The problem with optional arguments in C# is whether the callee sees the object as a TestClass or an ITestInterface. In the first case, the values declared in the class apply. In the second case the values declared in the interface apply. It is because the compiler uses the statically available type information to construct the call. In case of an explicit interface implementation the method is never called ‘for a class’, always ‘for an interface’
The C# Language Specification in 10.6.1 states:
If optional parameters occur in an implementing partial method declaration (§10.2.7) , an explicit interface member implementation (§13.4.1) or in a single-parameter indexer declaration (§10.9) the compiler should give a warning, since these members can never be invoked in a way that permits arguments to be omitted.