because it is being used by another process.

 //最后一块文件块上传完成，文件块合并好之后
                    if (currentBlock == FileBlocks)
                    {
                        if (File.Exists(absolutePath))
                        {
                            AddTxtLog("开始压缩包MD5校验。");

                            #region MD5校验及压缩包文件解压
                            //压缩包MD5校验
                            string MD5String = GetFileMD5(absolutePath);

                            if (MD5String.ToUpper() != toVerifyString.ToUpper())
                            {
                                result = "压缩包（" + absolutePath + "）上传不完整！传过来的MD5校验码：" + toVerifyString + "，服务器生成的校验码：" + MD5String + "。";
                                ExceptionHandler.HandleWebException(logger, "405", result, Context, username);
                                Context.Response.Write(JsonHelper.ToJson(result));
                                AddTxtLog(result);
                                return;
                            }

                            //文件解压
                            if ((absolutePath.EndsWith(".zip") || absolutePath.EndsWith(".rar")
                                || absolutePath.EndsWith(".ZIP") || absolutePath.EndsWith(".RAR")))
                            {
                                AddTxtLog("开始压缩包文件解压。");
                                DeCompressFile(absolutePath, absolutePath.Substring(0, absolutePath.LastIndexOf('.')), "");  //传：压缩包文件绝对路径，压缩包解压文件夹绝对路径
                            }

                            //JSON字符串反序列化为对象
                            DataContractJsonSerializer deSerializer = new DataContractJsonSerializer(typeof(List<Obj_ImageValidation>));
                            MemoryStream memoryStre = new MemoryStream(Encoding.UTF8.GetBytes(MD5str));

                            List<Obj_ImageValidation> MD5check = deSerializer.ReadObject(memoryStre) as List<Obj_ImageValidation>;
                            memoryStre.Close();
                            memoryStre.Flush();
                            memoryStre.Dispose();

                            AddTxtLog("对解压后的每张图片进行MD5校验");

                            //对解压后的每张图片进行MD5校验
                            string MD5Code = "";
                            string PicfolderName = absolutePath.Substring(0, absolutePath.LastIndexOf('.'));  //图片存放的文件夹名称
                            foreach (Obj_ImageValidation iv in MD5check)
                            {
                                if (File.Exists(PicfolderName + "\\" + iv.fileName))
                                {
                                    MD5Code = GetFileMD5(PicfolderName + "\\" + iv.fileName);
                                    if (MD5Code.ToUpper() != iv.CheckCode.ToUpper())
                                    {
                                        result = "图片（" + PicfolderName + "\\" + iv.fileName + "）上传不完整！传过来的MD5校验码：" + iv.CheckCode + "，服务器生成的校验码：" + MD5Code + "。";
                                        ExceptionHandler.HandleWebException(logger, "406", result, Context, username);
                                        Context.Response.Write(JsonHelper.ToJson(result));
                                        AddTxtLog(result);
                                        return;
                                    }
                                }
                                else
                                {
                                    result = "在对每张图片做MD5校验时发现，根据从APP传过来的图片名称（" + iv.fileName + "）找不到文件！";
                                    AddTxtLog(result);
                                    Context.Response.Write(JsonHelper.ToJson(result));
                                    return;
                                }
                            }
                            #endregion
                        }
                        else
                        {
                            result = "压缩包文件（" + absolutePath + "）不存在！";
                            AddTxtLog(result);
                            Context.Response.Write(JsonHelper.ToJson(result));
                            return;
                        }
                    }

当代码执行到“MD5Code = GetFileMD5(PicfolderName + "\" + iv.fileName);”这一句的时候会报错，我觉得是我的程序对文件流没控制好，还望大神们指教。谢谢！

GetFileMD5 怎么定义的

 #region MD5文件校验
        /// <summary>
        /// MD5文件校验
        /// </summary>
        /// <param name="path"></param>
        /// <returns></returns>
        public string GetFileMD5(string path)
        {
            string ret = "";
            FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read);
            MD5CryptoServiceProvider md5 = new MD5CryptoServiceProvider();
            byte[] md5byte = md5.ComputeHash(fs);
            int i, j;
            foreach (byte b in md5byte)
            {
                i = Convert.ToInt32(b);
                j = i >> 4;
                ret += Convert.ToString(j, 16);
                j = ((i << 4) & 0x00ff) >> 4;
                ret += Convert.ToString(j, 16);
            }
            fs.Flush();
            fs.Close();
            fs.Dispose();
            return ret;
        }
        #endregion

FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read);
将这句替换成：

using( FileStream fs = new FileStream(path, FileMode.Open, FileAccess.Read))
{
}

你是不是用到多线程了？如果是，请处理好多线程同步问题。