alert if the user didn't fill the required fields - asp.net

Question

I have a field in asp and he has RequiredFieldValidator.
I have also a button that on click I send mail to the user.
my code:
<asp:TextBox class="form-control" ID="mail" runat="server"></asp:TextBox>
<asp:RequiredFieldValidator ID="RequiredFieldValidator3" runat="server"
ControlToValidate="ShemPrati"
ErrorMessage="you must enter your mail"
Display="Dynamic"
ForeColor="Red"> </asp:RequiredFieldValidator>
<asp:Button ID="Button4" class="btn btn-default" runat="server" Text="send mail" OnClick="Button4_Click" />
When I click on the button it's give me the ErrorMessage (if the field is empty).
I want also to alert a massage that he will be notify that there are a required fields in the form.
How can I do it?
thanks!

Answer 1

You can achieve your requirement by adding a client-side onclick event to the button that submits the form:
<script language="javascript" type="text/javascript">
function checkIfFormIsValid() {
//this will check if the form is valid
if (Page_IsValid == false) {
alert('Your form is not valid!');
}
}
</script>
Then you can call the function when someone clicks your button:
<asp:Button ID="Button4" OnClientClick="JAVASCRIPT:checkIfFormIsValid();" class="btn btn-default" runat="server" Text="send mail" OnClick="Button4_Click" />
BONUS:
You can add an onblur client-side event to a specific text box and do some custom stuff that way:
txtMail.Attributes.Add("onblur","javascript:checkIfFormIsValid();");