当我使用 datetime.parseexact() 时无法识别字符串

当我尝试在sql参数中输入日期时，它不断弹出无法识别的字符串，我什至尝试使用ParseExact()但它仍然显示错误。

我的.aspx ：

<asp:TextBox ID="txtfromdate" runat="server" Width="90px" Height="20px">
</asp:TextBox>
<cc1:CalendarExtender ID="calender1" runat="server" 
                      TargetControlID="txtfromdate" 
                      Format="dd/MM/yyyy">

aspx.cs ：

SqlParameter[] prm = {      
    new SqlParameter("@ContractName",SqlDbType.NVarChar){Value=txtContractname.Text.Trim()},
    new SqlParameter("@FromDate",DateTime.ParseExact(txtfromdate.Text,"dd/MM/yyyy",null)),
    new SqlParameter("@ToDate",DateTime.ParseExact(txtToDate.Text,"dd/MM/yyyy",null)) ,
    new SqlParameter("@VenID",SqlDbType.Int){Value=Convert.ToInt32(ddlsupplier.SelectedValue)} ,
    new SqlParameter("@CreatedBy",SqlDbType.Int){Value=Convert.ToInt32(Session["UserID"])}, //chng
    new SqlParameter("@OrgID",SqlDbType.Int){Value=Convert.ToInt32(Session["orgID"])},
    new SqlParameter("@Termsnconditions",SqlDbType.NVarChar){Value=txtTermsandconditions.Text} ,
    new SqlParameter("@Module",SqlDbType.NVarChar){Value=ddlModule.SelectedValue} ,
    new SqlParameter("@Filename",SqlDbType.NVarChar){Value=ddlAttachment.SelectedValue} ,
    new SqlParameter("@Currency",SqlDbType.NVarChar){Value=ddlCurrency.SelectedItem.Text} ,
    new SqlParameter("@ContractNumber",SqlDbType.NVarChar){Value =txtContractNumber.Text.Trim()} 
   };