Question

I have a simple Metro style app that’s giving me an issue with (async & await).

List<string> fileNames = new List<string>();
...
...
LoadList();
...
...
(Problem) Code that accesses the elements of the fileNames List
...
...

private async void LoadList()
{
    // Code that loops through a directory and adds the 
    // file names to the fileNames List using GetFilesAsync()
}

The problem is that the fileNames List is accessed prematurely – before it is fully loaded with items.
This is because of the async method – the program continues with the next line of code while the async method continues its processing.

How can I access the List after it is fully loaded (After the async method is done)?

Is there a way to accomplish what I’m trying to do without using async in Metro apps ?

Answer 1

You need the calling method to be asynchronous too – and rather than having a variable of fileNames, I’d make the LoadList method return it. So you’d have:

public async Task ProcessFiles()
{
    List<string> fileNames = await LoadList();
    // Now process the files
}

public async Task<List<string>> LoadList()
{
    List<string> fileNames = new List<string>();
    // Do stuff...
    return fileNames;
}

This does mean that you need to wait for all the files to be found before you start processing them; if you want to process them as you find them you’ll need to think about using a BlockingCollection of some kind. EDIT: As Stephen points out, TPL Dataflow would be a great fit here too.