DropDownList连动写不进数据库

前台：<asp:DropDownList ID="tsdlDropDownList" runat="server" AutoPostBack="True" 
                        DataSourceID="SqlDataSource1" DataTextField="tsdl_TypeName" 
                        DataValueField="tsdl_TypeId" ondatabound="tsdlDropDownList_DataBound">
                    </asp:DropDownList>
                    <asp:SqlDataSource ID="SqlDataSource1" runat="server" 
                        ConnectionString="<%$ ConnectionStrings:ZHLbookshop1 %>" 
                        SelectCommand="SELECT * FROM [BooksType]"></asp:SqlDataSource>
                        
                    <asp:DropDownList ID="tsxlDropDownList" runat="server" 
                       
                       ondatabound="tsxlDropDownList_DataBound" DataSourceID="SqlDataSource2" 
                        DataTextField="tsxl_TypeName" DataValueField="tsxl_TypeId">
                    </asp:DropDownList>
                    <asp:SqlDataSource ID="SqlDataSource2" runat="server" 
                        ConnectionString="<%$ ConnectionStrings:ZHLbookshop1 %>" 
                        SelectCommand="SELECT * FROM [BooksType2] WHERE ([tsdl_TypeId] = @tsdl_TypeId)">
                        <SelectParameters>
                            <asp:ControlParameter ControlID="tsdlDropDownList" Name="tsdl_TypeId" 
                                PropertyName="SelectedValue" Type="Int32" />
                        </SelectParameters>
                    </asp:SqlDataSource>后台：   protected void AddButton_Click(object sender, EventArgs e)
    {
        if (tsdlDropDownList.Text == "" || tsdlDropDownList.Text == null || tsxlDropDownList.Text == "" || tsxlDropDownList.Text == null
    || addition.Text == "" || addition.Text == null)
        {
            common.ShowMsg(this, "请输入要添加的内容！");
            return;
        }
        BooksType3 b = new BooksType3();
        b.tsxxl_TypeName = addition.Text;
        b.tsdl_TypeId = int.Parse(tsdlDropDownList.SelectedValue);
        b.tsxl_TypeId = int.Parse(tsxlDropDownList.SelectedValue);
        if (b.Add() > 0){            common.ShowMsg(this, "成功");
        
        
        }
            
        }还有一个添加类： public int Add()
        {
            StringBuilder strSql = new StringBuilder();
            strSql.Append("insert into BooksType3(");
            strSql.Append("tsxxl_TypeName,tsxl_TypeId,tsdl_TypeId)");
            strSql.Append(" values (");
            strSql.Append("@tsxxl_TypeName,@tsxl_TypeId,@tsdl_TypeId)");
            strSql.Append(";select @@IDENTITY");
            SqlParameter[] parameters = {
new SqlParameter("@tsxxl_TypeName", SqlDbType.VarChar,100),
new SqlParameter("@tsxl_TypeId", SqlDbType.Int,4),
new SqlParameter("@tsdl_TypeId", SqlDbType.Int,4)};
            parameters[0].Value = tsxxl_TypeName;
            parameters[1].Value = tsxl_TypeId;
            parameters[2].Value = tsdl_TypeId;            return dbHelper.ExecuteCommand(strSql.ToString(), parameters);
        }