The following example will save the uploaded file in the root folder of your application.
Default.aspx
============
<%@ Page Language="C#" AutoEventWireup="true"  CodeFile="Default.aspx.cs" Inherits="_Default" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Untitled Page</title>
</head>
<body>
    <form id="form1" runat="server">
    <div>
        <asp:FileUpload ID="fupTest" runat="server" /> 
        <asp:Button ID="btnUpload" runat="server" OnClick="btnUpload_Click" Text="Upload" />
        <br />
        <asp:Label ID="lblStatus" runat="server"></asp:Label></div>
    </form>
</body>
</html>
Default.aspx.cs
============
using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.IO;
public partial class _Default : System.Web.UI.Page 
{
     
    protected void btnUpload_Click(object sender, EventArgs e)
    {
        if (fupTest.HasFile )
        {
            try
            {
                string strFileName = Path.GetFileName(fupTest.FileName);
                fupTest.SaveAs(Server.MapPath("~/") + strFileName);
                lblStatus.Text = "File Uploaded Successfully";              
            }
            catch (Exception ex)
            {
                lblStatus.Text = "Error File could not be uploaded. The Reason for Failure is: " + ex.Message;
            }
        }
    }
}
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