[Solved] ASP.Net Validator Error Messages hides on PostBack

By default this will not happen and you need to do the following trick i.e. make the validators validate On PostBack

HTML

<asp:DropDownList runat="server" AutoPostBack = "true">
    <asp:ListItem Text="text1" />
    <asp:ListItem Text="text2" />
</asp:DropDownList>
<asp:TextBox ID = "txtName" runat="server" />  
<asp:RequiredFieldValidator ErrorMessage="Required" ControlToValidate="txtName"
    runat="server" ValidationGroup = "registration"/>
    <br />
<asp:Button Text="Submit" runat="server" ValidationGroup = "registration" />

Code

protected void Page_Load(object sender, EventArgs e)
{
    if (this.IsPostBack)
    {
        ClientScript.RegisterClientScriptBlock(this.GetType(), "Validation", "window.onload = function() { Page_ClientValidate('registration'); };", true);
    }
}

Please run this code

HTML

<asp:DropDownList ID="ddlItems" runat="server" AutoPostBack="true">
    <asp:ListItem Text="Item1" Value="Item1" />
    <asp:ListItem Text="Item1" Value="Item2" />
    <asp:ListItem Text="Item2" Value="Item3" />
</asp:DropDownList>
<asp:TextBox ID="txtProductName" runat="server" />
<asp:RequiredFieldValidator runat="server" ID="reqProductName" Display="Dynamic"
    ControlToValidate="txtProductName" ErrorMessage="Enter Product Name" ValidationGroup="registration"></asp:RequiredFieldValidator>
<asp:Button Text="Save" runat="server" OnClick="Redirect" ValidationGroup="registration" />

C#

protected void Page_Load(object sender, EventArgs e)
{        
    if (!this.IsPostBack)
    {

    }
    else
    {
        foreach (BaseValidator validator in Page.Validators)
        {
            validator.Validate();
        }
    }
}
protected void Redirect(object sender, EventArgs e)
{
    //Do some work here
}