private void Get(Uri uri)
        {
            var client = new System.Net.WebClient();
            client.DownloadStringAsync(uri);
            client.DownloadStringCompleted += (s, ea) => Function(uri.ToString());
        }

var t1 = Get(uri1);
var t2 = Get(uri2);

Task.Factory.ContinueWhenAll(new[] { t1, t2 }, tt => Dispatcher.BeginInvoke(() => MessageBox.Show("Complete")));

...

private Task Get(Uri uri)
{
    return Task.Factory.FromAsync(........);
}

    public static class WebClientHelper
    {
        public static Task<string> DownloadStringAsyncEx(this WebClient wc, Uri uri)
        {
            var tcs = new TaskCompletionSource<string>();
            wc.DownloadStringCompleted += DownloadStringCompleted;
            wc.DownloadStringAsync(uri, tcs);
            return tcs.Task;    
        }

        static void DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            var wc = (WebClient)sender;
            wc.DownloadStringCompleted -= DownloadStringCompleted;

            var tcs = (TaskCompletionSource<string>)e.UserState;

            if (e.Cancelled)
            {
                tcs.SetCanceled();
                return;
            }

            if (e.Error != null)
            {
                tcs.SetException(e.Error);
                return;
            }

            tcs.SetResult(e.Result);
        }
    }

    public partial class MainPage
    {
        public MainPage()
        {
            InitializeComponent();
            var task = DownloadStringAsyncEx(new Uri("http://localhost:1732/data1.txt"));
            MessageBox.Show(string.Format("Got {0}", task.Result));
        }

    public partial class MainPage
    {
        public MainPage()
        {
            InitializeComponent();
            
            DownloadStringAsyncEx(new Uri("http://localhost:1732/data1.txt"))
                .ContinueWith(t => Dispatcher.BeginInvoke(() =>
                       MessageBox.Show(string.Format("Got {0}", t.Result)
                )); // + обработку ошибок добавить
        }

Task<int> F1()
{ return ... }

Task<int> F2(int v1)
{ return ... }

Task<int> F3(int v2)
{ return ... }

...


Task<int> Execute()
{
    return F1()
        .ContinueWith(t => F2(t.Result))
        .Unwrap()
        .ContinueWith(t => F3(t.Result))
        .Unwrap();
}

var res = from t1 in Observable.Return(Task1())
              from t2 in Observable.Return(Task2())
              select t1+t2;
or
    Observable.Return(Task1()).And(Observable.Return(Task2()).Then((t1,t2) = > t1+t2)      

var res = from t1 in Observable.Return(Task1())
              from t2 in Observable.Return(Task2(t1))
              from t3 in Observable.Return(Task2(t2))
              etc
              select t48;

var t1 = Task1()
    .ContinueWith(t => Task2(t.Result))
    .Unwrap();

var t2 = Task3();

Task.Factory
    .ContunueWhenAll(
        new[] {t1, t2},
        tt =>
        {
              var items = t1.Result.Concat(t2.Result);
              Console.WriteLine(items.Min());
              Console.WriteLine(items.Max());
        }
    )
    .LogExceptions();

////////////////////

public static class TaskHelper
{
    public static void LogExceptions(this Task task)
    {
        if (task.Exception != null)
            Log.AddException(task.Exception);
    }
}