<form id="form1" runat="server">
 <div>
 <asp:FileUpload ID="FileUpload1" runat="server" />
 <asp:Button ID="Button1" runat="server" Text="上傳" Width="54px" OnClick="Button1_Click" />
 <asp:Label ID="Label1" runat="server" Text="" Style="color: Red"></asp:Label>
 <asp:Image runat="server" ID="Image1" Style="z-index: 102; left: 20px; position: absolute;
 top: 49px" Width="73px" />
 </div>
 </form>

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;

namespace Web.File
{
 public partial class WebForm1 : System.Web.UI.Page
 {
 protected void Page_Load(object sender, EventArgs e)
 {

 }
 #region 文件上傳
 /// <summary>
 /// 文件上傳
 /// </summary>
 protected void Button1_Click(object sender, EventArgs e)
 {
 if (FileUpload1.FileName == "")
 {
 this.Label1.Text = "上傳文件不能為空";
 return;
 }

 bool fileIsValid = false;
 //如果確認了上傳文件，則判斷文件類型是否符合要求 
 if (this.FileUpload1.HasFile)
 {
 //獲取上傳文件的后綴 
 String fileExtension = System.IO.Path.GetExtension(this.FileUpload1.FileName).ToLower();
 String[] restrictExtension = { ".gif", ".jpg", ".bmp", ".png" };
 //判斷文件類型是否符合要求 
 for (int i = 0; i < restrictExtension.Length; i++)
 {
 if (fileExtension == restrictExtension[i])
 {
 fileIsValid = true;
 }
 //如果文件類型符合要求,調用SaveAs方法實現上傳,并顯示相關信息 
 if (fileIsValid == true)
 {
 //上傳文件是否大于10M
 if (FileUpload1.PostedFile.ContentLength > (10 * 1024 * 1024))
 {
 this.Label1.Text = "上傳文件過大";
 return;
 }
 try
 {
 this.Image1.ImageUrl = "~/File/" + FileUpload1.FileName;
 this.FileUpload1.SaveAs(Server.MapPath("~/File/") + FileUpload1.FileName);
 this.Label1.Text = "文件上傳成功!";
 }
 catch
 {
 this.Label1.Text = "文件上傳失敗！";
 }
 finally
 {

 }
 }
 else
 {
 this.Label1.Text = "只能夠上傳后綴為.gif,.jpg,.bmp,.png的文件";
 }
 }
 }
 }
 #endregion
 }
}

<!--因為FileUpload 控件上傳最大為4M,如果要上傳更大文件,改下maxRequestLength的大小-->
<configuration>
 <system.web>
 <compilation debug="true" targetFramework="4.0" />
 <httpRuntime requestValidationMode="2.0" maxRequestLength="10485760" executionTimeout="3600" appRequestQueueLimit="10000"/>
 </system.web>
</configuration>