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Issue with Postback and Custom Validators in asp.net

I need some help on an issue, related with asp.net validators and postback.
The issue is that I have a page say 'p1' and it has a master page 'm1'. In addition to these I also have a usercontrol 'u1'.
Now, the issue is that on master page 'm1', I have a button that initiates a postback. And on my usercontrol 'u1', I have some fields which I need to check/validate and stop the postback if the fields are invalid.
I have tried using customvalidators and forcefully calling Page.Validate() method. But by doing this, I can see the page.IsValid property is false but sill the postback happens. I have even tried to write a return statement if the control fields are invalid but it doesn't helps..
Please note that I do not want to make any changes in the masterpage as this may have high impact on the other pages.

If clicking a button causes postbacks even when the page is invalid (as in your case it is proven with Page.IsValid property is False), then this means the button isn't configured to check for validations on the client side.
Solution 1:
You need to intialize the "CausesValidation" Property of the Button with the value True"
However, this requires changing the markup of the "Button" on the markup. And this isn't something you wish to perform.
Solution 2:
You can from within your page, access the Button of the Master page from within the Load Event and Initialize the CausesValidation property of the button to True.
Have a look at the following link to find the button from within the master page:
https://msdn.microsoft.com/en-us/library/xxwa0ff0.aspx

How to show validation errors after a postback

I am having some textboxes and dropdownlist controls but if i select the ddl value the validation errors are disappearing and after the button click they are reappearing but i want to show the errors even after a postback how can i do this??

Could you post your code so we can perhaps see what the specific issue might be? Without seeing any code, I would say try adding this into the Page_Load function:
if (IsPostBack)
Page.Validate();
Alternatively, add this to your DropDownList or whatever controls are initiating the postback:
CausesValidation="true"
.. as per the answer here: Validators do not Validate after postback occurs

Please remove your asp.net validators if added with the controls and also remove any client side validation in Java script. Now add validation code on the page you are redirecting to. If that validation fails redirect back to the controls page with proper messages to be shown

If you dont like the default behavior - don't use validation controls and implement them by yourself using client side programming.

More detailed explanation about GetPostBackEventReference method

I understand that this will causes a page reload (partial or full, depending on how your UpdatePanels are set up)
But,
where in the code I should put it (client or server side)?
which control should I send to the method? Is it must be inside the UpdatePanel?
does this method work only for controls inside update panels?
must the control have a postback capability?
what is the engine behind this? How does this method work, so I could use it properly.
Thanks.

The function call returns a string of executable JavaScript, which you need to write to the client somewhere in your response.
Typically, you're sending your Page (this/Me) unless you have a control that you specifically want to handle the postback (ie, that implements IPostBackEventHandler)
GetPostBackEventReference is not related to UpdatePanels; if you have one, it will handle the postback.
No (see #2)
This makes a postback to the page. If you want it to raise an event when it posts back, you need to implement IPostBackEventHandler, either on your page or on one of your controls.
http://msdn.microsoft.com/en-us/library/ms153112.aspx

How can I validate that several textboxes contain data when I click submit?

I have 10 defined textboxes with strings.
I have to check all if they are not empty while clicking ok button
whats the cleanest way to check them all and when function is at end. each checkbox which was empty to give this a specific CSSclass. perhaps. ClassError. ( which highlights red)
I'm happy for answers.

I would add RequiredFieldValidators to them, as well as a ValidationSummary control.
edit: You can also add fancy AJAX effects with the ValidatorCallout from the AJAX toolkit.
edit: Validator controls also support client-side validation.

Using javascript or C#?
Javascript I will create an array of textbox and loop through it.
C# just go FindControl within a Panel or the Container of the text box and go something like this
foreach(Control C in ContainerID.Controls)
{
if ( C is TextBox )
{
if ( String.IsNullOrEmpty((C as TextBox).Text))
{
// Do things this way
}
}
}

Something like this would work on the client side using jquery:
$('input').filter(function(){return this.value=="";}).css("CSSclass");
edit: I just saw the C# tag, I'll leave this here for posterity though.

What you're doing might be simple enough to do easily with some custom javascript code, though I would say that in general the built in validator controls are both easier to use and more robust than a simple validation routine that someone might write. In addition to client-side validation, the validator controls also perform server-side validation to ensure that the data submitted is truly valid, in case someone has javascript disabled in their browser.
If validator controls are included on the page, then they will include some javascript functions that you can invoke, such as Page_ClientValidate(). I believe this will return a boolean telling you whether validation passed and will trigger the visual indicators that identify what the errors are. You can execute Page_ClientValidate('') to trigger only a group of validator controls; actually I think you must do that in order to trigger validation on any validator controls that have a value in their ValidationGroup property, as I don't think Page_ClientValidate() will trigger their validation logic.
There is a CustomValidator control that you can point to your own client-side function if you want, in case you do have some special validation logic that you can only implement through a custom javascript function. This is nice to use, because then your custom javascript function will be executed by the built-in validator framework along with any other built-in validator controls that you might choose to include on the page.
Side note regarding client-side validation: I suggest that you avoid doing the following to trigger validation:
onclick="return myValidationFunction();"
because if there is any other javascript code being injected into the onclick event, your return statement will prevent it from executing. So instead, I suggest doing this:
onclick="if(!myValidationFunction()) return false;"
That's bitten me enough times that I thought I'd just throw that out there. This problem is particularly noticeable if you have an ASP.Net button on which you've set the UseSubmitBehavior property to false, as it will cause the button to render as an HTML "button" control instead of a "submit" control, and as a result, executing a return statement, either true or false, will prevent the button from triggering a postback.

ASP.NET catch the submit action

I'm a super beginner ASP.NET developer. I read that on every submit the request parameters are being populated to the controls and instead of reading the "Response.Form[]" parameters I can read the input parameters from the control itself.
Are there any events that I can catch all the submits before and after the magic happens?
Which method on the server side is activated that perform this magic?
Can I override it (for fun)?
Thanks,
Ronny

I believe that you are talking about the function of Viewstate and how control values are persisted.
This is a diagram that will show you the page load order for ASP.NET
For you, if you want to look before viewstate is loaded, you can work inside the Page_Init method.

see few tutorials
http://aspalliance.com/quickstart/aspplus/
http://quickstart.developerfusion.co.uk/QuickStart/
In asp.net all event, result in post-back. So, you can handle them in Page_Load, but it's classic way. For fun you can try it.

ASP.NET maps HTML input field values from the Request.Form collection to server control properties, such as TextBox.Text between the Page.InitComplete and Page.PreLoad events, as detailed in the ever-linkable ASP.NET Page Life Cycle Overview.
The actual mapping takes place in the non-virtual private method Page.ProcessPostData, so there's no real hook for modifying that process. (You can see this by downloading Reflector and reviewing the Page.ProcessRequestMain method.)
If you want to perform custom processing before or after the mapping, you can add a handlers to the appropriate events or override the associated virtual methods (Page.OnInitComplete and Page.OnPreLoad).