必填字段验证器在后退列表中消失

Question

I populate two dropdownlist in asp.net. Both are assigned to a required field validator.

The codebehind is as below

 if (!Page.IsPostBack)
        {
            DataTable dt = new DataTable();
            dt.Columns.Add("emp");
            dt.Columns.Add("ename");
            for (int i = 0; i < 5; i++)
            {
                DataRow dr = dt.NewRow();
                dr["emp"] = (i + 1).ToString();
                dr["ename"] = (i + 1).ToString();
                dt.Rows.Add(dr);
            }
            ddlEmp.DataSource = dt;
            ddlEmp.DataTextField = "emp";
            ddlEmp.DataValueField = "ename";
            ddlEmp.DataBind();
            ListItem l1 = new ListItem("--Select--", "0");
            ddlEmp.Items.Insert(0, l1);
            DropDownList1.DataSource = dt;
            DropDownList1.DataTextField = "emp";
            DropDownList1.DataValueField = "ename";
            DropDownList1.DataBind();
            DropDownList1.Items.Insert(0, l1);

        }

the designer code is as below

 <asp:DropDownList ID="ddlEmp" AutoPostBack="true" runat="server"></asp:DropDownList>
    <asp:RequiredFieldValidator ID="rfvEmp" runat="server" ControlToValidate="ddlEmp" ErrorMessage ="employee" InitialValue="0">
    </asp:RequiredFieldValidator>

    <asp:DropDownList ID="DropDownList1" AutoPostBack="true" runat="server"></asp:DropDownList>
    <asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="DropDownList1" ErrorMessage ="DropDownList1" InitialValue="0">
    </asp:RequiredFieldValidator>
    <asp:Button ID="btn" runat="server" CausesValidation="true" />

Now what happens is when I choose a field, and then once again go and choose"-- Select--", the validator appears and disappears.

Why doesnt the validator stay? Where am I going wrong?

Hema

Answer 1

This issue has bitten me a bunch of times and just because it's a little wacky how they designed it in my opinion.

The problem is your using the InitialValue property to compare to the value property of the list item when it should be comparing to the text value. They should have named the property InitialText or something...

Change your RequiredFieldValidator to the following:

<asp:RequiredFieldValidator ID="rfvEmp" runat="server" ControlToValidate="ddlEmp" ErrorMessage="employee" InitialValue="--Select--">
</asp:RequiredFieldValidator>

<asp:RequiredFieldValidator ID="RequiredFieldValidator1" runat="server" ControlToValidate="DropDownList1" ErrorMessage ="DropDownList1" InitialValue="--Select--">  
</asp:RequiredFieldValidator>

The client side code is comparing the value that is being displayed, not the value attached to the selection behind the scenes.

Answer 2

Workaraund：如果所选项是默认项，则在autopostback上执行的codebehind方法上将RequiredFieldValidator1.IsValid属性设置为false。