Question

I have an extension method for DropDownList that’s throwing Extension method can only be declared in non-generic, non-nested static classes on the keyword this. I’m not quite sure why though since I’ve seen plenty of examples with extension methods on DropDownList

public static class DropDownListExtensions {
    public static void populateDropDownList(this DropDownList source, Action delegateAction) {
        source.DataValueField = "Key";
        source.DataTextField = "Value";
        source.DataSource = delegateAction;
        source.DataBind();
    }
}

Answer 1

Possibly your DropDownListExtensions class declared as nested of some other class. Move it to namespace level.