How to Use _Dopostback()

What is the meaning of __doPostBack function, and when is it used?

simply said, it is used mainly by controls with AutoPostBack property

http://www.dotnetspider.com/resources/189-AutoPostBack-What-How-works.aspx

if you want to implement autopostback for your custom control, then you need to implement IPostBackDataHandler

How to use __doPostback in C#

You could call the .click() event on your button that would cause the postback you are searching for:

Page.ClientScript.RegisterStartupScript(this.GetType(), "postback", "<script>$(document).ready( function() {$('#" + PrintExcemptionsButton.ClientID + "').click()});</script>", false);

However I don't see the sense of this. You could simply call the OnButtonClick() function in your Page_Load function from codebehind.

ASP.NET how to use __doPostBack from Custom JavaScript with Master/Content Pages

are you hard coding the call to __doPostBack?

try using Page.GetPostBackEventReference

to obtain a valid handle in your javascript.

once you write out a valid post back event reference, you could wrap it inside a local function, which you can than call from master page.

if (window.myPostBackfunction) // check if it exists
  myPostBackFunction();

Is it OK to use __doPostBack()?

I would advice against it, since it's internal stuff of ASP.NET and was never meant to be used directly.

Instead, what I'm doing when I need to "manually" trigger PostBack is adding hidden "server side" button with the proper OnClick:

<asp:Button id="btnDummy" runat="server" OnClick="Foo" style="display: none;" />

Then the JS is:

document.getElementById("<%=btnDummy.ClientID%>").click();

This way I don't care how post back happens, I just trigger the natural flow of events.

ASP.NET postback with JavaScript

Here is a complete solution

Entire form tag of the asp.net page

<form id="form1" runat="server">
    <asp:LinkButton ID="LinkButton1" runat="server" /> <%-- included to force __doPostBack javascript function to be rendered --%>

    <input type="button" id="Button45" name="Button45" onclick="javascript:__doPostBack('ButtonA','')" value="clicking this will run ButtonA.Click Event Handler" /><br /><br />
    <input type="button" id="Button46" name="Button46" onclick="javascript:__doPostBack('ButtonB','')" value="clicking this will run ButtonB.Click Event Handler" /><br /><br />

    <asp:Button runat="server" ID="ButtonA" ClientIDMode="Static" Text="ButtonA" /><br /><br />
    <asp:Button runat="server" ID="ButtonB" ClientIDMode="Static" Text="ButtonB" />
</form>

Entire Contents of the Page's Code-Behind Class

Private Sub ButtonA_Click(sender As Object, e As System.EventArgs) Handles ButtonA.Click
    Response.Write("You ran the ButtonA click event")
End Sub

Private Sub ButtonB_Click(sender As Object, e As System.EventArgs) Handles ButtonB.Click
    Response.Write("You ran the ButtonB click event")
End Sub

• The LinkButton is included to ensure that the __doPostBack javascript function is rendered to the client. Simply having Button controls will not cause this __doPostBack function to be rendered. This function will be rendered by virtue of having a variety of controls on most ASP.NET pages, so an empty link button is typically not needed

What's going on?

Two input controls are rendered to the client:

<input type="hidden" name="__EVENTTARGET" id="__EVENTTARGET" value="" />
<input type="hidden" name="__EVENTARGUMENT" id="__EVENTARGUMENT" value="" />

• __EVENTTARGET receives argument 1 of __doPostBack
• __EVENTARGUMENT receives argument 2 of __doPostBack

The __doPostBack function is rendered out like this:

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

• As you can see, it assigns the values to the hidden inputs.

When the form submits / postback occurs:

• If you provided the UniqueID of the Server-Control Button whose button-click-handler you want to run (javascript:__doPostBack('ButtonB',''), then the button click handler for that button will be run.

What if I don't want to run a click handler, but want to do something else instead?

You can pass whatever you want as arguments to __doPostBack

You can then analyze the hidden input values and run specific code accordingly:

If Request.Form("__EVENTTARGET") = "DoSomethingElse" Then
    Response.Write("Do Something else") 
End If

Other Notes

• What if I don't know the ID of the control whose click handler I want to run?
  
  • If it is not acceptable to set ClientIDMode="Static", then you can do something like this: __doPostBack('<%= myclientid.UniqueID %>', '').
  • Or: __doPostBack('<%= MYBUTTON.UniqueID %>','')
  • This will inject the unique id of the control into the javascript, should you wish it

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