我应该如何在错误输入时清除密码框值？我看到过一些答案，比如“将密码框发送到视图模型”，但这只是打破了MVVM模式。
当前xaml

<PasswordBox x:Name="PasswordInput" Style="{DynamicResource PWbox}" cal:Message.Attach="[Event PasswordChanged] = [Action PasswordChanged($source)]" Grid.Row="3" Grid.Column="2"/>

字符串
viewmodel

public void PasswordChanged(PasswordBox source)
        {
            OldPassword = source.Password;
        }

private string _oldPassword;
        public string OldPassword
        {
            get { return _oldPassword; }
            set
            {

                _oldPassword = value;
                NotifyOfPropertyChange(() => OldPassword);
            }
        }