Auto-implemented properties with non null guard clause?

I do agree with Mark Seeman's notion that Automatic Properties are somewhat evil as they break encapsulation. However I do like the concise syntax, readability and convenience they bring.

I quote:

public string Name { get; set; }

The problem with the code snippet isn’t that it contains too much ceremony. The problem is that it breaks encapsulation. In fact

“[…] getters and setters do not achieve encapsulation or information hiding: they are a language-legitimized way to violate them.”

James O. Coplien & Gertrud Bjørnvig. Lean Architecture. Wiley. 2010. p. 134.

Most of the time, adding a non-null guard clause is good enough for a property setter and I would like to know if there is a better way of doing it than one of the below. By better, I mean in a more concise/less repetitive way.

Using Code Contracts:

private string _username;
public virtual string Username
{
    get { return _username; }
    set 
    {  
        Contract.Requires(value != null);
        _username = value; 
    }
}

Using vanilla .NET:

private string _username;
public virtual string Username
{
    get { return _username; }
    set 
    {
        if (value == null) throw new ArgumentNullException("Username");
        _username = value; 
    }
}