C# は Xamarin を使用してアプリケーションを開発する -- アラート ボックス

public static class AlertExtension
	{
		public static void ShowAlert(this Activity activity ,string msg,string title = "", string btnText = "OK"){
			AlertDialog.Builder builder = new AlertDialog.Builder(activity);
			builder.SetTitle(title);
			builder.SetMessage(msg);
			builder.SetCancelable(false);
			builder.SetPositiveButton(btnText, delegate {
					// it is alert ,keep it simple , do nothing here 
				});
			builder.Show();
		}
	}

if(txtPwd.Text != txtCfmPwd.Text){
					this.ShowAlert("Password and confirmed password not match");
				}