dropdownvalue
is a DropDownList object. You can't assign a string to it, that would be meaningless.
What you are actually trying to do is change the
.Text
displayed on the control, so try something like ...
dropdownvalue.Text = "Unavailable"
.
Note this will only work if "Unavailable" is already in the list of items on the control. If it isn't there you will have to add it.
What I think you
meant to do was say
dropdownvalues = "Unavailable";