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Editorial Team
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Asked: 2026-06-15T06:23:14+00:00

I am building a CustomValidator to handle my own application’s logic on time fields

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I am building a CustomValidator to handle my own application’s logic on time fields (“09:00”, “15:35”, …) but I am stumbling on a behaviour that I haven’t found any explanation for online.

My focus right now is the validation logic that gets executed client-side.

The problem is, as said in the title, that, if and only if I set the ControlToValidate property in the Validator with the ID of the textbox I am validating, the validation gets fired as soon as focus leaves the textbox; it even fires before the onblur event, which is absolutely detrimental for me, since I am using the onblur event to standardize the time formats (eg “9:00” -> “09:00”, “11.45” -> “11:45”) and thus, the validation logic potentially receives an incorrect value. If, on the other hand, the ControlToValidate property remains blank, the ClientValidationFunction is fired only on a submit/postback.

The only related answer I’ve found is this https://stackoverflow.com/a/8649697/450684, but still, to me it makes no sense at all. Why should the presence of a ControlToValidate indicate that I want client-side validation to execute before onblur? I don’t want it! Is there any way to supress this behaviour?

Here is an example page:

<asp:TextBox runat="server" AutoPostBack="false" ID="txtBox1" onblur="FormatText(this);" />
<asp:CustomValidator runat="server" ID="CV1" ControlToValidate="txtBox1" ClientValidationFunction="Test1" />
<asp:CustomValidator runat="server" ID="CV2" ClientValidationFunction="Test2" />
<asp:Button ID="btn1" Text="postback" runat="server" OnClick="btn1_Click" />
<asp:Label ID="lbl1" runat="server" />
<script type="text/javascript">
    function FormatText(txtBox1)
    {
        alert('FormatText');
    }
    function Test1(val, args)
    {
        alert('Test1');

    }
    function Test2(val, args)
    {
        alert('Test2');
    }</script>

What I want is both Test1 and Test2 to execute only on btn1’s click; instead on txtBox1’s onblur event I get Test1 and FormatText executing in this order

ASP.NET’s client validation was really fun for me to write and study, don’t let this ruin everything 🙂

Thx

PS: .NET framework’s version is 4.0. Plus, the server-side language is C#, if it matters

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1 Answer

  1. Editorial Team

    I think what is going on is this:

    The standard behavior for ASP.NET client-side validation is to validate when the field is exited. That’s an observation, not a reference to a published standard (although there may be one.) The out-of-the-box validators all behave this way. All of them require specifying a particular field to validate.

    The custom validator allows you to validate a single control (by specifying it with ControlToValidate), or lets you validate a combination of controls, in which case you set ControlToValidate to an empty string. Unless things have changed, you do have to specify it; if you omit the attribute, no validation will occur.

    So … if you specify a control to validate, the custom validator behaves like every other validator and reacts to the user exiting the field. If you don’t specify a control to validate, it doesn’t know what controls you’re interested in, and doesn’t do that.

    You may be able to work around this by writing a truly custom validator: inherit from BaseValidator. That can actually be a lot of fun.

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