System.Threading.Tasks.TaskCanceledException: A task was canceled.

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Engg wrote:

... Because in all the Call we need Stream how to get the remaining Stream without uploading it again.

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Hm..🤔🤷 What kind of "remaining Stream" do you need at all and why? Honestly, I cannot understand you.

Probably you don't understand how Dropbox upload works in general and that's what confuses you. In all cases (either using explicitly upload session or using simplified upload) the actual upload is performed as from your stream (either provided in one step for small files or in many steps for large files) to a POST HTTP request body gets formed, which fills (at once or step by step) the upload session body (residing on Dropbox server and represented with session id, that you may have or not). Once upload session gets filled upload may be finished (gets associated with file name and path) to users account. 😉 That's it. This flow happens even when you just use simple upload - then it just happens in one call and you don't have access to session id (you don't need it).

No one file can be uploaded directly to particular place in users account (even though such your use of 'UploadAsync' builds such an illusion)! Upload always happens to something like temporary file - upload session! At the end, this session' content can be copied to desired place in users account - upload finishing. About expected streams - all this is option and such a stream always can be empty.

Some example, showing all explained, follows:

private readonly long CHUNK_SIZE = 4 * 1024 * 1024;

private async Task UploadSessionAppenV2Async(DropboxClient dbx, UploadSessionCursor cursor, byte[] buffer, int bytesRead, bool close = false)
{
    try
    {
        var memStream = new MemoryStream(buffer, 0, bytesRead);
        await dbx.Files.UploadSessionAppendV2Async(cursor, close, null, memStream);
    }
    catch(Exception ex)
    {
        Console.WriteLine("Error while append at offset {0}: {1}", cursor.Offset, ex);
    }
}

async Task TestBigUpload(DropboxClient dbx, string filePath)
{
    Console.WriteLine("Започва сесия за качване на {0} като {1}", filePath, filePath.Substring(filePath.LastIndexOf('/')));
    var fileStream = File.OpenRead(filePath);
    Console.WriteLine("Размерът на файла е {0} байта.", fileStream.Length);
    var numChunks = (int)Math.Ceiling(((double)fileStream.Length) / CHUNK_SIZE);
    Console.WriteLine("Ще се качва на {0} части.", numChunks);
    var buffer = new byte[CHUNK_SIZE];
    var memStream = new MemoryStream(buffer, 0, 0);
    var sessionId = (await dbx.Files.UploadSessionStartAsync(body: memStream)).SessionId;
    Console.WriteLine("Отвори се сессия с идентификатор: {0}", sessionId);
    for (var i = 0; i < numChunks - 1; ++i)
    {
        var bytesRead = fileStream.Read(buffer, 0, (int)CHUNK_SIZE);
        var cursor = new UploadSessionCursor(sessionId, (ulong)(i * CHUNK_SIZE));
        Console.WriteLine("Качва се блок с размер {0} на позиция {1}", bytesRead, i * CHUNK_SIZE);
        await UploadSessionAppenV2Async(dbx, cursor, buffer, bytesRead);
    }
    var bytesReaded = fileStream.Read(buffer, 0, (int)CHUNK_SIZE);
    Console.WriteLine("Последият прочетен блок е с размер {0}", bytesReaded);
    var lastCursor = new UploadSessionCursor(sessionId, (ulong)((numChunks - 1) * CHUNK_SIZE));
    await UploadSessionAppenV2Async(dbx, lastCursor, buffer, bytesReaded, true);
    lastCursor = new UploadSessionCursor(sessionId, (ulong)fileStream.Length);
    var commitInfo = new CommitInfo(filePath.Substring(filePath.LastIndexOf('/')), WriteMode.Overwrite.Instance, false);
    memStream = new MemoryStream(buffer, 0, 0);
    await dbx.Files.UploadSessionFinishAsync(lastCursor, commitInfo, body: memStream);
    Console.WriteLine("Качването на {0} приключи.", filePath.Substring(filePath.LastIndexOf('/')));
    Console.WriteLine("Започва приключване и на неговото копие {0} \ud83d\ude09...", "/testSubfolder" + filePath.Substring(filePath.LastIndexOf('/')));
    lastCursor = new UploadSessionCursor(sessionId, (ulong)fileStream.Length);
    commitInfo = new CommitInfo("/testSubfolder" + filePath.Substring(filePath.LastIndexOf('/')), WriteMode.Overwrite.Instance, false);
    memStream = new MemoryStream(buffer, 0, 0);
    await dbx.Files.UploadSessionFinishAsync(lastCursor, commitInfo, body: memStream);
}

This code can upload file of any size. In some places empty streams are used. Just call 'TestBigUpload' from your code to see the result. The temporary file will appear as 2 copies - showing how the session can be used later again without real stream (including in exception handler - not used here). 🙂

Hope this sheds some more light.

PS: The code is just example and is far from optimal - implement it in your code better.