Question

I am currently developing a web site using ASP.NET 3.5. On a page there are some situations I don’t want that a specific control will cause a postback to the server. I wrote a function to return false if that condition is met which will be called when the onsubmit-Event occurs. But I somehow need to determine which control will cause the postback, because the postback should be cancelled only if this specific control caused it under certain conditions. How it is possible to do that?

Thanks for your time.

Answer 1

See:

How does ASP.NET recognize the control
responsible for handling the postback?
When no controls referenced in the
request body implement the
IPostBackEventHandler interface, the
page class looks for the __EVENTTARGET
hidden field, if any. The contents of
the field is assumed to be the ID of
the control that caused the postback.
If this control implements the
IPostBackEventHandler interface, the
RaisePostbackEvent method is invoked.

this is from here – The Client Side of ASP.NET Pages.

So at the client-side __EVENTTARGET is all you need. At the server-side you could either override the Page.RaisePostBackEvent method (this is protected method, so you could inherit from System.Web.UI.Page class):

protected override void RaisePostBackEvent(IPostBackEventHandler sourceControl, 
                                           string eventArgument)
{
    // sourceControl is a control that caused postback
    base.RaisePostBackEvent(sourceControl, eventArgument);
}

or perform the same without inherining:

var controlName = page.Request.Params["__EVENTTARGET"];
Control postbackControl = null;
if (!string.IsnullOrEmpty(controlName))
{
    postbackControl = this.Page.FindControl(controlName);
}

EDIT: regarding the author’s comment to my answer: if __EVENTTARGET value is an empty string, it seems you’re getting this value before it is been set in __doPostBack function. So the workaround could be in overriding __doPostBack function or a similar way; you could find an example of doing it in this SO quesion.