Ref and Out parameters in Asynchronous Method

As for why async methods don't support out-by-reference parameters? 

That's a limitation of the CLR. There are good reasons for that; the most obvious is that if you pass a local variable by reference, it is stored on the stack, but the current stack won’t remain available during the whole execution of the async method (only until the first await), so the location of the variable won’t exist anymore.

However, it’s pretty easy to work around that limitation : you only need to create a Ref<T> class to hold the value, and pass an instance of this class by value to the async method:

public class Ref<T>

{

    public Ref() { }

    public Ref(T value) { Value = value; }

    public T Value { get; set; }

    public override string ToString()

    {

        T value = Value;

        return value == null ? "" : value.ToString();

    }

    public static implicit operator T(Ref<T> r) { return r.Value; }

    public static implicit operator Ref<T>(T value) { return new Ref<T>(value); }

}










async void Button_Click(object sender, EventArgs e)

{

    var count = new Ref<int>();

    var size = new Ref<ulong>();

    await GetFileStats(tbPath.Text, count, size);

    txtFileStats.Text = string.Format("{0} files ({1} bytes)", count, size);

}

 

async Task GetFileStats(string path, Ref<int> totalCount, Ref<ulong> totalSize)

{

    var folder = await StorageFolder.GetFolderFromPathAsync(path);

    foreach (var f in await folder.GetFilesAsync())

    {

        totalCount.Value += 1;

        var props = await f.GetBasicPropertiesAsync();

        totalSize.Value += props.Size;

    }

    foreach (var f in await folder.GetFoldersAsync())

    {

        await GetFilesCountAndSize(f, totalCount, totalSize);

    }

}







As you can see, it’s pretty straightforward. This approach can also be used in iterator blocks (i.e. yield return), that also don’t allow ref and out parameters. It also has an advantage over standard ref and out parameters: you can make the parameter optional, if for instance you’re not interested in the result (obviously, the callee must handle that case appropriately).