The return type of the members on an Interface Implementation must match exactly the interface definition?

Question

According to CSharp Language Specification.

An interface defines a contract that can be implemented by classes and structs. An interface does not p

Answer 1

Simple fact is, if an interface says:

IInterface{
   Animal A { get; }
}

Then an implementation of that property must match the type exactly. Trying to implement it as

MyClass : IInterface{
  Duck A { get; }
}

Does not work - even though Duck is an Animal

Instead you can do this:

MyClass : IInterface{
  Duck A { get; }
  Animal IInterface.A { get { return A; } }
}

I.e. provide an explicit implementation of the IInterface.A member, exploiting the type relationship between Duck and Animal.

In your case this means implementing, the getter at least, ITest.Integers as

IEnumerable ITest.Integers { get { return Integers; } }

To implement the setter, you will need to cast optimistically or use .ToList() on the input value.

Note that the use of A and Integers inside these explicit implementations is not recursive because an explicit interface implementation is hidden from the public view of a type - they only kick in when a caller talks to the type through it's IInterface/ITest interface implementation.