Question

How can I still have a Placeholder text (.Text = “Whatever”) while also binding the textbox to a ListBox’s Selected Item?

Answer 1

When do you want this placeholder text to display? When there is no SelectedItem?

You could use PriorityBinding which will allow you to provide a list of bindings and it will use the first that produces a result.

Something like:

<TextBox> 
   <TextBox.Text>
     <PriorityBinding>
       <Binding Path="myListBox.SelectedItem"/>
       <Binding Source="Default Text"/>
     </PriorityBinding>
   </TextBox.text>
 </TextBox>