"Calling a method on null a reference" by Jon Skeet

Two months ago, I wrote a blog post called Extension Methods and null Objects. In this post, I described how to write Extension Methods in C# 3 and wrote a small sample code to add IsNullOrEmpty as an extension method to the String class. Well, guess what, I found that particular example in a book.

A few weeks ago, I bought Jon Skeet’s book C# in Depth, and have been reading it trough since then (I have to say that it is very well written and a pure joy to read, highly recommended if you are somewhat familiar with C# and would like to polish your knowledge of the language).

Reading part 10.2.4, "Calling a method on a null reference", I was surprised to discover that Jon used exactly the same example as I did! Well, I have to admit that it is I that used the same example as him, as it was already long printed when I wrote that post.

At first it seems odd to be able to call IsNullOrEmpty on a variable that is null without an exception being thrown, particularly if you’re familiar with it as a static method from .NET 2.0. In my view, code using the extension method is more easily understandable. For instance, if you read the expression if (name.IsNullOrEmpty()) out loud, it says exactly what it’s doing.

Nevertheless, I was very pleased with myself and, dare I say it, immediately thought "Great Minds Think Alike" :p.

January 18, 2009 | Tags: Tags: , , , | 1 Comment 

Extension Methods and null Objects

C# 3.0 allows adding new methods to existing types. These are called Extension Methods. This allow programmers to add methods to existing types, even if they are not partial or event sealed.

These special methods have to be declared in a special way:

  1. They must be declared within a static class
  2. They must be static
  3. The first parameter of the method has to be the type that is "extended", preceded by the this keyword

So, for example, let’s copy the very useful IsNullOrEmpty method as an extension method.

public static class Extensions
{
    public static bool IsNullOrEmpty(this String s)
    {
        return String.IsNullOrEmpty(s);
    }
}

Simple. Calling this

String s = "";
s.IsNullOrEmpty();

returns true. You can even do

"".IsNullOrEmpty();

which returns true as well.

Now, all of this is actually an illusion. When the compilers finds

"".IsNullOrEmpty();

he will translates it in 

Extensions.IsNullOrEmpty("");

So, when an extension method is used, there is one more parameter, that is of the type the extension method is defined on. In this case, String.

But, if it as parameter, it can be null, right? Yes!

String s = null;
s.IsNullOrEmpty();

or even

((String) null).IsNullOrEmpty()

will both work, without trowing a NullReferenceException.

Nothing revolutionary here, but I found funny to "call" method on a null object and not have an exception raised.

November 6, 2008 | Tags: Tags: , , | 2 Comments