GetMultipartBoundary()
private byte[] GetMultipartBoundary()
{
    string attributeFromHeader = GetAttributeFromHeader(this.ContentType, "boundary");
    if (attributeFromHeader == null)
    {
        return null;
    }
    attributeFromHeader = "--" + attributeFromHeader;
    return Encoding.ASCII.GetBytes(attributeFromHeader.ToCharArray());
}

FileStream fs = File.Create(HttpContext.Current.Server.MapPath("TempUpload/" +  strFileName));
//以上是创建一个文件
fs.Write(FileByte, FileStartPos, FileReadedLength);
//以上是将读取到的部分文件数据写入该文件

将除去文件数据的其余HTTP请求写入缓存
    /**//// <summary>
    /// 将除去文件数据的其余HTTP请求写入缓存
    /// </summary>
    /// <param name="bWriteByte">要被写入缓存的数组</param>
    /// <param name="iStartIndex">被写入数组的起始位置</param>
    /// <param name="iLength">被写入数组的长度</param>
    /// <param name="bHttpRequestByte">HTTP缓存</param>
    private void WriteHttpRequestWithoutFileData(byte[] bWriteByte,int iStartIndex, int iLength,ref byte[] bHttpRequestByte)
    {
        if (bHttpRequestByte == null)
        {
            bHttpRequestByte = new byte[iLength];
            Array.Copy(bWriteByte, iStartIndex, bHttpRequestByte, 0, iLength);//将文件数据之前的所有内容放入缓存用于稍后封装HTTP请求
        }
        else
        {
            byte[] newbyte = new byte[iLength];
            Array.Copy(bWriteByte, iStartIndex, newbyte, 0, iLength);
            bHttpRequestByte = UnionByte(newbyte, bHttpRequestByte);
        }
    }

    private byte[] UnionByte(byte[] bnewbyte, byte[] boldbyte)
    {
        int iLen = 0;
         
        if (bnewbyte != null)
        {//要添加到原数组中的字节如果不为空
            if (boldbyte != null)
            {
                iLen = boldbyte.Length;
            }
            byte[] unionbyte = new byte[bnewbyte.Length + iLen];
            if (boldbyte != null)
            {
                boldbyte.CopyTo(unionbyte, 0);
            }
            bnewbyte.CopyTo(unionbyte, iLen);
            return unionbyte;
        }
        else
        {
            return boldbyte;
        }
    }

InjectTextParts
    private void InjectTextParts(HttpRequest request, byte[] textParts)
    {
        BindingFlags flags1 = BindingFlags.NonPublic | BindingFlags.Instance;
        Type type1 = request.GetType();
        FieldInfo info1 = type1.GetField("_rawContent", flags1);
        FieldInfo info2 = type1.GetField("_contentLength", flags1);

        if ((info1 != null) && (info2 != null))
        {
            Assembly web = Assembly.GetAssembly(typeof(HttpRequest));
            Type hraw = web.GetType("System.Web.HttpRawUploadedContent");
            object[] argList = new object[2];
            argList[0] = textParts.Length + 1024;
            argList[1] = textParts.Length;

            CultureInfo currCulture = CultureInfo.CurrentCulture;
            object httpRawUploadedContent = Activator.CreateInstance(hraw,
                                                                     BindingFlags.NonPublic | BindingFlags.Instance,
                                                                     null,
                                                                     argList,
                                                                     currCulture,
                                                                     null);

            Type contentType = httpRawUploadedContent.GetType();
            contentType.GetField("_completed", flags1).SetValue(httpRawUploadedContent, true);//不加上这句就会有问题

              FieldInfo dataField = contentType.GetField("_data", flags1);
            dataField.SetValue(httpRawUploadedContent, textParts);
          
            FieldInfo lengthField = contentType.GetField("_length", flags1);
            lengthField.SetValue(httpRawUploadedContent, textParts.Length);
            FieldInfo fileThresholdField = contentType.GetField("_fileThreshold", flags1);
            fileThresholdField.SetValue(httpRawUploadedContent, textParts.Length + 1024);
            info1.SetValue(request, httpRawUploadedContent);
            info2.SetValue(request, textParts.Length);

        }
    }