Gzip解压缩字符串错误，Gzip头中的幻数不正确

我试图在c#中复制php函数gzuncompress到目前为止，我得到了以下代码的一部分工作。参见下面的注释和代码。

我认为在byte[]和字符串转换期间发生了一些棘手的事情。我该如何解决这个问题?我错过了什么?

我使用的是。net 3.5环境

        var plaintext = Console.ReadLine();
        Console.WriteLine("string to byte[] then to string");
        byte[] buff = Encoding.UTF8.GetBytes(plaintext);
        var compress = GZip.GZipCompress(buff);
        //Uncompress working below
        try
        {
            var unpressFromByte = GZip.GZipUncompress(compress);
            Console.WriteLine("uncompress successful by uncompress byte[]");
        }catch
        {
            Console.WriteLine("uncompress failed by uncompress byte[]");
        }
        var compressString = Encoding.UTF8.GetString(compress);
        Console.WriteLine(compressString);
        var compressBuff = Encoding.UTF8.GetBytes(compressString);
        Console.WriteLine(Encoding.UTF8.GetString(compressBuff));
        //Uncompress not working below by using string
        //The magic number in GZip header is not correct
        try
        {
            var uncompressFromString = GZip.GZipUncompress(compressBuff);
            Console.WriteLine("uncompress successful by uncompress string");
        }
        catch
        {
            Console.WriteLine("uncompress failed by uncompress string");
        }

类Gzip的代码

public static class GZip
      {
        public static byte[] GZipUncompress(byte[] data)
        {
            using (var input = new MemoryStream(data))
            using (var gzip = new GZipStream(input, CompressionMode.Decompress))
            using (var output = new MemoryStream())
            {
                gzip.CopyTo(output);
                return output.ToArray();
            }
        }
        public static byte[] GZipCompress(byte[] data)
        {
            using (var input = new MemoryStream(data))
            using (var output = new MemoryStream())
            {
                using (var gzip = new GZipStream(output, CompressionMode.Compress, true))
                {
                    input.CopyTo(gzip);
                }
                return output.ToArray();
            }
        }
        public static long CopyTo(this Stream source, Stream destination)
        {
            var buffer = new byte[2048];
            int bytesRead;
            long totalBytes = 0;
            while ((bytesRead = source.Read(buffer, 0, buffer.Length)) > 0)
            {
                destination.Write(buffer, 0, bytesRead);
                totalBytes += bytesRead;
            }
            return totalBytes;
        }
}