Setting default text for a combo box

I'm wondering if there is a way in silverlight to set a placeholder value for a combo box.

The behavior I wish to achieve is such that when the combo box has no selection, a placeholder value is shown.

This value shouldn't be displayed when the 开发者_运维技巧user clicks to open the list of values, and if the selected index is cleared, the placeholder value is shown again.

Thanks!

Have a look at this How to display default text "--Select Team --" in combo box on pageload in WPF? and this Silverlight 4 ComboBox - Binding to Nullable data (tried TargetNullValue but not working as expected)