Understanding Task vs Task<T> as a return type

Question

Apologies if this has been addressed somewhere, but it is quite overwhelming the amount of resources on the topic.

I wrote the following method that awaits on an async call

public async Task GetAllObjectsNames(string bucketName)
{
    if (string.IsNullOrEmpty(bucketName))
    {
        throw new ArgumentNullException(nameof(bucketName), "bucket name can't be null or empty string");
    }

    var request = new ListObjectsRequest
    {
        BucketName = bucketName
    };

    await  s_client.ListObjectsAsync(request);
}

Working on wrapping my head around the difference between returning a Task and returning a Task<T>

public async Task<IEnumerable<string>> GetAllObjectsNames(string bucketName)
{
    if (string.IsNullOrEmpty(bucketName))
    {
        throw new ArgumentNullException(nameof(bucketName), "bucket name can't be null or empty string");
    }

    var request = new ListObjectsRequest
    {
        BucketName = bucketName
    };

    var response = await s_client.ListObjectsAsync(request);

    return response.S3Objects.Select(o => o.Key);
}

Could someone chime in?

Answer 1

Task represents an operation which might be in progress, or might have been cancelled, faulted, or completed. A completed Task does not contain any sort of result, it just represents the fact that the operation has finished.

Task<T> also represents an operation which might be in progress, or might have been cancelled, faulted, or might have completed. However, a completed Task<T> will contain the result of that operation, which you can then read.

You can write:

public async Task<IEnumerable<string>> GetAllObjectsNames(string bucketName)
// ...
var objects = await GetAllObjectsNames("Foo");
await GetAllObjectsNames("Foo"); // Throws away the result of the Task

However you can only write:

public async Task GetAllObjectsNames(string bucketName)
// ...
await GetAllObjectsNames("Foo");

The Task returned from this version of GetAllObjectsNames cannot contain a result, so you can only tell when that operation has finished - you cannot get a result out of it.

You might use Task as the return value from an UploadObject method - where you care when the upload operation finishes, but there's no result for you to read. You would probably use Task<T> for GetAllObjectsNames, since the point of that operation is to fetch some values for you to then use.

Conceptually it's the same as the difference between a method which returns void (does not return a result), or a method which returns anything else. Or the difference between Action and Func<T>.