How to Securely Bind a PasswordBox to a ViewModel in WPF?

Securely bind PasswordBox in WPF MVVM mode: detailed steps

Binding PasswordBox in MVVM architecture will cause security issues, but a safe and reliable binding method can be achieved. A common technique is illustrated in the code example provided at http://www.wpftutorial.net/PasswordBox.html.

PasswordBox binding practice

Let’s dive into the implementation of this technology. Assume that the ViewModel contains username and password properties. Binding the username to the TextBox is very simple, but binding the password to the PasswordBox requires some modification.

Using the provided code you can include a PasswordBox in XAML:

<code class="language-xml"><passwordbox ff:passwordhelper.attach="True" ff:passwordhelper.password="{Binding Path=Password}" width="130"></passwordbox></code>

With this setup, the following code demonstrates using the Command property in a ViewModel:

<code class="language-csharp">private DelegateCommand loginCommand;

public string Username { get; set; }
public string Password { get; set; }

public ICommand LoginCommand
{
    get
    {
        if (loginCommand == null)
        {
            loginCommand = new DelegateCommand(Login, CanLogin);
        }
        return loginCommand;
    }
}

private bool CanLogin()
{
    return !string.IsNullOrEmpty(Username);
}

private void Login()
{
    bool result = securityService.IsValidLogin(Username, Password);
}</code>

Hidden steps?

While the above code ensures binding, there is a critical step in PasswordBox binding that is often overlooked. When inspecting the XAML, you'll see that the TextBox's username binding works as expected, but the PasswordBox's password binding does not update the ViewModel property.

Secret Assistant

In fact, you set a breakpoint in the helper class, confirming that the code executed but failed to update the Password property of the ViewModel. This is where a critical step is missing.

Manual binding

To complete the implementation, a connection needs to be established between PasswordBox and ViewModel. In the code-behind file, define a handler for the PasswordBox's PasswordChanged event:

<code class="language-csharp">private void PasswordBox_PasswordChanged(object sender, RoutedEventArgs e)
{
    if (this.DataContext != null)
    {
        ((dynamic)this.DataContext).SecurePassword = ((PasswordBox)sender).SecurePassword;
    }
}</code>

Safety method

By defining a SecureString property in the ViewModel and handling the PasswordChanged event, you can securely retrieve the password value while maintaining MVVM principles. This approach avoids violating security guidelines and maintains a clear separation between Views and ViewModels.

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