Check if RadioButtonList is selected (checked) using C# and VB.Net in ASP.Net

I have created a sample for your reference.

HTML

<div>
    <asp:RadioButtonList ID="rblTest" runat="server">
        <asp:ListItem Text="Mango" Value="Mango"></asp:ListItem>
        <asp:ListItem Text="Banana" Value="Banana"></asp:ListItem>
        <asp:ListItem Text="Orange" Value="Orange"></asp:ListItem>
    </asp:RadioButtonList>
    <br />
    <asp:Button ID="btnValidate" OnClick="btnValidate_Click" runat="server" Text="Validate" />
</div>

C#

protected void btnValidate_Click(object sender, EventArgs e)
{
    if (rblTest.SelectedIndex == -1)
    {
        string message = "alert('" + "Please select an option." + "');";
        ClientScript.RegisterStartupScript(this.GetType(), "Error_message", message, true);
        return;
    }
}

VB.Net

Protected Sub btnValidate_Click(ByVal sender As Object, ByVal e As EventArgs)
    If rblTest.SelectedIndex = -1 Then
        Dim message As String = "alert('" & "Please select an option." & "');"
        ClientScript.RegisterStartupScript(Me.[GetType](), "Error_message", message, True)
        Return
    End If
End Sub

Screenshot