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";
}
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