i am getting the error as method error 500 in dropdownlist.

Code:

    [WebMethod]
    [System.Web.Script.Services.ScriptMethod]
    public static CascadingDropDownNameValue[] GetDropDownContentsPageMethod(string knownCategoryValues, string category)
    {
        return new categorylist().GetDropDownContents(knownCategoryValues, category);
    }

                 <div class="row">
                     <div class="form-group">
                          <div class="col-sm-2"></div>
                          <label class="control-label col-sm-2" for="Category">Category:</label>
                          <div class="col-sm-8">
                              <asp:ScriptManager ID="ScriptManager1" runat="server" />
                               <asp:DropDownList ID="DropDownList1" runat="server" EnableViewState="False" 
                                        AppendDataBoundItems="True" CssClass="form-control" style="width:50%;">
                                    </asp:DropDownList>
                                   <ajaxToolkit:CascadingDropDown ID="CascadingDropDown1" runat="server" TargetControlID="DropDownList1"
                                        PromptText="Please select" Category="category1" ServicePath="categorylist.asmx"
                                        ServiceMethod="GetDropDownContents" LoadingText="[Loading sections&#8230;]">
                                    </ajaxToolkit:CascadingDropDown>
                                <asp:RequiredFieldValidator ID="RequiredFieldValidator11" runat="server" ErrorMessage="* Select Category"
                                        ControlToValidate="DropDownList1"></asp:RequiredFieldValidator>
                         </div>
                     </div>
                 </div>