HttpClient - A task was cancelled?

I ran into this issue because my Main() method wasn't waiting for the task to complete before returning, so the Task<HttpResponseMessage> was being cancelled when my console program exited.

C# ≥ 7.1

You can make the main method asynchronous and await the task.

public static async Task Main(){
    Task<HttpResponseMessage> myTask = sendRequest(); // however you create the Task
    HttpResponseMessage response = await myTask;
    // process the response
}

C# < 7.1

The solution was to call myTask.GetAwaiter().GetResult() in Main() (from this answer).

There's 2 likely reasons that a TaskCanceledException would be thrown:

1. Something called Cancel() on the CancellationTokenSource associated with the cancellation token before the task completed.
2. The request timed out, i.e. didn't complete within the timespan you specified on HttpClient.Timeout.

My guess is it was a timeout. (If it was an explicit cancellation, you probably would have figured that out.) You can be more certain by inspecting the exception:

try
{
    var response = task.Result;
}
catch (TaskCanceledException ex)
{
    // Check ex.CancellationToken.IsCancellationRequested here.
    // If false, it's pretty safe to assume it was a timeout.
}