In my experience, setting the value can be tricky, as it is a variant. For example, when KeyValue has been set to a string value then you want set it to a numeric value, you first have to call the Clear() method to clear the variant type, then set it to new value (can be string or numeric). The same is true if it was set to a numeric value and you want to set it to a string. If KeyValue has been cleared without having been set to a valid value, the control may throw an exception when you attempt to manipulate the combobox value (via mouse or keyboard).