@Html.DropDownListFor(m => m.SelectedValue,
new SelectList(Model.CollectionOfItemsForDropdown, "ValueField", "NameField"),
@Model.CollectionOfItemsForDropdown.Count > 1
? (object)new { @class = "form-control", required = "true", }
: new { @class = "form-control", required = "true", disabled = "disabled" })

You have to use the conditional operator here for the anonymous objects because the dropdown list will be disabled if the word ‘disabled’ is rendered in the tag in any way.