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: 
There is most likely another constructor in your class that looks like this:

```
public AccountMembershipService(MembershipProvider provider)
{
    _provider = provider;
}

```

Your code calls this constructor first and passes `null` as the argument for `provider`, then executes the original constructor.
It calls a different constructor with a `null` argument.