Question

I got an exception when user submitted a form without selecting any value in the dropdownlistfor as follows:

    <div class="editor-field">
        @Html.DropDownListFor(
            x => x.TypeID,
            new SelectList(ViewBag.EventType, "ID", "Name"),
            "-- Select Event Type --"
            )
        @Html.ValidationMessageFor(model => model.TypeID)
    </div>

I got an exception:

Value cannot be null. Parameter name: items

I think I have to check if user selected a value or make an alert, but I’m not sure how to make it.

Answer 1

You should use a nullable type for the TypeID property on your view model if you want to handle the non-selected case in your drop down list ("-- Select Event Type --"):

public int? TypeID { get; set; }

You are getting the exception because if the user doesn’t select any option in the dropdown, the default one is used:

<option value="">"-- Select Event Type --"</option>

Notice how the value is empty string. This obviously cannot be bound to a non-nullable integer type on your model.