While working in Windows you have seen that, while focusing on any TextBox the entire text inside it has been selected. This is a default behaviour of Windows, but in WPF this behaviour is not present. For this you have to write code.

If you want to add the same behaviour in your WPF application then you have to register the TextBox.GotFocus event for each textbox inside your Window and in that event implementation you have to write the code to select all the text inside the focused TextBox, right? If you do like this, then you have to write so many event registration for each one of the TextBox.

 

 

Lets say, you have a single Window with 100 TextBoxs in it. In this case though you can write the event definition inside a single implementation but you have to register the event for 100 times. Now imagine the same behaviour for 10 Windows inside the same application. Here you have to write the event implementation for each and every Window & register the event for each TextBox. This will definately clutter your code.

So, how can you get the said behaviour in your WPF application? It is a simple trick inside your App.xaml.cs file. This will be a global event handler for any TextBox in your application. If you open your App.xaml.cs you will find a overridable method named "OnStartUp". Inside this register the TextBox.GotFocus event for every TextBox before calling the OnStartup method for the base class. After doing this trick, your OnStartup method will look like this:

 

 

protected override void OnStartup(StartupEventArgs e)
{
    EventManager.RegisterClassHandler(typeof(TextBox), TextBox.GotFocusEvent, new RoutedEventHandler(TextBox_GotFocus));
    base.OnStartup(e);
}

Here, the EventManager class will be responsible to register the event handler for all TextBox. So simple right? Now add the event handler implementation for the TextBox in the same class. This will look like this: 
private void TextBox_GotFocus(object sender, RoutedEventArgs e)
{
    TextBox tb = sender as TextBox;
    tb.Focus();
    tb.SelectAll();
}
Now run your application with multiple Windows having multiple TextBox on them. Focus on any TextBox having some texts inside it. You will see the same behaviour has been added to all of your TextBox.

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