How does Page.IsValid work?

Question

I have following code with a `RequiredFieldValidator`. The `EnableClientScript` property is set as "false" in the validation control. Also I have disabled script in browser. I am NOT using `Page.IsValid` in the code behind. Still, when I submit without any value in textbox I will get `error message`. From comments of @Dai, I came to know that this can be an issue, if there is any code in `Page_Load` that is executed in a `postback`. There will be no validation errors thrown. (However, for button click handler, there is no need to check `Page.IsValid`) ``` if (Page.IsPostBack) { string value = txtEmpName.Text; txtEmpName.Text = value + "Appended"; } ``` QUESTION - Why does not the server side validation happen before `Page_Load`? - Why does it work fine when I use `Page.IsValid`? - Can you provide any reference to an article that explains this? (Not something that says - always use `Page.IsValid`; but something that says what are the mandatory scenarios to use `Page.IsValid` UPDATE 1 R…

Accepted Answer

Validation occurs after `Page_Load`, but before event handlers (See http://msdn.microsoft.com/en-us/library/ms178472(v=VS.100).aspx).

If your button does not cause validation, you must manually fire `Page.Validate.`

You may not interrogate `Page.IsValid` until after (1) you have called `Page.Validate` or (2) a control that causes validation was the source of/included in a postback.

If you require validation to occur before event handlers fire, you may use:

```
if (Page.IsPostback) 
{
   Page.Validate( /*Control Validation Group Name Optional*/ );
   if (Page.IsValid)
   {
       //Do some cool stuff
   }
}
```

You may also want to consider redesigning so you aren't required to do so.

In an event handler that handles a control which causes validation, `Page.IsValid` is guaranteed to be available. In all other cases, it is generally safer to re-request validation. One model for handling submissions on a form that has validators:

```
void btnSubmit_Click(object sender, EventArgs e)
{
   this.UpdateGUIWithSubmitRequest();
   if (Page.IsValid)
   {
      this.ProcessSuccessfulSubmission();
   }
   else
   {
      this.ProcessInvalidSubmission();
   }
}
```

If you are using a `CustomValidator` that has a very expensive validation step, you may consider caching the result in the `HttpResponse.Cache` so you do not have to re-validate if multiple calls to Page.Validate occur.

```
void CustomValidator_ServerValidate(object source, ServerValidateEventArgs args)
{
   CustomValidator self = (CustomValidator)source;
   string validatorResultKey = self.ClientID;
   bool? validatorResult = Context.Items[validatorResultKey] as bool?;
   if (validatorResult.HasValue)
   {
      args.IsValid = validatorResult.Value;
      return;
   }

bool isValid = this.DoSomethingVeryTimeConsumingOrExpensive();
   Context.Items[validatorResultKey] = isValid;
   args.IsValid = isValid;
}
```

This, of course, depends 100% on your architecture and whether or not you are able to assume that a passed/failed validation during initial validation still passes/fails during subsequent validations of the same Page Life Cycle.

How does Page.IsValid work?

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