validate a combination of different fields - asp.net

Question

aspx page
<asp:Label ID="Label1" runat="server" Text="Date of Birth: "></asp:Label>
<asp:Label ID="dob_msg_lbl" runat="server" Font-Size="Medium" ForeColor="Red" Text="invalid date of birth" visible="False"></asp:Label>
<br />
<asp:TextBox ID="day_tb" runat="server" MaxLength="2" Width="15px"></asp:TextBox> <asp:Label ID="Label4" runat="server" Text="/" Font-Bold="True"></asp:Label>
<asp:DropDownList ID="month_ddl" runat="server">
<asp:ListItem Value=""></asp:ListItem>
<asp:ListItem Value="Jan">January</asp:ListItem>
<asp:ListItem Value="Feb">Feb</asp:ListItem>
<asp:ListItem Value="Mar">Mar</asp:ListItem>
<asp:ListItem Value="Apr">Apr</asp:ListItem>
<asp:ListItem Value="May">May</asp:ListItem>
<asp:ListItem Value="Jun">Jun</asp:ListItem>
<asp:ListItem Value="Jul">Jul</asp:ListItem>
<asp:ListItem Value="Aug">Aug</asp:ListItem>
<asp:ListItem Value="Sep">Sep</asp:ListItem>
<asp:ListItem Value="Oct">Oct</asp:ListItem>
<asp:ListItem Value="Nov">Nov</asp:ListItem>
<asp:ListItem Value="Dec">Dec</asp:ListItem>
</asp:DropDownList> <asp:Label ID="Label5" runat="server" Text="/" Font-Bold="True"></asp:Label> <asp:TextBox ID="year_tb" runat="server" Width="30px" MaxLength="4"></asp:TextBox>
aspx.vb page
Dim dobStr As String = day_tb.Text + " " + month_ddl.SelectedValue + " " + year_tb.Text
Try
dob = Convert.ToDateTime(dobStr)
Catch ex As Exception
dob_msg_lbl.Visible = True
End Try
Currently I am mimicking the asp.net's validation functions by setting dob_msg_lbl.visible to true when Convert.ToDateTime results in an exception.
The problem with such a method is that the validation only occurs when the form is submitted.
I want to validate it on the fly, just like what happens when you use RequiredFieldValidator and RegularExpressionValidator.
Is it to possible to use something such as the CustomValidator to validate a combination of textbox and dropdownlist on the fly?

Answer 1

You can try to use CustomerValidator as explained here
http://www.codeproject.com/Articles/9522/CustomValidator-dependent-on-multiple-controls
Or
you can create a validate function with your logic, and call it on the Selection change event for dropdown and text change event of TextBox. So anytime, any of those thing changes that will trigger the event and will call your validate function.