Question

I believe the following VB.Net code is the equivalent of the proceeding C# code; however the VB.Net test fails – the event handling Lambda is never called.

What is going on?

VB.Net version – fails:

<TestFixture()> _ Public Class TestClass     <Test()> _     Public Sub EventTest()         Dim eventClass As New EventClass         Dim eventRaised As Boolean = False         AddHandler eventClass.AnEvent, Function() (eventRaised = True)         eventClass.RaiseIt()         Assert.IsTrue(eventRaised)     End Sub     End Class  Public Class EventClass     Public Event AnEvent()     Public Sub RaiseIt()         RaiseEvent AnEvent()     End Sub End Class 

C# version – passes:

[TestFixture]     public class TestClass     {         [Test]         public void EventTest()         {             var eventClass = new EventClass();             var eventRaised = false;             eventClass.AnEvent += () => { eventRaised = true; };              eventClass.RaiseIt();             Assert.IsTrue(eventRaised);         }     }      public class EventClass     {         public delegate void EventHandler();         public event EventHandler AnEvent;         public void RaiseIt()         {             AnEvent();         }     } 

Answer 1

Note: This relates to older versions of VB.net Prior to Visual Studio 2010 and VB.net 10

The difference is that in VB.Net a lambda expression must return a value i.e. they must be functions not subs. The lambda expression eventRaised = true is being interpreted as a boolean expression rather than an assignment i.e. is evaluating to false rather than setting to true.

Further details on MSDN.

I’m don’t think the c# pattern for testing events used in the example can be done in VB.Net without introducing another function e.g.

<TestFixture()> _ Public Class Test     <Test()> _     Public Sub EventTest()         Dim eventClass As New EventClass         Dim eventRaised As Boolean = False         AddHandler eventClass.AnEvent, Function() (SetValueToTrue(eventRaised))         eventClass.RaiseIt()         Assert.IsTrue(eventRaised)     End Sub      Private Function SetValueToTrue(ByRef value As Boolean) As Boolean         value = True         Return True     End Function  End Class  Public Class EventClass     Public Event AnEvent()     Public Sub RaiseIt()         RaiseEvent AnEvent()     End Sub End Class