how to use __doPostBack function in asp.net

Question

i am trying to use __doPostBack function so i can force my page to do post pack on the page load but i am having some difficulties understanding.

when i was looking at the examples online. On button click, i want to do the post back but not sure how to complete the code in the code behind.

Here is what i have so far:

<script type="text/javascript">
        function DoPostBack() {
            __doPostBack('btnNew', 'MyArgument');
        }
    </script>

Here is my button

 <asp:Button ID="btnNew" runat="server" CausesValidation="False" CommandName="New" OnClick="DoPostBack()" Text="Create" />

I don't seem to understand to use "MyArgument" in the code behind. What do i need to do in the code behind so it does post back on the page load? thanks in advance for the assistance.

Answer 1

Scenario 1

If you want to use your JavaScript function to trigger the postback, you need to replace the OnClick with OnClientClick. Modify the button definition like this (I'm already assuming that it's nested inside an UpdatePanel):

<asp:Button ID="btnNew" 
    runat="server" 
    CausesValidation="False" 
    CommandName="New" 
    OnClientClick="DoPostBack();" 
    Text="Create" />

In the code behind, in the Page_Load method, read the Request["__EVENTTARGET"] and the Request["__EVENTARGUMENT"]:

protected void Page_Load(object sender, EventArgs e)
{
    if (Page.IsPostBack)
    {
        if (Request["__EVENTTARGET"] == "btnNew" && 
            Request["__EVENTARGUMENT"] == "MyArgument")
        {
            //do something
        }
    }
}

Scenario 2

If you don't necessarily want to use JavaScript, modify the button's definition like this:

<asp:Button ID="btnNew"
    runat="server"
    CausesValidation="False"
    CommandName="New"
    OnClick="DoPostback"
    CommandArgument="MyArgument"
    Text="Create" />

Then, add the following method in the code behind:

protected void DoPostback(object sender, EventArgs e)
{
    var target = ((Button)(sender)).ClientID; //"btnNew"
    var argument = ((Button)(sender)).CommandArgument; //"MyArgument"
    if (target == "btnNew" &&
        argument == "MyArgument")
    {
        //do something
    }
}

Answer 2

The values that are passed to the function __doPostBack as arguments will be sent to the server as request parameters named __EVENTTARGET and __EVENTARGUMENT. ASP.NET uses these values to determine what control has fired an event and what arguments should be passed as EventArgs object. But you can access them directly using object HttpRequest:

string eventTarget = this.Request.Params.Get("__EVENTTARGET");
string eventArgument = this.Request.Params.Get("__EVENTARGUMENT");