why?comboBox1.DropDownStyle will conflict with combobox.SelectionStart property ?

when i want to get the combobox.selectionStart or combobox.selectionLength value , the project will throw an exception after run this code:

this.comboBox1.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDownlist;

when i change the code like this :

this.comboBox1.DropDownStyle = System.Windows.Forms.ComboBoxStyle.DropDown;

i can get the value of the combobox.selectionStart or combobox.selectionLength ;

why

The exception is like this : combobox.selectionStart = -1448642680

in other's computer will not throw the exception !

my operate system is english-xp , and .net is 2005 for c#

thank for your reply . !