How do you know that an async method has done its job?

Question

C++ / C#

I have 2 methods. The first downloads audio and video, and the second combines them.
I have an error - video and audio start to merge before the video is completely downloaded. What should I do?

public async void DownloadVideo1080P(string path, ProgressBar progressBar, string link)
{
    await ytClient.Videos.Streams.DownloadAsync(audioInfo, Path.Combine(newLink, $"audio.wav"));
    await ytClient.Videos.Streams.DownloadAsync(videoInfo, Path.Combine(newLink, $"video.mp4"), progressHandler);
}

public async void MergeAudioVideo(string link)
{
    await ffmpeg.ExecuteAsync([email protected]"-i video.mp4  -i audio.wav -c:v copy -c:a aac newVideo.mp4");
}

Answer 1

async void

will return control back without waiting for the execution of asynchronous code inside. you need to return the task

public async Task DownloadVideo1080PAsync(string path, ProgressBar progressBar, string link)
{
    await ytClient.Videos.Streams.DownloadAsync(audioInfo, Path.Combine(newLink, $"audio.wav"));
    await ytClient.Videos.Streams.DownloadAsync(videoInfo, Path.Combine(newLink, $"video.mp4"), progressHandler);
}

public Task MergeAudioVideoAsync(string link)
{
    return ffmpeg.ExecuteAsync([email protected]"-i video.mp4  -i audio.wav -c:v copy -c:a aac newVideo.mp4");
}

....
  await DownloadVideo1080PAsync(....);
  await MergeAudioVideoAsync(...);
....

Answer 2

Remove await, return tasks and wait for them to complete - https://docs.microsoft.com/en-us/dotnet/api/system...
You need to understand that an asynchronous method returns control as soon as it encounters await. Therefore, it is easy to call two of your methods in a row and hope for the best.