Question

I have the below class I want to unit test:

public class MyClass
{
  BackgroundWorker _worker = new BackgroundWorker();
  ObservableCollection<Product> _data = new ObservableCollection<Product>();

  public MyClass() 
  {
    LoadProducts();
  }

  public ObservableCollection<Product> Data
  {
    get
    {
      return _data;
    }
  }

  void LoadProducts()  
  {
     _worker.DoWork += worker_DoWork;
     _worker.RunWorkerCompleted += worker_RunWorkerCompleted;
     _worker.RunWorkerAsync();
  }  

  void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e)
  {
    UpdateData();
  }

  void UpdateData()  
  {
    _data.Clear();
    //Do some work with the retrieved products
  }
}

Here is my unit test:

[SetUp]
public void Setup()
{
  _myClass = new MyClass();
}

[Test]
public void Data_Count_Should_Contain_Same_Number_Of_Items_As_Source()
{
  Assert.AreEqual(_myClass.Data.Count, 100, "Data item counts do not match");
}

The problem is that once _worker.RunWorkerAsync() is called, the unit test continues and does not wait for the async result so the data is not ready.

I tried adding a Thread.Sleep call before the Assert, but it looks like that causes worker_RunWorkerCompleted to be called on another thread. Because of that, when UpdateData is called, I get an exception because a thread other than the thread that created the collection is trying to modify it.

"This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread."

How can I test this code? Is it possible to do if I cannot change the class itself?

Thanks.

Answer 1

You need to add a ManualResetEvent to signal the completion of the background work which consumers can hook into

public class MyClass
{
  ManualResetEvent _doneEvent = new ManualResetEvent(true);
  ...

  public ManualResetEvent BackgroundWorkComplete {
    get { return _doneEvent; }
  }

  void LoadProducts() {
    _doneEvent.Reset();  
    _worker.DoWork += worker_DoWork;
    _worker.RunWorkerCompleted += worker_RunWorkerCompleted;
    _worker.RunWorkerAsync();
  }  

  void worker_RunWorkerCompleted(object sender, RunWorkerCompletedEventArgs e) {
    UpdateData();
    _doneEvent.Set();
  }
}

Now you can wait on this event from the unit test code.

_myClass.BackgroundWorkComplete.WaitOne();
Assert.AreEqual(_myClass.Data.Count, 100, "Data item counts do not match");