Why does calling an Action, that is null, throw NullReferenceException?

Question

Why do I have to check if an Action is not null to avoid getting a NullReferenceException? Isn't it logical if there is no action, then it's okay to just do nothing and proceed? I don't understand why it has to throw an exception. Action is a class, why doesn't it handle this itself?

Answer 1

You're trying to invoke an instance method (Invoke) on an object. Doing that always fails with a NullReferenceException if you're using a null reference¹. For example:

 object x = null;
 string y = x.ToString();

Should object.ToString() handle that too?

Basically, this is consistent with the way the rest of the type system works. The C# language could have been designed in a different way - potentially only for the "shorthand" of action() meaning action.Invoke() - but it wasn't, and it's not going to change now.

It's easy enough to add your own extension method to Action if you want to:

public static class ActionExtensions
{
    public static void NullSafeInvoke(this Action action)
    {
        if (action != null)
        {
            action();
        }
    }
}

Or use the C# 6 null-conditional operator to only invoke the delegate when the reference is non-null:

myAction?.Invoke();

(This works for any delegate type, not just actions - for EventHandler for example, you'd use something like handler?.Invoke(this, new EventArgs()). The EventArgs() constructor won't even be called if handler is null.)

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¹ At least using C#. There are ways of invoking instance methods non-virtually "on" null references in IL, but it's far from the normal.