Setting the caretindex on a maskedtextbox (WPF)

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C#

XAML

WPF

I have a MaskedTextBox from the Extended WPF Toolkit and I want to set the CaretIndex to 0 every time the user clicks on the TextBox.

What I have tried:

I have tried using the events MouseUp and GotFocus and tried setting the CaretIndex and the SelectionStart like so within these events;

DateJoinedTextBox.SelectionStart = 0;

and

DateJoinedTextBox.CaretIndex = 0;

but even after trying this the Caret position is still decided on where the user clicks, rather than being forced to 0.