下拉列表的扩展方法导致错误

我有一个扩展方法，用于DropDownList，它Extension method can only be declared in non-generic, non-nested static classes关键字this。我不太确定为什么，因为我已经看到了很多关于扩展方法的示例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();
    }
}