Using async/await to get information

Question

I have a list that I need to load with product names like:

List<string> prodNames= new List<string>();

Product data does not exist on my system. So, I must download them first.

Product data is downloaded one-by-one to local file system. Downloading a product fires an event handler which I handle and the handler provides the product name I need to add to the list.

I know number of products to get this way at all times, say 5.

I am trying to figure out how do I know when all 5 product names have been added to my list so I can proceed to display them. Here is the pseudocode

List ids = // list of product ids 
List<string> prodNames= new List<string>();  //list to fill with prod names once they are downloaded

this.EventHandler += OnDownloadedProduct; //event fired once each product is downloaded

void GetProducts()
{
  foreach (var id in ids)
  {
     // NOTE that I cannot modify neither downloader object nor GetProduct() 
     // method.  These are not my code but come from a component for which
     // I have no code access.
     downloader.GetProduct(id);  //fires OnDownloadedProduct once product is downloaded
  } 
}

//this event is fired once each product data is downloaded.
// ProductDataArgs is provided to me and I cannot modify it.
void OnDownloadedProduct(object sender, ProductDataArgs e)
{
   // this is where I get product name and add it to the list
   string prodName = e.getProdName();
   prodNames.Add(prodName);
   // <-- I could show prodName here in a message box but that
   // would show one message box for all 5 products.  I don't
   // want that.  I want to show all 5 product names in one
   // message box.
}

I need to display all 5 product names in a form or message box. I could do it in above event but for each product. This would show 5 message boxes (or forms). But I need to show them all 5 in a single message box. So, I need to so something like this in caller:

List<string> prodNames = await.GetProducts(ids);
var names = null;
foreach(var prod in prodNames)
{
  names += string.Format("{0}\n");
}
MesageBox.Show(names);

PROBLEM: The problem I face is that the operation may take time and that I dont know when the list is loaded with all 5 product names (because names are not available in GetProducts method but in OnDownloadedProduct event).

After some research on my own, Tasks/async/await seem to offer solution for this but all examples I have seen so far are not addressing the situation I described above where I depend on an event handler to get what I need (in this case, all 5 product names).

UPDATE Thanks to @Kris comment below, I have been able to rewrite my logic into this but it is not working properly:

public class ProductRetriver
{
    List<int> ids = new List<int>();  // product ids
    List<string> prodNames = new List<string>();  // product names

    private TaskCompletionSource<List<string>> prodsTask;

    this.EventHandler += OnDownloadedProduct; 

    Task<List<string>> async GetProducts(List<int> _ids)
    {
        ids = _ids;
        prodsTask = new TaskCompletionSource<List<string>>();
        foreach (var id in ids)
        {
            //PROBLEM: GetProduct() is not awaited, so method will finish
            //before list is filled.  Also, it seem to add duplicates for
            //some reason to the list.  I cannot modify the method as it is
            //not code available to me.
            downloader.GetProduct(id);  //fires OnDownloadedProduct 
        }
        return prodNames;
    }

    //this event is fired once each product data is downloaded
    void OnDownloadedProduct(object sender, ProductDataArgs e)
    {
        // this is where I get product name and add it to the list
        string prodName = e.getProdName();
        prodNames.Add(prodName);
        if (prodNames.Count == ids.Count)
        {
            prodsTask.SetResult(prodNames);
        }
    }
}

Answer 1

If I right understand, you don't need received result from downloader.GetProduct(id) ? If so - for what you want awaiting ? Just create list of tasks and wait while them all finished like this:

public class ProductRetriver
{
List<int> ids = new List<int>();  // product ids
List<string> prodNames = new List<string>();  // product names

private TaskCompletionSource<List<string>> prodsTask;

this.EventHandler += OnDownloadedProduct; 
public static List<Task> TaskList = new List<Task>();
Task<List<string>> async GetProducts(List<int> _ids)
{
    ids = _ids;
    prodsTask = new TaskCompletionSource<List<string>>();

    foreach (var id in ids)
    {

       TaskList.Add(downloader.GetProduct(id));
    }
    Task.WaitAll(TaskList.ToArray());
    return prodNames;
}

//this event is fired once each product data is downloaded
void OnDownloadedProduct(object sender, ProductDataArgs e)
{
    // this is where I get product name and add it to the list
    string prodName = e.getProdName();
    prodNames.Add(prodName);
    if (prodNames.Count == ids.Count)
    {
        prodsTask.SetResult(prodNames);
    }
}

}

Answer 2

i don't know what is this downloader and where it comes from, if eventhandler is out side downloader try this

 public class ProductRetriver
{

    List<int> ids = new List<int>();
    List<string> prodNames = new List<string>();  //list to fill with prod names once they are downloaded
    private TaskCompletionSource<List<string>> prodsTask;
        this.EventHandler += OnDownloadedProduct; //event fired once each product is downloaded

        Task<List<string>> async GetProducts(List<int> _ids)
    {
        ids = _ids;
        prodsTask = new TaskCompletionSource<List<string>>();
        foreach (var id in ids)
        {
            downloader.GetProduct(id);  //fires OnDownloadedProduct once product is downloaded
        }

    }

    //this event is fired once each product data is downloaded
    void OnDownloadedProduct(object sender, ProductDataArgs e)
    {
        // this is where I get product name and add it to the list
        string prodName = e.getProdName();
        prodNames.Add(prodName);
        if (prodNames.Count == ids.Count)
        {
            prodsTask.SetResult(prodNames);
        }
    }
}

if eventhandler is a part of downloader you can do this

Task<List<string>> async GetProducts(List<int> ids)
    {
        List<string> prodNames = new List<string>();

        downloder.EventHandler += (s, e) => {

            string prodName = e.getProdName();
            prodNames.Add(prodName);
            if (prodNames.Count == ids.Count)
            {
                prodsTask.SetResult(prodNames);
            }
        };
        foreach (var id in ids)
        {
            downloader.GetProduct(id);  //fires OnDownloadedProduct once product is downloaded
        }

        return prodNames;

    }

you can call this like this

Products = await getproducts(ids);