RadGrid, dropDown SelectedIndexChanged事件在asp FormView中不工作

在我的网页中有一个RadGrid，其中有4个字段:
1)公司列表(下拉列表)
2)金额(文本框)
3)备注(文本框)

company下拉列表下的数据如下:
TX - 0%税率
GRE - 11%税
EP - 0%税率
BL - 6%税
TL - 0%税

需求1:

在"添加新记录"时更改下拉列表值当"编辑"记录时，"金额"answers"备注"文本框变为空。

为此，我试图将"DropDown_SelectedIndexChanged"事件代码但是因为我的RadGrid在asp:FormView中，所以我的页面从来没有发布回来的时候我从RadGrid中重新选择项目，代码不起作用

需求2:

如果下拉框中所选项目的文本值为0%，则"金额"文本框应为

我尝试了下面的代码:

protected void rggst_ItemDataBound(object sender, GridItemEventArgs e)
    {
        if (e.Item is GridEditableItem && e.Item.IsInEditMode)
        {
                GridEditableItem item = e.Item as GridEditableItem;
                DropDownList list = item.FindControl("ddlCompany") as DropDownList;
                list.DataTextField = "TaxDescription";
                list.DataValueField = "TaxId";
                list.DataSource = IPRRequest.DLTaxCode(CorporateGroupId);
                list.DataBind();
                if (list.Items.FindByValue("0%"))
                {
                    if (e.Item is GridEditFormInsertItem)
                    {
                        GridEditFormInsertItem insertItem = (GridEditFormInsertItem)e.Item;
                        TextBox txt = (TextBox)insertItem["Amount"].Controls[0];
                        txt.Text = "0.00";
                        txt.Enabled = false;
                    }
                }
        }
    }

但是每次我在下面出错:
不能隐式转换类型"System.Web.UI.WebControls"。ListItem' to 'bool'

下面是我用于RadGrid的HTML和c#代码:

<asp:FormView ID="fvIPRForm" runat="server" DefaultMode="Insert" DataKeyNames="RequestID" DataSourceID="odsIPRForm" EnableModelValidation="True" OnItemInserting="fvIPRForm_ItemInserting" OnDataBound="fvIPRForm_DataBound" OnItemUpdating="fvIPRForm_Updating" OnItemCommand="fvIPRForm_ItemCommand">
 <InsertItemTemplate>
   <asp:Panel ID="pnlApprover" runat="server" Visible="true">
   <telerik:RadMultiPage ID="RadMultiPage3" runat="server" SelectedIndex="0" Width="100%">
   <telerik:RadPageView ID="RadPageView1" runat="server" Width="100%">
   <telerik:RadAjaxPanel ID="Main" runat="server">                                                                                       
      <telerik:RadGrid ID="RGGST" runat="server" AutoGenerateColumns="false"
        ShowStatusBar="true" EnableEmbeddedSkins="true" Skin="Outlook" ShowFooter="True"
        OnItemDataBound="rggst_ItemDataBound"
        OnInsertCommand="rggst_InsertCommand" OnUpdateCommand="rggst_UpdateCommand"
        OnDeleteCommand="rggst_DeleteCommand" OnNeedDataSource= "rggst_NeedDataSource">
      <mastertableview commanditemdisplay="Top" autogeneratecolumns="false" datakeynames="Amount" 
        insertitempageindexaction="ShowItemOnCurrentPage" ShowFooter="True" >                                                                                           
         <CommandItemSettings AddNewRecordText="New" />
           <Columns>
              <telerik:GridEditCommandColumn UniqueName="imagebutton1" ButtonType="ImageButton"></telerik:GridEditCommandColumn>  
              <telerik:GridTemplateColumn UniqueName="Company" HeaderText="Company">
                 <ItemTemplate>
                   <asp:Label ID="lblCompany" Text='<%# Eval("Company") %>' runat="server"></asp:Label>
                 </ItemTemplate>
                 <EditItemTemplate>
                   <asp:DropDownList ID="ddlCompany" runat="server"/>                                                
                 </EditItemTemplate>
              </telerik:GridTemplateColumn>
              <telerik:GridBoundColumn aggregate="SUM" DataField="Amount" HeaderText="Amount" FooterAggregateFormatString="Total : {0:###,##0.00}" 
              UniqueName="Amount" SortExpression="Amount" DataFormatString="{0:n}"></telerik:GridBoundColumn>                                                                                                                        
              <telerik:GridBoundColumn DataField="Remark" HeaderText="Remark"
              UniqueName="Remark" SortExpression="Remark" maxlength ="30"></telerik:GridBoundColumn>                                                                                
              <telerik:GridButtonColumn ConfirmText="Delete this Tax Code?" ConfirmDialogType="RadWindow"
              ConfirmTitle="Delete" ButtonType="ImageButton" CommandName="Delete" ConfirmDialogHeight="160px" ConfirmDialogWidth="250px">
              </telerik:GridButtonColumn>
          </Columns>                                    
          <EditFormSettings>
            <EditColumn ButtonType="ImageButton" />
          </EditFormSettings>
          <PagerStyle AlwaysVisible="True" PageSizeControlType="RadComboBox" />
       </mastertableview>
       </telerik:RadGrid>
   </telerik:RadAjaxPanel>
   </telerik:RadPageView>
</telerik:RadMultiPage>
</asp:Panel>
 </InsertItemTemplate>
<EditItemTemplate>
   //same RadGrid in this section
 </EditItemTemplate>
 <ItemTemplate>
   //same RadGrid in this section
 </ItemTemplate>
</asp:FormView>

这是我为需求1尝试的代码:

protected void ddlTaxCodes1_SelectedIndexChanged(object sender, EventArgs e)
    {
        DropDownList taxCodesList = (DropDownList)sender;
        GridEditableItem item = (GridEditableItem)taxCodesList.NamingContainer;
        string grossAmountTxt = (item["LIGrossAmt"].Controls[0] as TextBox).Text;
        string TaxAmt = (item["LITaxAmt"].Controls[0] as TextBox).Text;
        string TaxableAmt = (item["LITaxableAmt"].Controls[0] as TextBox).Text;
        string Description = (item["LIDescription"].Controls[0] as TextBox).Text;
        grossAmountTxt = "";
        TaxAmt = "";
        TaxableAmt = "";
        Description = "";
    }

请让我知道我在我的代码做什么错误。
请注意，我是新来的电视。

protected void ddlTaxCodes1_SelectedIndexChanged(object sender, EventArgs e)
    {
        if (ddlAcCode.SelectedValue != null || ddlAcCode.SelectedValue != "")
        {
            ddlAcCode.Enabled = false;
            string selItem = ddlAcCode.SelectedItem.Text;
            if (selItem.Contains("0%"))
            {
                txtAmount.Text = "0.00";
                txtAmount.Enabled = false;
                txtRemark.Text = "";
            }
            else
            {
                txtAmount.Text = "";
                txtRemark.Text = "";
            }
        }
    }