How to write code for enter only digits in dropdownlist ?

Hi,
I have written windows application
My requirement is I have to enter only digits in DropdownList, for this I have writter code as following

private void BranchCodeRadDropDownList_KeyPress(object sender, KeyPressEventArgs e)
{
BranchCodeRadDropDownList.DropDownStyle = RadDropDownStyle.DropDown;
BranchCodeRadDropDownList.AutoCompleteMode = AutoCompleteMode.SuggestAppend;
Regex regex = new Regex("^[0-9\b]*$");
e.Handled= (!regex.IsMatch(e.KeyChar.ToString())); }
How to write code for enter only digits in dropdownlist ?