C# 3.0 Extension Methods

One of the new features in .NET 3.5 and C# 3.0 is extension methods. Extension methods, as their names state, let you extend any existing type by adding new methods without having to inherit from it.

For instance, one of the common issues we encounter when retrieving data from the database, is to check whether the value is DBNULL or not before we can use it. I used to write a DbReaderHelper class that actually implements ‘dbnull safe’ data retrieval methods. The syntax to declare an extension method is very simple. It is a static method which its first argument starts with the keyword “this” followed by the type we want to extend.

For instance, if I want to add a new method called “nGetInt32” which returns an Int32.MinValue if the field is dbnull otherwise the field value (an Int32), I should write it as follows:

public static Int32 nGetInt32(this DbDataReader reader, int index)
{
if (reader.IsDBNull(index))
return Int32.MinValue;
return reader.GetInt32(index);
}

How to use it?

The extension must be part of a static class (see the example at the end of this post). To use it, simply add the namespace the class belongs to as part of the “using” directives and that’s it. Even intellisense will take it into account;

Why using extension methods

If we want to extend the DataReader class to make it dbnull safe, we would have to inherit from one of its implementations and therefore would not be able to extend all inherited classes. The other advantage is the fact that you can extend any class (even those marked as “sealed”. On the other hand, extending a class has a limitation which is that the extension method can only use public methods and therefore there’s no access to the inner state of the object.

Methods resolution

- Instance methods have priority
- If the same extension method is declared more than once, the compiler raises an error.
- The compiler looks into the current namespace and all the namespaces included with the using directive.