Javascript GZIP Compressed string is invalid when read by .NET Deflate stream and GZipStream

So, try as I might, I’m having issues decompressing a string in .NET.

The string is being compressed in Javascript with the following:

const compressedFileText = await compressArrayBuffer(await readFileIntoArrayBuffer(selectedFile));

selectedFile is an object defined as

class selectedFile {
    readonly file: File;
    constructor(file: File) {

    }
    // remainder omitted...
}

The other functions involved here are:

async function compressArrayBuffer(arrBuffer: ArrayBuffer): Promise<string> {

    const stream = new Blob([arrBuffer]).stream();
    const compressedReadableStream = stream.pipeThrough(new CompressionStream("gzip"));
    const compressedResponse = await new Response(compressedReadableStream);
    const blob = await compressedResponse.blob();

    tempDownloadFunction(blob);

    const buffer = await blob.arrayBuffer();
    const bufferView = new Uint8Array(buffer);
    const compressedString = new TextDecoder().decode(bufferView);
    return compressedString;
}

function tempDownloadFunction(blob: Blob): void {
    const elem = window.document.createElement("a");
    elem.href = window.URL.createObjectURL(blob);
    elem.download = '';
    document.body.appendChild(elem);
    elem.click();
    document.body.removeChild(elem);
}

async function readFileIntoArrayBuffer(selectedFile: SelectedFile): Promise<ArrayBuffer> {

    return new Promise<ArrayBuffer>((resolve, reject) => {

        const reader = new FileReader();
        reader.onload = () => {

            resolve(reader.result as ArrayBuffer);
        }
        reader.onerror = reject;
        reader.readAsArrayBuffer(selectedFile.file);
    });
}

This all works fine, and the compressed string is sent to the server as part of a JSON object.

However, when try to decompress on the server I get an Exception with the message:

System.IO.Compression: "The archive entry was compressed using an unsupported compression method."

I’ve tried decompressing via DeflateStream and GZipStream as below:

public static string Decompress(string compressedString)
{
    byte[] decompressedBytes;

    byte[] buffer = Encoding.UTF8.GetBytes(compressedString);

    var compressedStream = new MemoryStream(buffer);

    using (var decompressorStream = new DeflateStream(compressedStream, CompressionMode.Decompress))
    {
        using (var decompressedStream = new MemoryStream())
        {
            decompressorStream.CopyTo(decompressedStream);

            decompressedBytes = decompressedStream.ToArray();
        }
    }

    return Encoding.UTF8.GetString(decompressedBytes);
}

and…

public static string UnZip(string value)
{
    //Transform string into byte[]
    byte[] byteArray = new byte[value.Length];
    int indexBA = 0;
    foreach (char item in value.ToCharArray())
    {
        byteArray[indexBA++] = (byte)item;
    }

    //Prepare for decompress
    System.IO.MemoryStream ms = new System.IO.MemoryStream(byteArray);
    System.IO.Compression.GZipStream sr = new System.IO.Compression.GZipStream(ms,
        System.IO.Compression.CompressionMode.Decompress);

    //Reset variable to collect uncompressed result
    byteArray = new byte[byteArray.Length];

    //Decompress
    int rByte = sr.Read(byteArray, 0, byteArray.Length);

    //Transform byte[] unzip data to string
    System.Text.StringBuilder sB = new System.Text.StringBuilder(rByte);
    
    for (int i = 0; i < rByte; i++)
    {
        sB.Append((char)byteArray[i]);
    }
    sr.Close();
    ms.Close();
    sr.Dispose();
    ms.Dispose();
    return sB.ToString();
}

I’m not sure what I’m doing wrong, and no searching that I’ve tried has yielded a result. However, taking the compressed string downloaded via the temp function and running it through WinZip correctly decompresses it and yields my input file with all data intact.

Can anyone help, please?

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