How to Set Focus to a WPF TextBox from the ViewModel?

Set TextBox focus from ViewModel in WPF

Question:

In a WPF application, after displaying an error message to the user when a condition fails, the goal is to set focus back to a specific TextBox. However, the current method using System.Windows.Input.Keyboard.Focus does not work.

Question:

How to set focus of TextBox from ViewModel in WPF?

Answer:

Part One: Accessibility Considerations

It is important to note that directly referencing UI elements (e.g., cs.txtCompanyID) in the ViewModel violates the MVVM pattern and may cause accessibility issues. Instead, rely on the data binding mechanism to communicate with the UI.

Part 2: Debugging .NET Source Code

To effectively troubleshoot focus issues, consider debugging .NET source code. Please refer to Shawn Bruke's blog to learn how to enable .NET source code debugging.

Part 3: Using additional attributes

An efficient way to set focus from a ViewModel is to use attached properties. Create an attached property called IsFocused and use the following implementation:

public static class FocusExtension
{
    public static bool GetIsFocused(DependencyObject obj) {...}
    public static void SetIsFocused(DependencyObject obj, bool value) {...}

    public static readonly DependencyProperty IsFocusedProperty =
        DependencyProperty.RegisterAttached(...);

    private static void OnIsFocusedPropertyChanged(...) {...}
}

In your XAML view, bind the IsFocused attached property to the property in the ViewModel that triggers focus:

<TextBox local:FocusExtension.IsFocused="{Binding IsUserNameFocused}" />

When this property is updated in the ViewModel, the UI element will automatically gain focus.

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