将行从gridview传输到数据集不起作用

Question

我根据选择从gridview中选择记录并将其传递给datatable但不起作用。它抛出错误并且无法正常工作，如何根据frop down选择复制这些行？

代码：

protected void btnGenerateReport_Click(object sender, EventArgs e)
    {
        dtRoom.Columns.Add(new DataColumn("ID", typeof(Int64)));
        dtRoom.Columns.Add(new DataColumn("RecievingDate", typeof(string)));
        dtRoom.Columns.Add(new DataColumn("FromMobileNo", typeof(string)));
        dtRoom.Columns.Add(new DataColumn("Message", typeof(string)));
        dtRoom.Columns.Add(new DataColumn("IsComplaint", typeof(short)));

        for (int i = 0; i <= GridViewSmsComplaints.Rows.Count; i++)
        {
            //drw = dtRoom.NewRow();
            DropDownList IsValid = (DropDownList) GridViewSmsComplaints.Rows[i].FindControl("ddlValidity");
            if (IsValid.SelectedValue == "1")
            {

                DataRow datarw = null;
                dtRoom = new DataTable();
                datarw = dtRoom.NewRow();
                for (int j = 1; j < GridViewSmsComplaints.Rows[i].Cells.Count; j++)
                {
                    datarw[j] = GridViewSmsComplaints.Rows[i].Cells[j].Text;
                }
                dtRoom.Rows.Add(datarw);



            }
        }
        GridViewSmsComplaints.DataSource = dtRoom;
        GridViewSmsComplaints.DataBind();
        Response.Write("Done");
    }
}

<asp:GridView ID="GridViewSmsComplaints" AllowPaging="True" PageSize="6" runat="server" OnPageIndexChanging="GridViewSmsComplaints_PageIndexChanging" AutoGenerateColumns="False" CssClass="mGrid" BorderColor="#333333"  Width="550px">
               <Columns>
                    <asp:BoundField HeaderText="ID" DataField="ID" /> 
                    <asp:BoundField HeaderText="Recieving Date" DataField="RecievingDate" />  
                    <%--<asp:BoundField HeaderText="ToMobileNo" DataField="ToMobileNo" />  --%>
                    <asp:BoundField HeaderText="FromMobileNo" DataField="FromMobileNo" /> 
                    <asp:BoundField HeaderText="Message" DataField="Message" >   
                     <ItemStyle Wrap="True" />
                    </asp:BoundField>
                    <asp:TemplateField HeaderText="IsComplaint">
                     <ItemTemplate>
                       <asp:DropDownList ID="ddlValidity" runat="server">
                           <asp:ListItem Value="1" Text="Valid"></asp:ListItem>
                           <asp:ListItem Value="0" Text="Invalid"></asp:ListItem>
                       </asp:DropDownList>
                     </ItemTemplate>
                    </asp:TemplateField>
               </Columns>
    </asp:GridView>

错误： 对象引用未设置为对象的实例。

第77行：dtRoom.Columns.Add（新的DataColumn（＆＃34; ID＆＃34;，typeof（Int64）））;