Saving fields to nsuserdefaults xamarin

user defaults.cs

NSUserDefaults userdefaults = NSUserDefaults.StandardUserDefaults;

public override void ViewDidAppear(bool animated)
        {
            base.ViewDidAppear(animated);
            this.txtUserName.Text = NSUserDefaults.StandardUserDefaults.StringForKey("username");
            this.txtPassword.Text = NSUserDefaults.StandardUserDefaults.StringForKey("password");
        }
        
          NSUserDefaults.StandardUserDefaults.SetString(credentials.username.ToString(), "username");
                            NSUserDefaults.StandardUserDefaults.SetString(credentials.password.ToString(), "password");
                            NSUserDefaults.StandardUserDefaults.Synchronize();