Question: ```
    public class AccountMembershipService : IMembershipService
    {
        private readonly MembershipProvider _provider;

        public AccountMembershipService()
            : this(null)
        {
        }

```

I took this bit of code from the AccountModels.cs class automatically created with the MVC3 project.

Can you explain what the 'this(null)' bit is doing?

Answer: 
It will call the single-argument constructor for `AccountMembershipService`, passing a `null` as the argument, before processing the body of the constructor you list.

From [MSDN](http://msdn.microsoft.com/en-us/library/ms173115.aspx):

> 
> A constructor can invoke another constructor in the same object by using the this keyword. Like base, this can be used with or without parameters, and any parameters in the constructor are available as parameters to this, or as part of an expression.
> 
> 
> 

More detail in section 17.10.1 (Constructor initializers) of the [C# spec](http://msdn.microsoft.com/en-us/netframework/aa569283).
It calls a different constructor with a `null` argument.