How to use __doPostBack()

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.

Answers:

Answer

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.

Answer

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);
Answer

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.

Answer

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.

Answer

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"].

Answer

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

Answer

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);               
    }

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