File download from Google Drive to byte array failing - C# HttpClient

Question

I'm new to HttpClient. The following code to download a binary file returns a byte array of the wrong length and containing the wrong data:

public static async Task<byte []> ReadWebFileBinaryAsync ( HttpClient httpClient, string webUrl )
{
    byte [] bytes;

    try
    {
        HttpResponseMessage response = await httpClient.GetAsync ( webUrl );
        response.Content.Headers.ContentType = new System.Net.Http.Headers.MediaTypeHeaderValue( "application/octet-stream" );
        using ( MemoryStream memoryStream = new MemoryStream () )
        {
            using ( Stream stream = await response.Content.ReadAsStreamAsync () )
            {
                stream.CopyTo ( memoryStream );
            }
            memoryStream.Seek ( 0, SeekOrigin.Begin );
            bytes = memoryStream.ToArray ();
        }

        return bytes;
    }
    catch ( Exception ex )
    {
        // ...
    }
}

httpClient is initialised app-wide as follows:

httpClientHandler = new HttpClientHandler
{
    AllowAutoRedirect = true,           // Allegedly the default, but to make sure
    MaxAutomaticRedirections = 20
};

httpClient = new HttpClient ( httpClientHandler );
httpClient.Timeout = TimeSpan.FromSeconds ( 60 );

// Accept all languages
httpClient.DefaultRequestHeaders.AcceptLanguage.Clear();
httpClient.DefaultRequestHeaders.AcceptLanguage.Add ( new System.Net.Http.Headers.StringWithQualityHeaderValue ( "*" ) );

// Accept binary data
httpClient.DefaultRequestHeaders.Accept.Add ( new System.Net.Http.Headers.MediaTypeWithQualityHeaderValue ( "application/octet-stream" ) );

The web file is 688kb and has a public url of the form

https://drive.google.com/file/d/xxxxx/view?usp=share_link

Anyone who has the link can open it in a web browser.

The returned array contains only 75K bytes, and as mentioned the data is completely wrong (comparing with the web file in a hex editor). Another file of size just 14KB likewise returns an array of 75K bytes! The exact number of bytes varies slightly each time the app is run, but between 75,400 and 75,600.

This is in .NET 7.0.1, Xamarin.Forms.

I don't know if my code is wrong or there is something funny about downloading from Google Drive. Maybe the Google API must be used? A way to transform the url maybe? The files I care about will be less than 1MB. Any help appreciated!

Answer 1

For Google Drive links, Google interposes a page with a Download button, causing the code above to fail.

The problem is solved by first calling the following method, which works for files up to around 25Mb in size. Larger files would probably require a Google API solution.

private string CheckForGoogleDriveUrl ( string url )
// If given url is a Google Drive url, returns an equivalent direct url, avoiding initial page with download button
// Transforms https://drive.google.com/file/d/xxxxx/view?usp=share_link to https://drive.google.com/uc?export=download&id=xxxxx
// Note: Probably does not work for files that Google considers too large to virus check, around 25MB, as Google will issue a prompt
{
    if ( url.Contains ( "https://drive.google.com/file" ) )
    {
        string [] s = url.Split ( '/' );
        url = s [5];
        s = url.Split ( '/' );
        url = "https://drive.google.com/uc?export=download&id=" + s [0];
    }

    return url;
}