Async await how to use return values

Question

I have a windows service that I have inherited from another developer, it runs very slow and has numerous slow call to the eBay API. I wish to speed it up without too much refac

Answer 1

If the task returned by Task.WhenAll has completed, that means all of the tasks that you passed to it have completed too. That in turn means that you can use the Result property of each task, with no risk of it blocking.

string details = additionalProductDetials.Result;

Alternatively, you could await the tasks, for consistency with other async code:

string details = await additionalProductDetials;

Again, this is guaranteed not to block - and if you later remove the Task.WhenAll for some reason (e.g. you're happy to use the details to kick off another task before you've got the part number collection) then you don't need to change the code.

Answer 2

Use Result property on Task class:

await Task.WhenAll(partNumberCollection, additionalProductDetials);

var partNumberCollectionResult = partNumberCollection.Result;
var additionalProductDetialsResult = additionalProductDetials.Result;

Answer 3

Try this code:

public async Task<String> ProcessAdditionalProductDetialsAsync(ItemType oItem) { 
    String additionalProductDetails = await Task.Run(() => {
       if (oItem.ItemSpecifics.Count > 0) {
          foreach (NameValueListType nvl in oItem.ItemSpecifics) { 
             if (nvl.Value.Count > 0) {
                 string retval = String.Empty;

                 foreach (string s in nvl.Value) {
                     retval += "<li><strong>" 
                       + nvl.Name + ":</strong>&nbsp;" + s + "</li>";
                 }
             }
          }
       }
       return retval;
     }
     return additionalProductDetails;
 }

Usage:

private async void GetAdditionalProductDetailsAsync(Action<string> callback) {
   string apd = await ProcessAdditionalProductDetialsAsync();
   callback(apd);
}

private void AdditionalProductDetailsRetrieved(string apd) {
    // do anything with apd
}

Answer 4

Your async method lacks of await operators and will run synchronously. while you are calling non blocking API you could use Task.Run() to do cpu-bound work on background thread.

public async Task<String> ProcessAdditionalProductDetialsAsync(ItemType oItem)
{
    return await Task.Run(() =>
    {
        String additionalProductDetails = string.Empty;

        if (oItem.ItemSpecifics.Count > 0)
        {
            foreach (NameValueListType nvl in oItem.ItemSpecifics)
            {
                if (nvl.Value.Count > 0)
                {
                    foreach (string s in nvl.Value)
                    {
                        additionalProductDetails += "<li><strong>" + nvl.Name + ":</strong>&nbsp;" + s + "</li>";
                    }
                }
            }
        }
        return additionalProductDetails;
    });
}

and get result

var detail = await ProcessAdditionalProductDetialsAsync(itemType);
var result = ProcessAdditionalProductDetialsAsync(itemType).Result;