//小文件public static string ComputeMD5(byte[] bytes)        {            var alg = HashAlgorithmProvider.OpenAlgorithm("MD5");            IBuffer buff = CryptographicBuffer.CreateFromByteArray(bytes);            var hashed = alg.HashData(buff);            var res = CryptographicBuffer.EncodeToHexString(hashed);            return res;        }        public static byte[] StreamToBytes(Stream stream)        {            byte[] bytes = new byte[stream.Length];//这个地方会报内存不够，文件太大了            stream.Read(bytes, 0, bytes.Length);            return bytes;        }//大文件的话必须用数据流，不然会内存不够，一般大于50M的最好用流的形式比较好        public static async Task<string> ComputeMD5(Stream stream)        {            var alg = HashAlgorithmProvider.OpenAlgorithm(HashAlgorithmNames.Md5);            var inputStream = stream.AsInputStream();            const uint capacity = 1000000000;//100M这个参数可以自己修改，根据自己的配置决定            var buffer = new Windows.Storage.Streams.Buffer(capacity);            var hash = alg.CreateHash();            while (true)            {                await inputStream.ReadAsync(buffer, capacity, InputStreamOptions.None);                if (buffer.Length > 0)                    hash.Append(buffer);                else                    break;            }            string hashText = CryptographicBuffer.EncodeToHexString(hash.GetValueAndReset()).ToLower();                        return hashText;        }

使用例子:
StorageFile MYFile = await KnownFolders.DocumentsLibrary.GetFileAsync("791d5b3b142c6ba114eecc12ac2ff751.mp4");
            Stream mystream = (await MYFile.OpenAsync(FileAccessMode.Read)).AsStream();
            string txt2 = ComputeMD5(StreamToBytes(mystream));//使用正常获取，可能报错
            string txt = await ComputeMD5(mystream);//使用流方式获取