After some struggle I managed to make RadMultiColumnComboBox and RadToolStripItem to live together. Here is what you need to do:
|// Create the combo box and the RadHostItem which contains it.
|RadMultiColumnComboBox multiColumn = new RadMultiColumnComboBox();
|RadHostItem h = new RadHostItem(multiColumn);
|h.MinSize = multiColumn.Size;
|h.AutoSize = true;
|// Add the host item to the toolstrip item.
Note that it is VERY important not to add any other elements to the toolstrip item which contains the multi-column combo, otherwise an infinite layout loop is triggered which causes a stack overflow exception. I hope this is a solution that the users can live with. We will fix the multi-column combo to behave as expected in a future release.
Let us know if this works for you.
the Telerik team