how does the __doPostBack method get called? Where is the calling method?

Question

I used an <asp:Button /> control, and after rendering in the browser that control doesn't have a click event property assigned. How exactly is it calling the sever side event?

ASPX code:

<asp:Button ID="Button1" runat="server" Text="Button" OnClick="TestClickEvent" />

The above control was rendered in browser as following code:

<input type="submit" name="Button1" value="Button" id="Button1">

The following code is rendered in the browser, which sets __EVENTTARGET. My doubt is how does the __doPostBack method get called? Where is the calling method?

 function __doPostBack(eventTarget, eventArgument) {
        if (!theForm.onsubmit || (theForm.onsubmit() != false)) {
            theForm.__EVENTTARGET.value = eventTarget;
            theForm.__EVENTARGUMENT.value = eventArgument;
            theForm.submit();
        }
    }

Answer 1

The simple answer: The __doPostBack JavaScript function is called based on specific <asp /> controls and the events that it handles.

The detailed answer: It depends.

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First, let's cover your example. You have an <asp:Button /> which is rendered as a standard <input type="submit" />. Everything in ASP.NET WebForms revolves around the standard HTML <form> tag. An HTML <form> is submitted without the use or assistance of JavaScript through clicking an <input type="submit" /> button.

With this in mind, you can very well see (which you've already noticed) that the rendered <input type="submit" /> button does not have an onclick event assigned. And, as you can see, the form is submitted when the button is clicked.

When it comes to how the back end (C#/VB.NET/etc.) code is executed when the <input type="submit" /> button is clicked: it is all handled by the ASP.NET Framework itself, and is beyond the scope of this question/answer.

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Second, now let's cover what __doPostBack is, and how it is used. __doPostBack is simply a helper JavaScript function used to submit the HTML <form>. Due to the reasons outlined above, you now know why the <input type="submit" /> button does not need to call the __doPostBack function.

For simplicity, let's take a look at an ASP.NET page which has an <asp:DropDownList /> control, and it has the SelectedIndexChanged event handler assigned:

<asp:DropDownList ID="MyDropDownList" AutoPostBack="true" OnSelectedIndexChanged="MyDropDownList_SelectedIndexChanged" runat="server" />

The <asp:DropDownList /> is rendered as follows:

<select id="ctl00_MyDropDownList" onchange="javascript:setTimeout('__doPostBack(\'ctl00$MyDropDownList\',\'\')', 0)" name="ctl00$MyDropDownList"></select>

^{let's ignore the setTimeout function in the onchange event - it's merely a hacky workaround used by ASP.NET - and let's focus on the __doPostBack function inside of it.}

As you can see here, the __doPostBack function is being called by the onchange event handler. The key difference is that changing the value of an <asp:DropDownList /> or <select /> control does not cause the browser to submit the form!

Once again the ASP.NET Framework handles internally how the back end code is executed when the form is submitted (whether through the __doPostBack function or not).

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Lastly, as for the details of __doPostBack: it accepts two parameters - eventTarget and eventArgument. eventTarget contains the rendered HTML id property of the control which is causing the postback; and eventArgument is an optional parameter which can be used to pass additional data to the back end code.

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Edit Additional Info: the OP asked a very interesting question - what happens when there is more than one submit button?

Well, during a POST operation, browsers include the value of the <input type="submit" /> which caused the operation to initiate.

This means, that just as you obtain the values of your <input /> elements, you can also query for which button caused the submit!

Answer 2

Your control is a submit button, it can directly submit the form to the server without calling the __dopostback method. So in this case __dopostback is not getting called. __dopostback is used for controls not having a default submit behaviour, like dropdownlist.

The wireing to your server side event happens through the ipostbackeventhandler interface that your control implements. For more info please check out this http://msdn.microsoft.com/en-us/library/system.web.ui.ipostbackeventhandler.raisepostbackevent.aspx