您可以从PasswordBox.PasswordChanged事件处理程序中清除PasswordBox：

<Window>
  <PasswordBox PasswordChanged="OnPasswordChanegd" />
</Window>

private void OnPasswordChanged(object sender, RoutedEventArgs e)
{
  var passwordBox = sender as PasswordBox;
  this.ViewModel.AuthenticateUser(passwordBox.SecureString);

  passwordBox.SecureString.Dispose();

  // Clear the password box
  passwordBox.Clear();
}

或者，让视图观察视图模型，以便在Passwordbox准备好被清除时被触发。

ViewModel.cs

class ViewModel : INotifyPropertyChanged
{
  public event EventHandler Authenticated;

  private bool isAuthenticationSuccessful 
  public bool IsAuthenticationSuccessful 
  {
    get => this.isAuthenticationSuccessful;
    private set
    {
      this.isAuthenticationSuccessful = value;
      OnPropertyChanged();
      OnAuthenticated();
    }
  }

  public void AuthenticateUser(SecureString clientPassword)
  {
    // TODO::Validate credentials
    this.IsAuthenticationSuccessful = ?;
  }

  private void OnAuthenticated()
    => this.Authenticated?.Invoke(this, EventArgs.Empty);
}

MainWindow.xaml.cs

class MainWindow : Window
{
  public MainWindow()
  {
    var viewModel = new ViewModel();
    viewModel.Authenticated += OnAuthenticated;

    this.DataContext = viewModel;
  }

  private void OnPasswordChanged(object sender, RoutedEventArgs e)
  {
    var passwordBox = sender as PasswordBox;
    this.ViewModel.AuthenticateUser(passwordBox.SecureString);
  }

  private void OnAuthenticated(object sender, EventArgs e)
  {    
    var viewModel = sender as ViewModel;
    viewModel.Authenticated -= OnAuthenticated;
    this.PasswordBox.SecureString.Dispose();
    this.PasswordBox.Clear();
  }
}