It would be usefull to have a Property on the RadMultiColumnComboBox which represents the inputted searchtext. This Property should be TwoWay-Bindable.

A usecase for it could be to programmatically manipulate the searchtext.
For example setting it to NULL on LostFocus:

private void mccb_LostFocus(object sender, RoutedEventArgs e)
{
    this.mccb.ChildrenOfType<RadWatermarkTextBox>().First().Text = string.Empty;
}

Instead of hacking around with the visualTree etc. one could simply bind the property. This would also be MVVM compliant.

Proposed Name for the Property "SearchText".