用ASP.net判断上传文件类型的三种方法

一、 安全性比较低，把文本文件1.txt改成1.jpg照样可以上传，但其实现方法容易理解，实现也简单，所以网上很多还是采取这种方法。

Boolean fileOk = false;

          string path = Server.MapPath("~/images/");

          //判断是否已经选取文件

          if (FileUpload1.HasFile)

          {

              //取得文件的扩展名,并转换成小写

              string fileExtension = System.IO.Path.GetExtension(FileUpload1.FileName).ToLower();

              //限定只能上传jpg和gif图片

              string[] allowExtension = { ".jpg", ".gif" };

              //对上传的文件的类型进行一个个匹对

              int j = 0;

              for (int i = 0; i < allowExtension.Length; i++)

              {

                  if (fileExtension == allowExtension[i])

                  {

                      fileOk = true;

                      return;

                  }

                  else

                  {

                      j++;

                  }

              }

              if (j > 0)

              {

                  Response.Write("<script>alert('文件格式不正确');</script>");

                  return;

              }

          }

          else

          {

              Response.Write("<script>alert('你还没有选择文件');</script>");

              return;

          }

          //如果扩展名符合条件，则上传

          if (fileOk)

          {

              FileUpload1.PostedFile.SaveAs(path + FileUpload1.FileName);

              Response.Write("<script>alert('上传成功');</script>");

          }

二、不检测文件后缀而是检测文件MIME内容类型。

Boolean fileOk = false;

           string path = Server.MapPath("~/images/");

           //判断是否已经选取文件

           if (FileUpload1.HasFile)

           {

               //取得文件MIME内容类型

               string type = this.FileUpload1.PostedFile.ContentType.ToLower();

               if (type.Contains("image"))    //图片的MIME类型为"image/xxx"，这里只判断是否图片。

               {

                   fileOk = true;

 

               }

               else

               {

                   Response.Write("<script>alert('格式不正确')</script>");

               }

           }

           else

           {

               Response.Write("<script>alert('你还没有选择文件');</script>");

           }

           //如果扩展名符合条件，则上传

           if (fileOk)

           {

               FileUpload1.PostedFile.SaveAs(path + FileUpload1.FileName);

               Response.Write("<script>alert('上传成功');</script>");

           }

三、可以实现真正意义上的文件类型判断

try

            {

                //判断是否已经选取文件

                if (FileUpload1.HasFile)

                {

                    if (IsAllowedExtension(FileUpload1))

                    {

                        string path = Server.MapPath("~/images/");

                        FileUpload1.PostedFile.SaveAs(path + FileUpload1.FileName);

                        Response.Write("<script>alert('上传成功');</script>");

                    }

                    else

                    {

                        Response.Write("<script>alert('您只能上传jpg或者gif图片');</script>");

                    }

 

                }

                else

                {

                    Response.Write("<script>alert('你还没有选择文件');</script>");

                }

            }

            catch (Exception error)

            {

                Response.Write(error.ToString());

            }

            #endregion

        }

//真正判断文件类型的关键函数

        public static bool IsAllowedExtension(FileUpload hifile)

        {

            System.IO.FileStream fs = new System.IO.FileStream(hifile.PostedFile.FileName, System.IO.FileMode.Open, System.IO.FileAccess.Read);

            System.IO.BinaryReader r = new System.IO.BinaryReader(fs);

            string fileclass = "";

            //这里的位长要具体判断.

            byte buffer;

            try

            {

                buffer = r.ReadByte();

                fileclass = buffer.ToString();

                buffer = r.ReadByte();

                fileclass += buffer.ToString();

 

            }

            catch

            {

 

            }

            r.Close();

            fs.Close();

            if (fileclass == "255216" || fileclass == "7173")//说明255216是jpg;7173是gif;6677是BMP,13780是PNG;7790是exe,8297是rar

            {

                return true;

            }

            else

            {

                return false;

            }

 

        }