How to fetch dropdown selected value from view in controller (MVC)

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MVC4

How to fetch dropdown selected value from view in controller to add ModelState error as we added for textbox values for being null?
if (string.IsNullOrEmpty(employee.Name))
{
ModelState.AddModelError("Name","The Name field is required");
}
Ex:When we are creating an Employee if the user selects "Select Department" value from Department dropdown list,we need to add the error stating "Please select the valid Department"
How do we achieve this?