Determine PostBack Control or AJAX Request Initiator

Description

In some scenarios it might prove helpful to know which control has caused the PostBack or partial request action to the server.

Solution

You can use one of these approaches to achieve this requirement.

C#

protected void Page_Load(object sender, EventArgs e)
{
    string id1 = Request.Params["__EVENTTARGET"];
    string id2 = Request.Form["__EVENTTARGET"];
    string id3 = ScriptManager.GetCurrent(Page).AsyncPostBackSourceElementID;
    bool isAJAX = ScriptManager.GetCurrent(Page).IsInAsyncPostBack;
    if (isAJAX)
    {
        string id4 = DeterminePostBackMode()["__EVENTTARGET"];
    }
}

VB

Protected Sub Page_Load(ByVal sender As Object, ByVal e As EventArgs)
    Dim id1 As String = Request.Params("__EVENTTARGET")
    Dim id2 As String = Request.Form("__EVENTTARGET")
    Dim id3 As String = ScriptManager.GetCurrent(Page).AsyncPostBackSourceElementID
    Dim isAJAX As Boolean = ScriptManager.GetCurrent(Page).IsInAsyncPostBack

    If isAJAX Then
        Dim id4 As String = DeterminePostBackMode()("__EVENTTARGET")
    End If
End Sub