type is determined at runtime. Assuming the following

interface Ibase 
{
   void B()
}


interface Ichild1: Ibase
{
    void C1()
}

interface Ichild1: Ibase
{
    void C2()
}

class Class1 : Ichild1
{
    // needs to provide implementation for methods B() and C1()
}

class Class2: Ichild2
{
    // needs to provide implementation for methods B() and C2()
}

IBase getClass(credentials)
{
     case 1:
        return Class1()

     case 2:
        return Class2()
}

MainProgram

IBase ref = getClass("XQE");
ref. //-> only B() method is available...

How can i work around this situation in this specific case Thank you.