DataTemplate中的Silverlight Combobox在第一次单击时打开

I've got a combobox in a datatemplate on my grid, I would like to open it on the first click but it seems that the row is selected first. I have tried changing the isDropDown open on rowchange but for some reason it's not working. I have also tried opening the dropdown from PreparingCellForEdit but that was never fired. Any suggestions?I've got a combobox in a datatemplate on my gri