Using MultiBinding to bind elements

Question

I have a login form that contains a username textbox and a password box.

I want the ok button to be enabled only when both the fields contain a value.

I have a converter that check for all the strings if they're null or empty.

I placed a breakpoint on the first line of the Convert method, and it stops only when the MenuItem initializes, afterwords, i.e. when I change the text it doesn't.

The following example works good, the problem is that the multibinding is not triggered when i change the text; it's only bound when initializing the form:

<!--The following is placed in the OK button-->
<Button.IsEnabled>
    <MultiBinding Converter="{StaticResource TrueForAllConverter}">
        <Binding ElementName="tbUserName" Path="Text"/>
        <Binding ElementName="tbPassword" Path="Password"/>
    </MultiBinding>
</Button.IsEnabled>

I think the issue is that you don't get notified when the remote binding source is changed (e.g. there is no an option to set UpdateTargetTrigger="PropertyChanged".

Any ideas?

Answer 1

I would suggest you look into command binding. A command can enable or disable your Login button automatically depending on some condition (ie. user name and password is not empty).

public static RoutedCommand LoginCommand = new RoutedCommand();

private void CanLoginExecute(object sender, CanExecuteRoutedEventArgs e)
{
    e.CanExecute = !string.IsNullOrEmpty(_userInfo.UserName) && !string.IsNullOrEmpty(_userInfo.Password);
    e.Handled = true;
}

private void LoginExecute(object sender, ExecutedRoutedEventArgs e)
{
    MessageBox.Show("Loging in...");
    // Do you login here.
    e.Handled = true;
}

XAML command binding will look something like this

<TextBox Text="{Binding UserName, UpdateSourceTrigger=PropertyChanged}" />
<TextBox Text="{Binding Password, UpdateSourceTrigger=PropertyChanged}" />
<Button Command="local:LoginWindow.LoginCommand" >Login</Button>

To register the command in XAML

<Window.CommandBindings>
    <CommandBinding Command="local:LoginWindow.LoginCommand" CanExecute="CanLoginExecute" Executed="LoginExecute" />
</Window.CommandBindings>

Or in code behind

public LoginWindow()
{
    InitializeComponent();

    CommandBinding cb = new CommandBinding(LoginCommand, CanLoginExecute, LoginExecute);
    this.CommandBindings.Add(cb);
}

More readigin here.