实现真正意义上的文件类型判断

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;

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

    }
    protected void btn_upload_Click(object sender, EventArgs e)
    ...{
        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());
        }
    }
    //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 = "";
        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;
        }

    }
}

 
关键代码是优快云的月儿写的,我只是做了一些适当的修改,可以实现真正意义上的文件类型判断,推荐使用这种方法.
优快云的月儿的博客
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http://blog.youkuaiyun.com/21aspnet/archive/2007/04/01/1548696.aspx