Set focus on textbox in WPF

In case you haven't found the solution on the other answers, that's how I solved the issue.

Application.Current.Dispatcher.BeginInvoke(new Action(() =>
{
    TEXTBOX_OBJECT.Focus();
}), System.Windows.Threading.DispatcherPriority.Render);

From what I understand the other solutions may not work because the call to Focus() is invoked before the application has rendered the other components.

In Code behind you can achieve it only by doing this.

 private void Window_Loaded(object sender, RoutedEventArgs e)
        {
            txtIndex.Focusable = true;
            txtIndex.Focus();
        }

Note: It wont work before window is loaded

Another possible solution is to use FocusBehavior provided by free DevExpress MVVM Framework:

<TextBox Text="This control is focused on startup">
    <dxmvvm:Interaction.Behaviors>
        <dxmvvm:FocusBehavior/>
    </dxmvvm:Interaction.Behaviors>
</TextBox>

It allows you to focus a control when it's loaded, when a certain event is raised or a property is changed.