How to post back to the server using JavaScript in ASP.NET

• Download JavaScriptPostback.zip - 1.5 KB

Introduction

The code in the original article works great and causes postback from JavaScript. However, it doesn't let you pass a variable argument back from JavaScript. I made some slight modifications to the code to allow it to pass a single string variable from JavaScript to codebehind.

Background

Basically, as Scott Norberg noted in his original article, http://www.codeproject.com/Articles/42183/How-to-post-back-to-the-server-using-JavaScript-in, were trying to avoid using __doPostback to cause a postback from javascript in .Net, and instead use documented .Net framework class "PostBackOptions" to cause the postback. This extension to his code allows for passing a variable from JavaScript also.

Using the code

Here are my changes (my code is in VB). First step is to write a small EventArg class:

Public Class JavaScriptPostbackEventArgs
    Inherits EventArgs

    Public args As String = ""

    Public Sub New(args As String)
        Me.args = args
    End Sub
End Class

Next, change the postback event's type to use this new class:

Public Event Postback As EventHandler(Of JavaScriptPostbackEventArgs)

Now the next task is to get the JavaScript to pass a single string variable. This is done by changing the Render method:

The first step is to add an argument to the option:

options.Argument = "arg"

Next, also add it as a variable in the javascript function call:

writer.Write([String].Format("{0} = function(arg) {1}", functionName__1, "{"))

When GetPostBackEventReference renders this, it renders arg is a string value, not a variable. The following will remove the single quotes off of arg and use the variable:

writer.Write(Me.Page.ClientScript.GetPostBackEventReference(options).Replace("'arg'", "arg"))

Protected Overrides Sub Render(writer As HtmlTextWriter)
    MyBase.Render(writer)
    Dim options As New PostBackOptions(PostbackButton)
    options.PerformValidation = Me.CausesValidation
    options.ValidationGroup = Me.ValidationGroup
    options.Argument = "arg"

    Dim functionName__1 As [String] = If(JavaScriptNamespaces.Equals([String].Empty), FunctionName, [String].Concat(JavaScriptNamespaces, ".", FunctionName))
    Dim arg As String = "e"
    writer.Write("<script type='text/javascript'>")
    AddNamespaces(JavaScriptNamespaces, writer)
    writer.Write([String].Format("{0} = function(arg) {1}", functionName__1, "{"))
    writer.Write(Me.Page.ClientScript.GetPostBackEventReference(options).Replace("'arg'", "arg"))
    writer.Write("};")
    writer.Write("</script>")
    Me.Page.ClientScript.RegisterForEventValidation(options)
End Sub

Private Sub PostbackButton_Click(sender As Object, e As EventArgs)
    RaiseEvent Postback(sender, New JavaScriptPostbackEventArgs(HttpContext.Current.Request("__EVENTARGUMENT")))
End Sub

That's all the changes.

Example of how to use

Here is an example on how to use it from the client side. Add the control the page markup:

<uc:JavascriptPostBack runat="Server" ID="ucPostback" FunctionName="javaPostback" />

A simple javascript example to call the function is: You can use this inside another function.

<script>
javaPostback('test');
</script>

And finally the code behind that handles the event:

Private Sub ucJavascriptpostback_Postback(sender As Object, e As JavaScriptPostbackEventArgs) Handles ucNodeClickPostback.Postback    


End Sub

In the above example, e.args will have the value 'test'.