Validate property in setter

See more:

MVVM

C#6.0

Say I have the property

private string _lastName;
[Required]
[StringLength(20)]
public string LastName
{
      get {return this._lastName;}
      set
      {
           if(value != this.-lastName)
           {
                ValidateProperty(value);
                this._lastName = value;
                RaisePropertyCanged("LastName");
           }
      }
}

The LastName comes from the textbox in a WPF view, two way binding.
When I don't fill it in the textbox, it is null. The code never reaches the setter in property.

What I have tried:

Try to move the Validation method out of setter. Not sure!