Set focus on TextBox in WPF from view model

Question

I have a TextBox and a Button in my view.

Now I am checking a condition upon button click and if the condition turns out to be false, displ

Answer 1

None of these worked for me exactly, but for the benefit of others, this is what I ended up writing based on some of the code already provided here.

Usage would be as follows:

And the implementation would be as follows:

/// 

/// Behavior allowing to put focus on element from the view model in a MVVM implementation.
/// 
public static class FocusBehavior
{
    #region Dependency Properties
    /// 

    /// IsFocused dependency property.
    /// 
    public static readonly DependencyProperty IsFocusedProperty =
        DependencyProperty.RegisterAttached("IsFocused", typeof(bool?),
            typeof(FocusBehavior), new FrameworkPropertyMetadata(IsFocusedChanged));
    /// 

    /// Gets the IsFocused property value.
    /// 
    /// The element.
    /// Value of the IsFocused property or null if not set.
    public static bool? GetIsFocused(DependencyObject element)
    {
        if (element == null)
        {
            throw new ArgumentNullException("element");
        }
        return (bool?)element.GetValue(IsFocusedProperty);
    }
    /// 

    /// Sets the IsFocused property value.
    /// 
    /// The element.
    /// The value.
    public static void SetIsFocused(DependencyObject element, bool? value)
    {
        if (element == null)
        {
            throw new ArgumentNullException("element");
        }
        element.SetValue(IsFocusedProperty, value);
    }
    #endregion Dependency Properties

    #region Event Handlers
    /// 

    /// Determines whether the value of the dependency property IsFocused has change.
    /// 
    /// The dependency object.
    /// The  instance containing the event data.
    private static void IsFocusedChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        // Ensure it is a FrameworkElement instance.
        var fe = d as FrameworkElement;
        if (fe != null && e.OldValue == null && e.NewValue != null && (bool)e.NewValue)
        {
            // Attach to the Loaded event to set the focus there. If we do it here it will
            // be overridden by the view rendering the framework element.
            fe.Loaded += FrameworkElementLoaded;
        }
    }
    /// 

    /// Sets the focus when the framework element is loaded and ready to receive input.
    /// 
    /// The sender.
    /// The  instance containing the event data.
    private static void FrameworkElementLoaded(object sender, RoutedEventArgs e)
    {
        // Ensure it is a FrameworkElement instance.
        var fe = sender as FrameworkElement;
        if (fe != null)
        {
            // Remove the event handler registration.
            fe.Loaded -= FrameworkElementLoaded;
            // Set the focus to the given framework element.
            fe.Focus();
            // Determine if it is a text box like element.
            var tb = fe as TextBoxBase;
            if (tb != null)
            {
                // Select all text to be ready for replacement.
                tb.SelectAll();
            }
        }
    }
    #endregion Event Handlers
}