Async/Await Iterate over returned Task<IEnumerable<SomeClass>>

Question

I want to use async/await when querying a database with a HTTP call in my WPF application. It's my first time using async/await, so if you see any obvious mistake, feel free to point them out.

The problem is, that im not able to iterate over the returned collecting anymore, because it's now a Task of <IEnumerable<SomeClass>>, and from what I found out, the Task doesn't implement IEnumerable/IEnumerator.

My code look like this: The method that calls the async/await method.

private void AddProjectDrawingAndComponentsFromServerToLocalDbAsync(CreateDbContext1 db, Project item)
{
   var drawings = client.GetDrawingsAsync(item.ProjectId);
   ..... (waiting for GetDrawingsAsync to return, so i can iterate over the drawings)
   db.SaveChanges();
}

The method GetDrawingsAsync:

public async Task<IEnumerable<Drawing>> GetDrawingsAsync(int projectId)
{
    var drawings = Task.Factory.StartNew(() => _client.Get(new GetDrawingsReq() { ProjectId = projectId }));
    return await drawings;
}

Back to the method that waits for the async method to finish:

private void AddProjectDrawingAndComponentsFromServerToLocalDbAsync(CreateDbContext1 db, Project item)
{
    var drawings = client.GetDrawingsAsync(item.ProjectId);  <-- The returned Task<Task<IEnumerable<Drawing>>
    Drawing prj_drawing = null;
    foreach (var draw in drawings)
    {
        prj_drawing = new Drawing() { DrawingKey = draw.DrawingKey, Name = draw.Name, ProjectId = item.ProjectId };
        item.AddDrawing(prj_drawing);
        db.Projects.Add(item);
    }
    db.SaveChanges();
}

How would I be able to convert the returned type Task<IEnumerable<Drawing>> to something that i can iterate over in the foreach loop.

Answer 1

To iterate over the result of the Task, you need to get the result of the Task. And to do that, you should use await. This means you should change AddProjectDrawingAndComponentsFromServerToLocalDbAsync() into an async method:

private async Task AddProjectDrawingAndComponentsFromServerToLocalDbAsync(
    CreateDbContext1 db, Project item)
{
    var drawings = await client.GetDrawingsAsync(item.ProjectId);
    foreach (var draw in drawings)
    {
        // whatever
    }
    db.SaveChanges();
}

This means that AddProjectDrawingAndComponentsFromServerToLocalDbAsync() (which is BTW quite a bad name for a method, I think) now returns a Task, so you should probably change the method that calls it into an async method too. This leads to “async all the way”, but that's unavoidable.