[Closed] Compress to GZip?

I am trying to compress a file with Gzip type,the codes here:

    file_a=@"C:\file_to_compress.txt"
	file_b=@"C:\need_compress.gz"    

	OriginFile = (dotnetClass "System.IO.File").open file_a (dotnetClass "system.io.filemode").open
    CompressedStream = (dotnetClass "System.IO.File").create file_b

	OriginStream = dotnetobject "System.IO.Compression.GZipStream" OriginFile (dotnetclass "System.IO.Compression.Compressionmode").compress
	OriginStream.CopyTo(CompressedStream)

    OriginFile.Close()
    CompressedStream.Close()

Then,I got an error:

– Runtime error: dotNet runtime exception: Stream does not support reading. <<

Any one can help me how to fix it?Thanks !