Question

I am making a base class that has a virtual method called “GetBaseAddresses()”. It has as a return type IEnumerable<Uri>. The base class will not yield any results if enumerated, but a derived class has the option to override the method and return as many items as it wants.

Here is the base method:

public virtual IEnumerable<Uri> GetBaseAddresses() { }

The problem is, that won’t compile. You must return a value for the compiler to be happy. So, because I want to have an empty result, I would just return null, right?

public virtual IEnumerable<Uri> GetBaseAddresses() { return null; }

The problem there is that if someone does a foreach on an instance of the base class, they will crash with a runtime error of “Object reference not set…”

So, recalling that the yield return keyword does some magic with the C# compiler… I came up with this compiler hack (that works by the way).

public virtual IEnumerable<Uri> GetBaseAddresses()
{
    if (false) { yield return new Uri(""); }
}

Strangely enough, even though the “if (false) { … }” code gets completely compiled away – the compiler is happy with me meeting the “must return a value” requirement, and does exactly what I want – that is, an empty result set that is safe to enumerate.

My question is – Is there a way to do this without my compiler trick?

Answer 1

Just return an empty enumerable in the base method.

    public virtual IEnumerable<Uri> GetBaseAddresses()
    {
        return Enumerable.Empty<Uri>();
    }

Or if you’re targeting a version of the .NET framework < 3.5 return an empty List.