How to use __doPostBack()

Question

I'm trying to create an asyncrhonous postback in ASP.NET using __doPostBack(), but I have no idea how to do it. I want to use vanilla JavaScript.

Something simple like a button click can cause the __doPostBack() event to fire. I'm just trying to learn how the mechanism works.

Weldonwelfare

possible duplicate of Asychnronous Message Send Asp.net – Derby

First of all __doPostBack doesn't cause asynchronous action unless it is triggered by control inside UpdatePanel. – Schulman

Why is this tagged c# and javascript. I understand the javascript part, but not the c# part. – Wrac

@Solomon Ucko - because __doPostBack() causes the page to POST to its server-side implementation, which will most likely be in C# (could be VB.NET) – Ligroin

While it usually works, manually inserting __doPostBack is not actually supported. If your page has nothing on it which triggers a postback, asp.net might omit defining the __doPostBack function. The supported approach is to call ClientScriptManager.GetPostBackEventReference, which returns a string in the form __doPostBack(...) and ensures that __doPostBack is defined. – Fence

Answer 1

You can try this in your web form with a button called btnSave for example:

<input type="button" id="btnSave" onclick="javascript:SaveWithParameter('Hello Michael')" value="click me"/>

<script type="text/javascript">
function SaveWithParameter(parameter)
{
  __doPostBack('btnSave', parameter)
}
</script>

And in your code behind add something like this to read the value and operate upon it:

public void Page_Load(object sender, EventArgs e)
{
  string parameter = Request["__EVENTARGUMENT"]; // parameter
  // Request["__EVENTTARGET"]; // btnSave
}

Give that a try and let us know if that worked for you.

Borrell

Just a quick question, __EVENTARGUMENT would be the parameter or btnSave? – Chenoweth

It is the parameter. __EVENTTARGET will give you the button. – Borrell

what if i have two or more buttons that do __doPostBack then how do i differentiate them on server side in page_load(). – Calamus

@khalidkhan if you are implementing your JavaScript as I have demonstrated then the __EVENTTARGET will still apply, depending on which button you clicked you would have a different value represented by __EVENTTARGET – Borrell

What is the case when i have more than one parameter? – Wunder

@Wunder you could try passing in an array and reading it as a form of IEnumerable instead of string in the C#, I have not tried so cannot guarantee it would work, but it should be possible. – Borrell

@Wunder as another thought your argument could be Json data which could map to an object in your C#, you could then use Json.net to deserialise/serialise to object etc. – Borrell

@khalidkhan Instead of explicitly specifying the id of the button, as shown in the answer, specify event.target.id.ClientID without quotes, and the correct ID that caused the event will be inserted. – Borrego

Answer 2

This is also a good way to get server-side controls to postback inside FancyBox and/or jQuery Dialog. For example, in FancyBox-div:

   <asp:Button OnClientClick="testMe('param1');" ClientIDMode="Static"  ID="MyButton"  runat="server" Text="Ok" >
</asp:Button>

JavaScript:

function testMe(params) {
    var btnID= '<%=MyButton.ClientID %>';          
    __doPostBack(btnID, params);
}

Server-side Page_Load:

 string parameter = Request["__EVENTARGUMENT"];
 if (parameter == "param1")
     MyButton_Click(sender, e);

Hydrostat

should use UniqueID, not ClientID. – Auriferous

Or set the ClientIDMode="Static" attribute of MyButton, so that ASP does not mangle the button's ID. – Suhail

Answer 3

Here's a brief tutorial on how __doPostBack() works.

To be honest, I don't use it much; at least directly. Many server controls, (e.g., Button, LinkButton, ImageButton, parts of the GridView, etc.) use __doPostBack as their post back mechanism.

Headroom

Fast forward 6 years later; I will be shocked if still use this. The approach towards web development has generally moved away from the mentality where things like this are used. – Internment

@Internment - haha, I was reading this thread yesterday while looking at a bug where this was implemented :) It does seem quite archaic in this day and age – Penutian

Fast forwarded 6 years... and ASP.NET Web Forms does still do this. Hence the overwhelming upvoting of the accepted answer. – Headroom

@kbrimington: I did not say ASP.NET will not "still do this". I do not expect it to change much. What I meant is that less people use ASP.NET WebForm for new development and others are migrating off it. – Internment

Answer 4

I'd just like to add something to this post for asp:button. I've tried clientId and it doesn't seem to work for me:

__doPostBack('<%= btn.ClientID%>', '');

However, getting the UniqueId seems to post back to the server, like below:

__doPostBack('<%= btn.UniqueID%>', '');

This might help someone else in future, hence posting this.

Fruiter

Yeah I was having trouble with ClientID, because I was using ClientIDMode = Static. Using UniqueID worked. – Tangible

I've been trying to get this to work and neither ClientID nor UniqueId work for me. – Oidium

OMG! Few days trying to solve this problem. UniqueID did solve the issue. Thanks dude! – Scow

Answer 5

Old question, but I'd like to add something: when calling doPostBack() you can use the server handler method for the action.

For an example:

__doPostBack('<%= btn.UniqueID%>', 'my args');

Will fire, on server:

protected void btn_Click(object sender, EventArgs e)

I didn't find a better way to get the argument, so I'm still using Request["__EVENTARGUMENT"].

Marrufo

Answer 6

Like others have said, you need to provide the UniqueID of the control to the __doPostback() method.

__doPostBack('<%= btn.UniqueID %>', '');

On the server, the submitted form values are identified by the name attribute of the fields in the page.

The reason why UniqueID works is because UniqueID and name are in fact the same thing when the server control is rendered in HTML.

Here's an article that describes what is the UniqueID:

The UniqueID property is also used to provide value for the HTML "name" attribute of input fields (checkboxes, dropdown lists, and hidden fields). UniqueID also plays major role in postbacks. The UniqueID property of a server control, which supports postbacks, provides data for the __EVENTTARGET hidden field. The ASP.NET Runtime then uses the __EVENTTARGET field to find the control which triggered the postback and then calls its RaisePostBackEvent method.

src: https://www.telerik.com/blogs/the-difference-between-id-clientid-and-uniqueid

Delrosario

Answer 7

This is how I do it

    public void B_ODOC_OnClick(Object sender, EventArgs e)
    {
        string script="<script>__doPostBack(\'fileView$ctl01$OTHDOC\',\'{\"EventArgument\":\"OpenModal\",\"EncryptedData\":null}\');</script>";
        Page.ClientScript.RegisterStartupScript(this.GetType(),"JsOtherDocuments",script);               
    }

Stellastellar