How? Editable DataGridComboBoxColumn

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C#

WPF

Hey Guys,
I'm a newbie with wpf and I'm kinda desperate .. I hope someone can help me..

I've created a datagrid, bound to datatable. The Datagrid has a DataGridComboBoxColumn. The Combobox is working, all values a correctly displayed and the selection ist stored. Everything fine, so far..
I want to make this box editable, so you can type a value.. This works for normal comboboxes, but there's no editable Property for DG comboboxes. I've been searching all day for a solution, but it havn't found anything yet..

Is there any way to do this?

(I'm using the .NET 4.0 Datagrid)