CODEWITHSUNDEEP
c#task
Percy
Percy

Reputation: 3115

C# Task.WaitAll isn't waiting

My aim is to download images from an Amazon Web Services bucket.

I have the following code function which downloads multiple images at once:

public static void DownloadFilesFromAWS(string bucketName, List<string> imageNames)
{
    int batchSize = 50;
    int maxDownloadMilliseconds = 10000;

    List<Task> tasks = new List<Task>();

    for (int i = 0; i < imageNames.Count; i++)
    {
        string imageName = imageNames[i];
        Task task = Task.Run(() => GetFile(bucketName, imageName));
        tasks.Add(task);
        if (tasks.Count > 0 && tasks.Count % batchSize == 0)
        {
            Task.WaitAll(tasks.ToArray(), maxDownloadMilliseconds);//wait to download
            tasks.Clear();
        }
    }

    //if there are any left, wait for them
    Task.WaitAll(tasks.ToArray(), maxDownloadMilliseconds);
}

private static void GetFile(string bucketName, string filename)
{
    try
    {
        using (AmazonS3Client awsClient = new AmazonS3Client(Amazon.RegionEndpoint.EUWest1))
        {
            string key = Path.GetFileName(filename);

            GetObjectRequest getObjectRequest = new GetObjectRequest() {
                 BucketName = bucketName,
                    Key = key
            };

            using (GetObjectResponse response = awsClient.GetObject(getObjectRequest))
            {
                string directory = Path.GetDirectoryName(filename);
                if (!Directory.Exists(directory))
                {
                    Directory.CreateDirectory(directory);
                }

                if (!File.Exists(filename))
                {
                    response.WriteResponseStreamToFile(filename);
                }
            }
        }
    }
    catch (AmazonS3Exception amazonS3Exception)
    {
        if (amazonS3Exception.ErrorCode == "NoSuchKey")
        {
            return;
        }
        if (amazonS3Exception.ErrorCode != null && (amazonS3Exception.ErrorCode.Equals("InvalidAccessKeyId") || amazonS3Exception.ErrorCode.Equals("InvalidSecurity")))
        {
            // Log AWS invalid credentials
            throw new ApplicationException("AWS Invalid Credentials");
        }
        else
        {
            // Log generic AWS exception
            throw new ApplicationException("AWS Exception: " + amazonS3Exception.Message);
        }
    }
    catch
    {
        //
    }
}

The downloading of the images all works fine but the Task.WaitAll seems to be ignored and the rest of the code continues to be executed - meaning I try to get files that are currently non existent (as they've not yet been downloaded).

I found this answer to another question which seems to be the same as mine. I tried to use the answer to change my code but it still wouldn't wait for all files to be downloaded.

Can anyone tell me where I am going wrong?

Upvotes: 3

Views: 1677

Answers (1)

NineBerry
NineBerry

Reputation: 28499

The code behaves as expected. Task.WaitAll returns after ten seconds even when not all files have been downloaded, because you have specified a timeout of 10 seconds (10000 milliseconds) in variable maxDownloadMilliseconds.

If you really want to wait for all downloads to finish, call Task.WaitAll without specifying a timeout.

Use

Task.WaitAll(tasks.ToArray());//wait to download

at both places.

To see some good explanations on how to implement parallel downloads while not stressing the system (only have a maximum number of parallel downloads), see the answer at How can I limit Parallel.ForEach?

Upvotes: 6

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