Question

We want to avoid having a NullReferenceException. Currently we have:

ISomeInterface interface = this.GetISomeInterfaceInstance();
(interface as ClassImplmentsISomeInterface).Method();

This works fine, but risks NullReferenceException. One solution would be:

ISomeInterface interface = this.GetISomeInterfaceInstance();
ClassImplmentsISomeInterface instance = interface as ClassImplmentsISomeInterface;
if (instance != null)
    instance.Method();

But this produces a lot of extra code for a simple check (according to resharper there are 100s of possible NREs.) A second solution method is:

ISomeInterface interface = this.GetISomeInterfaceInstance();
if (interface is ClassImplmentsISomeInterface)
    (interface as ClassImplmentsISomeInterface).Method();

But I gather that is actually uses as in the background, thus doing the cast twice, which I’d like to avoid. Does this matter? For example, is the C# compiler clever enough to optimise this performance issue away?

Is there some other technique I am missing here? Or is one of the above methods preferabled?

Answer 1

If you want to cast speculatively then your first option (as and null check) is recommended. However, you should also consider refactoring the code such that the cast is no longer required.