How to declare lambda event handlers in VB.Net?
Long story short, you cannot do that in VB for the time being (it is on the list of features considered for next release). You have to use a declared method and the AddressOf operator.
The VB team did not have the time to include anonymous delegates in the language (which is what you are trying to use, technically not a lambda expression).
Lambda expressions they had to implement so that Linq can actually work. Anonymous delegates are not required by anything (but would be quite useful). I guess they spent more time on wrapping up things like Linq To XML and XML litterals and integrating more query operators in the syntax...
For those finding this question now: since Visual Basic 2010 (VB 10.0), anonymous
Subs do work, so you can write something like:
Sub() eventRaised = True
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