Your code assigns the cast to a Control object, so you actually don't need the cast at all. You could just write:
Control c = sender as Control;
This works because ComboBox and TextBox both derive from Control, and you can always upcast ("up" refers to the direction of inheritance).
This works great if all you care about are the properties TextBox and ComboBox inherit from Control. If you need properties specific to those types, you need to do the cast, and assign to the appropriate type:
TextBox b = sender as TextBox;
if (b != null)
{
//Do stuff with it as a TextBox
}
else
{
ComboBox c = sender as ComboBox;
//You should still perform the check here as a matter of good practice.
if (c != null)
{
//Do stuff with it as a ComboBox
}
}