title: Continue Task after all tasks finish
body: <p>In some class I want to load 2 collection asynchronously with Task and stop busyindicator</p>
  
  <p>I try Something like this</p>
  
  <pre><code>var uiScheduler = TaskScheduler.FromCurrentSynchronizationContext();
  WaitingIndicatorViewModel.IsBusy = true;
  var loadData1 = new Task&lt;ObservableCollection&lt;Data1&gt;&gt;(GetData1FromService).ContinueWith(t =&gt; Data1Collection = t.Result, uiScheduler);
  var loadData2 = new Task&lt;ObservableCollection&lt;Data2&gt;&gt;(GetData2FromService).ContinueWith(t =&gt; Data2Collection = t.Result, uiScheduler);
  
  Task.Factory.StartNew(() =&gt;{
                  loadData1.Start();//&lt;--Exception here           
                  loadData2.Start();
                  Task.WaitAll(loadData1, loadData2);
          })
  .ContinueWith(o =&gt; WaitingIndicatorViewModel.IsBusy = false, uiScheduler);
  </code></pre>
  
  <p>But this throw an exception <code>InvalidOperationException:Start may not be called on a continuation task.</code></p>
  
  <p>Why this doesn't work, and how can I run continue task after finishing both tasks, without blocking current thread?</p>