Why the Task gets Cancelled-CodePudding

I'd like to spawn some threads and in each thread sequentially make calls to an API and aggregate the results (some sort of stress testing). Here is my attempt:

private async Task DoWork()
{
    var allResponses = new List<int>();
    for (int i = 0; i < 10; i  )
    {
        await Task.Run(() =>
        {
            var responses = Enumerable.Range(0, 50).Select(i => CallApiAndGetStatusCode());
            allResponses.AddRange(responses);
        });
    }

    // do more work
}


private int CallApiAndGetStatusCode()
{
    try
    {
        var request = new HttpRequestMessage(httpMethod.Get, "some url");

        var responseResult = httpClient.SendAsync(request).Result;
        return (int)responseResult.StatusCode;
    }
    catch (Exception e)
    {
        logger.LogError(e, "Calling API failed");
    }
}

However, this code always ends up the catch with the inner exception being {"A task was canceled."}. What am I doing wrong here?

CodePudding user response：

There is no benefit to using either Enumerable.Range or .AddRange in your example, since you do not need the seeded number. Your code must be converted to async/await to avoid deadlocks and in doing so, you can simply loop inside of each task and avoid any odd interactions between Enumerable.Select and await:

private async Task DoWork()
{
    var allTasks = new List<Task>(10);
    var allResponses = new List<int>();
    
    for (int i = 0; i < 10; i  )
    {
        allTasks.Add(Task.Run(async () =>
        {
            var tempResults = new List<int>();
            
            for (int i = 0; i < 50; i  )
            {
                var result = await CallApiAndGetStatusCode();
                if (result > 0) tempResults.Add(result);
            }
            
            if (tempResults.Count > 0)
            {
                lock (allResponses)
                {
                    allResponses.AddRange(tempResults);
                }
            }
        }));        
    }

    await Task.WhenAll(allTasks);

    // do more work
}


private async Task<int> CallApiAndGetStatusCode()
{
    try
    {
        var request = new HttpRequestMessage(HttpMethod.Get, "some url");

        var responseResult = await httpClient.SendAsync(request);
        return (int)responseResult.StatusCode;
    }
    catch (Exception e)
    {
        logger.LogError(e, "Calling API failed");
    }
    
    return -1;
}

Note that this code is overly protective, locking the overall batch before adding the temp results.

CodePudding user response：

I changed your code to this and work

async Task DoWork()
{
    var allResponses = new List<int>();
    for (int i = 0; i < 10; i  )
    {
        await Task.Run(() =>
        {
            var responses = Enumerable.Range(0, 3).Select(i => CallApiAndGetStatusCodeAsync());
            allResponses.AddRange(responses.Select(x => x.Result));
        });
    }

    // do more work
}

async Task<int> CallApiAndGetStatusCodeAsync()
{
    try
    {
        var request = new HttpRequestMessage(HttpMethod.Get, "http://www.google.com");
        var responseResult = await httpClient.SendAsync(request);
        return (int)responseResult.StatusCode;
    }
    catch (Exception e)
    {
        logger.LogError(e, "Calling API failed");
        return -1;
    }
}