How Show Panel conditionally on change of DropDownList in ASP.Net

Hi @band.atul,

Try this..

Html

<div>
        <asp:DropDownList ID="ddlUsers" runat="server" AutoPostBack="true" OnSelectedIndexChanged="TogglePanelVisibility">
            <asp:ListItem Text="Ashutosh" Value="1" />
            <asp:ListItem Text="Ramkrishna" Value="2" />
            <asp:ListItem Text="Sajid" Value="3" />
            <asp:ListItem Text="Adnaan" Value="4" />
        </asp:DropDownList>
        <asp:Panel ID="pnlUserDetails" runat="server" Visible="false">
            <asp:Label ID="lblUserDetails" runat="server" Text="User Details"></asp:Label>
            <asp:TextBox ID="txtUserDetails" runat="server"></asp:TextBox>
            <asp:CheckBox ID="chkUser" runat="server" Text="Do you want multiline text box" AutoPostBack="true"
                OnCheckedChanged="ToggleTextMode" />
        </asp:Panel>
    </div>

Code:

        protected void TogglePanelVisibility(object sender, EventArgs e)
        {
            if (this.ddlUsers.SelectedIndex == 3)
            {
                this.pnlUserDetails.Visible = true;
            }
            else
            {
                this.pnlUserDetails.Visible = false;
            }
        }

        protected void ToggleTextMode(object sender, EventArgs e)
        {
            if (this.chkUser.Checked)
            {
                this.txtUserDetails.TextMode = TextBoxMode.MultiLine;
            }
            else
            {
                this.txtUserDetails.TextMode = TextBoxMode.SingleLine;
            }
        }