Question

I have this interface

Interface IProDataSource

    Delegate Sub DstartingHandler(ByVal sender As Object, ByVal e As EventArgs)
    Event starting_Sinc As DstartingHandler

End Interface

Trying to use the interface like this

Public Class DataSource : Implements IProDataSource

    Public Event starting_Sinc As DstartingHandler Implements IProDataSource.starting_Sinc
    Public Delegate Sub DstartingHandler(ByVal sender As Object, ByVal e As EventArgs)

End Class

Gives me the next error

Event ‘starting_Sinc’ cannot implement
event ‘starting_Sinc’ on interface
‘IProDataSource’ because their
delegate types ‘DstartingHandler’ and
‘IProDataSource.DstartingHandler’ do
not match.

Answer 1

You’ll need to move the delegate declaration outside of the interface and declare it public. All types used by an interface must be public when the class that implements them is public. Necessarily so or the client code could never assign the event. Thus:

Public Delegate Sub DstartingHandler(ByVal sender As Object, ByVal e As EventArgs)

Interface IProDataSource
    Event starting_Sinc As DstartingHandler
End Interface

Public Class DataSource : Implements IProDataSource
    Public Event starting_Sinc(ByVal sender As Object, ByVal e As System.EventArgs) Implements IProDataSource.starting_Sinc
End Class

If you limit the accessibility of the class you can use your original approach:

Interface IProDataSource
    Delegate Sub DstartingHandler(ByVal sender As Object, ByVal e As EventArgs)
    Event starting_Sinc As DstartingHandler
End Interface

Friend Class DataSource : Implements IProDataSource
    Public Event starting_Sinc(ByVal sender As Object, ByVal e As System.EventArgs) Implements IProDataSource.starting_Sinc
End Class