Not able to unzip a .zip file using gzipstream in C# .NET 4.0

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C#4.0

I have .zip file having one file inside it in csv format. I have to use .net 4.0, so I am using GZipStream to unzip the file . But I am getting an error: "The magic number in GZip header is not correct. Make sure you are passing in a GZip stream.".

Note: I have to achieve this using .net 4.0 and I cannot use third party libraries.

What I have tried:

public MainWindow()
        {
            InitializeComponent();
            OpenFileDialog ofd = new OpenFileDialog();
            ofd.Multiselect = false;
            if(ofd.ShowDialog().HasValue && !string.IsNullOrEmpty(ofd.FileName))
            {
                FileInfo fi = new FileInfo(ofd.FileName);
                Decompress(fi);
            }
        }

        public static void Decompress(FileInfo fileToDecompress)
        {
            using (FileStream originalFileStream = fileToDecompress.OpenRead())
            {
                string currentFileName = fileToDecompress.FullName;
                string newFileName = currentFileName.Remove(currentFileName.Length - fileToDecompress.Extension.Length);

                using (FileStream decompressedFileStream = File.Create(newFileName))
                {
                    using (GZipStream decompressionStream = new GZipStream(originalFileStream, CompressionMode.Decompress))
                    {
                        decompressionStream.CopyTo(decompressedFileStream);
                        Console.WriteLine("Decompressed: {0}", fileToDecompress.Name);
                    }
                }
            }
        }