Unit testing that an event is raised in C#

manpreet Best Answer 2 years ago

I have some code that raises PropertyChanged events and I would like to be able to unit test that the events are being raised correctly.

The code that is raising the events">events is like

public class MyClass : INotifyPropertyChanged
{
   public event PropertyChangedEventHandler PropertyChanged;  

   protected void NotifyPropertyChanged(String info)
   {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
   }  

   public string MyProperty
   {
       set
       {
           if (_myProperty != value)
           {
               _myProperty = value;
               NotifyPropertyChanged("MyProperty");
           }
       }
   }
}

I get a nice green test from the following code in my unit tests, that uses delegates:

[TestMethod]
public void Test_ThatMyEventIsRaised()
{
    string actual = null;
    MyClass myClass = new MyClass();

    myClass.PropertyChanged += delegate(object sender, PropertyChangedEventArgs e)
    {
         actual = e.PropertyName;
    };

    myClass.MyProperty = "testing";
    Assert.IsNotNull(actual);
    Assert.AreEqual("MyProperty", actual);
}

However, if I then try and chain the setting of properties together like so:

public string MyProperty
{
    set
    {
        if (_myProperty != value)
        {
            _myProperty = value;
            NotifyPropertyChanged("MyProperty");
            MyOtherProperty = "SomeValue";
        }
    }
}

public string MyOtherProperty
{
    set
    {
        if (_myOtherProperty != value)
        {
            _myOtherProperty = value;
            NotifyPropertyChanged("MyOtherProperty");
        }
    }
}

My test for the event fails - the event that it captures is the event for the MyOtherProperty.

I'm pretty sure the event fires, my UI reacts like it does, but my delegate only captures the last event to fire.

So I'm wondering:
1. Is my method of testing events correct?
2. Is my method of raising chained events correct?

manpreet 2 years ago

[TestMethod]
public void Test_ThatMyEventIsRaised()
{
    List receivedEvents = new List();
    MyClass myClass = new MyClass();

    myClass.PropertyChanged += delegate(object sender, PropertyChangedEventArgs e)
    {
        receivedEvents.Add(e.PropertyName);
    };

    myClass.MyProperty = "testing";
    Assert.AreEqual(2, receivedEvents.Count);
    Assert.AreEqual("MyProperty", receivedEvents[0]);
    Assert.AreEqual("MyOtherProperty", receivedEvents[1]);
}