当你的内存不够用的时候你应该这样写入文件

 MemoryStream stream          
BinaryWriter bw = new BinaryWriter(stream);
            for (Int32 i = 0; i < 100; i++)
            {
                bw.Write(100);
            }
            FileStream dumpFile = new FileStream(System.Web.HttpContext.Current.Request.MapPath("~/a.txt"), FileMode.Create, FileAccess.ReadWrite);
            stream.WriteTo(dumpFile);
            dumpFile.Flush();
            dumpFile.Close();