Question

I am trying to implement the Strategy design pattern using interfaces.
However, while developing some code I stumbled upon something strange.
The type of the object is not verified in design-time.

Observe the following code.
Notice that Foo implements IFoo and Bar DOES NOT implement this interface.
No error is shown when trying this:

Dim fb2 As FooBar = New FooBar(bar)

The full code:

Module Module1
    Sub Main()
        Try
            Dim foo As Foo = New Foo()
            Dim bar As Bar = New Bar()

            Dim fb1 As FooBar = New FooBar(foo)
            fb1.DoIt()
            Dim fb2 As FooBar = New FooBar(bar)
            fb2.DoIt()
        Catch ex As Exception
            Console.WriteLine(ex.Message)
        End Try
        Console.ReadLine()
    End Sub
End Module

Public Class FooBar
    Private _f As IFoo
    Public Sub New(ByVal f As IFoo)
        _f = f
    End Sub

    Public Sub DoIt()
        _f.DoSomething()
    End Sub
End Class

Public Interface IFoo
    Sub DoSomething()
End Interface

Public Class Foo
    Implements IFoo

    Public Sub DoSomething() Implements IFoo.DoSomething
        Console.WriteLine("DoSomething() called in Foo")
    End Sub
End Class

Public Class Bar
    Public Sub DoSomething()
        Console.WriteLine("DoSomething() called in Bar")
    End Sub
End Class

This code compiles fine. No error is shown in Visual Studio.
However, when I run this piece of code, I receive an InvalidCastException.
The output of the console:

DoSomething() called in Foo
Unable to cast object of type 'InterfaceTest.Bar' to type 'InterfaceTest.IFoo'.

Can anyone explain this behavior?

Answer 1

Turn on Option strict in the project properties.