Question

What is the best way to throw a required validation error using a validation attribute with a dropdownlist that has a default value of 0? If the value is 0, or the default value, I want the attribute to throw the error for my model.

Answer 1

you can provide the option label ^Docs that will set the selected value to 0 if not specified other wise like @Iridio menttioned in his answer, anotate the view model property with [Required]

public class MyVieWModel
 {
   [Required]
   public int MyValue { get;set;}
   public SelectList MyValues {get;set;}
 }

and in the view

@Html.DropDownListFor(x=>x.MyValue,Model.MyValues,"-- Select --")