Validate either ASP.Net DropDownList or TextBox using C# and VB.Net

Hi Corobori,

Please refer below sample.

HTML

<asp:DropDownList ID="ddlDetails" runat="server">
    <asp:ListItem Value="0" Text="--Select Details--"></asp:ListItem>
    <asp:ListItem Value="1" Text="ASPSnippets"></asp:ListItem>
    <asp:ListItem Value="2" Text="ASPForum"></asp:ListItem>
    <asp:ListItem Value="3" Text="Excelasoft Solution"></asp:ListItem>
</asp:DropDownList>
<br />
<br />
<asp:TextBox ID="txtValue" runat="server"></asp:TextBox>
<br />
<asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="Submit" />
<br />
<asp:Label Text="Invalid" runat="server" ID="lblMesssage"  Visible="false"/>

Code

C#

protected void Submit(object sender, EventArgs e)
{
    lblMesssage.Visible = false;
    if (ddlDetails.SelectedIndex > 0 && !string.IsNullOrEmpty(txtValue.Text))
    {
        lblMesssage.Visible = true;
        return;
    }
    if (ddlDetails.SelectedIndex == 0 && string.IsNullOrEmpty(txtValue.Text))
    {
        lblMesssage.Visible = true;
        return;
    }
}

VB.Net

Protected Sub Submit(ByVal sender As Object, ByVal e As EventArgs)
    lblMesssage.Visible = False
    If ddlDetails.SelectedIndex > 0 AndAlso Not String.IsNullOrEmpty(txtValue.Text) Then
        lblMesssage.Visible = True
        Return
    End If
    If ddlDetails.SelectedIndex = 0 AndAlso String.IsNullOrEmpty(txtValue.Text) Then
        lblMesssage.Visible = True
        Return
    End If
End Sub

Screenshot