If you want to set the default value of a drop down list from a value being passed to you by a third party you should always include a check to ensure that your drop down contains the value so that you don't end up with some nasty compiler errors.

To keep things simple let's say I want to check if my drop down contains the value "MyValue". If it does I want to pre-select that option for the user. To do this I would use something like below:

if (MyDropDownList.Items.FindByValue("MyValue") != null)
{
    MyDropDownList.SelectedValue = "MyValue";
}

You can also search based on the text value of your drop down by using FindByText instead of FindByValue.