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 calls a different constructor with a `null` argument.
It calls another constructor in your class and passes `null` as a parameter.

You can also write `: base(...)` to explicitly call a constructor in your base class.