null check

A few years ago, I blogged about a way to automate unit testing of null argument validation. Its usage looked like this: [Fact] public void FullOuterJoin_Throws_If_Argument_Is_Null() { var left = Enumerable.Empty<int>(); var right = Enumerable.Empty<int>(); TestHelper.AssertThrowsWhenArgumentNull( () => left.FullOuterJoin(right, x => x, y => y, (k, x, y) => 0, 0, 0, null), "left", "right", "leftKeySelector", "rightKeySelector", "resultSelector"); } Basically, for each of the specified parameters, the AssertThrowsWhenArgumentNull method rewrites the lambda expression by replacing the corresponding argument with null, compiles and executes it, and checks that it throws an ArgumentNullException with the appropriate parameter name.

The problem Have you ever written code like the following ? X xx = GetX(); string name = "Default"; if (xx != null && xx.Foo != null && xx.Foo.Bar != null && xx.Foo.Bar.Baz != null) { name = xx.Foo.Bar.Baz.Name; } I bet you have ! You just need to get the value of xx.Foo.Bar.Baz.Name, but you have to test every intermediate object to ensure that it’s not null. It can quickly become annoying if the property you need is nested in a deep object graph….