gistfile1.cs

public class NewUserViewModel : ReactiveObject
{
  // This is ReactiveUI's version of implementing INotifyPropertyChanged
  string _Password;
  public string Password {
    get { return _Password; }
    set { this.RaiseAndSetIfChanged(x => x.Password, value); }
  }
 
  string _PasswordConfirmation;
  public string PasswordConfirmation {
    get { return _PasswordConfirmation; }
    set { this.RaiseAndSetIfChanged(x => x.PasswordConfirmation, value); }
  }
 
  ICommand OkCommand { get; protected set; }
 
  public NewUserViewModel()
  {
    // Here's the interesting part - we'll combine the change notifications
    // for Password and PasswordConfirmation, and that will determine when
    // we can hit the Ok button
    //
    // canHitOk is now a stream of True's and False's
    var canHitOk = this.WhenAny(
      x => x.Password, 
      x => x.PasswordConfirmation,
      (pass, confirm) => (pass.Value == confirm.Value && pass.Value.Length > 3));
 
    // Feed this to the canExecute of the OkCommand - now the button is
    // bound to the two properties and will disable itself until the 
    // Func above is true.
    OkCommand = new ReactiveCommand(canHitOk);
  }
}