AsyncFileUpload Properties:

·         FileName - Gets the name of a file on a client to upload using the control.

·         HasFile - Gets a bool value indicating whether the control contains a file.

·         OnClientUploadComplete - The name of a javascript function executed in the client-side after the file successfully uploaded

·         OnClientUploadError - The name of a javascript function executed in the client-side if the file uploading failed

·         OnClientUploadStarted - The name of a javascript function executed in the client-side on the file uploading started

·         PostedFile - Gets a HttpPostedFile object that provides access to the uploaded file.

·         ThrobberID - ID of control that is shown while the file is uploading.

Code:

<%-- ScriptManager control manages client script for AJAX enabled ASP.NET pages.This enables partial-page rendering and Web-service calls.You have to used this if you want to use ajax control toolkit--%>

Step1: write these code on aspx page we add ajax control and one label

<asp:ScriptManager ID="ScriptManager1" runat="server"></asp:ScriptManager>
 <%-- Add the AsyncFileUpload control which acces the file in asynchronous way--%>
  
<cc1:AsyncFileUpload ID="AsyncFileUpload1" runat="server"  CompleteBackColor="#66CCFF" OnUploadedComplete="AsyncFileUpload1_UploadedComplete" OnLoad="AsyncFileUpload1_Load"
 OnUploadedFileError="AsyncFileUpload1_UploadedFileError" UploadingBackColor="#CCCCFF" />
<asp:Label ID="Label1" runat="server" Text=""></asp:Label>

Step2: write the code on event load of ajax control

protected void AsyncFileUpload1_Load(object sender,  EventArgs e)
{
   if (AsyncFileUpload1.HasFile) // Check the file is exist or not by using HasFile properties
   {
AsyncFileUpload1.SaveAs(Server.MapPath("~/UploadedFile/" + AsyncFileUpload1.FileName)); // Save the file here
   }
}

Run the project

When you click Browse button then Browse window will open

Choose the file and click on open button

File is asynchronously uploaded whenever file is selected.