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Douglas Stockwell's Generics Quiz

Douglas Stockwell posed an interesting problem the other day. He wanted to know if there was a way to make a piece of code work without falling down to reflection.

Personally, I can't see any way to do it without reflection, so I thought I'd post my answers on how to do it with reflection here :)

The problem is like this.

You have to interfaces defined:

Public Interface IA
 
End Interface
 
Public Interface IB
 
End Interface

And you have a generic method that demands a generic type T such that T implements both IA and IB. There's also a situation that takes parameter of type IA.

Public Class SomeClass
    Sub AandB(Of T As {IA, IB})(ByVal anb As T)
        MsgBox("here i am")
    End Sub
 
    Sub Situation(ByVal a As IA)
        If TypeOf a Is IB Then
            'call AandB() here somehow.
        End If
    End Sub
End Class

So the question was: How do you call AandB() passing in the parameter 'a' from Situation()?

At first glance it's easy, but it's actually not. I put the code into visual studio thinking it would be a cinch, but then moment I actually tried to call AandB() it all broke :)

Using reflection however it's quite simple.

Public Class SomeClass
 
    Sub AandB(Of T As {IA, IB})(ByVal anb As T)
        MsgBox("here i am")
    End Sub
 
    Sub Situation(ByVal a As IA)
        If TypeOf a Is IB Then
            Dim paramType As System.Type = CObj(a).GetType()
            Dim classType As System.Type = GetType(SomeClass)
            Dim method As System.Reflection.MethodInfo = classType.GetMethod("AandB", _
                Reflection.BindingFlags.Public _
                Or Reflection.BindingFlags.Instance _
                Or Reflection.BindingFlags.DeclaredOnly)
            Dim genericMethod As System.Reflection.MethodInfo = method.MakeGenericMethod(paramType)
            genericMethod.Invoke(Me, New Object() {a})
        End If
    End Sub
End Class

There's obviously no error checking going on (which you really should have) but it works.

Posted: Friday, 6 October 2006 9:46 AM by Geoff Appleby

Filed under: Tricks, Geek

Comments

Darren Neimke said:

Geoff, why wouldn't you create such a class like so:

public class SomeClass<T> where T : IA, IB {

public void AandB(T anb) {

Console.WriteLine( "here i am" ) ;

}

public void Situation(IA a) {

if (a.GetType() == typeof(T)) {

T tmp = (T)a;

AandB(tmp);

}

public interface IA { string A { get; set; } }

public interface IB { string B { get; set; } }

# October 5, 2006 9:26 PM

Geoff Appleby said:

Ah, I'm glad you asked :)

The simple answer is 'because'.

The challenge here is not 'how can i change this code to make it work' but only 'how can i make it work'.

The difference is subtle - you can only insert code between the

If TypeOf a Is IB Then

and

End If

lines (or c# equivalents).

What you've done is demanded that the caller MUST have some type which implements both IA and IB - they can't instantiate and call methods on SomeClass without it. But the code I showed above does not DEMAND that the caller have a class that implements both IA and IB, it just takes advantage of the cases where they HAPPEN TO have a class that does.

To be honest, if I were to write this sort of situation in my own code, I would decide that it's time to change my API in some way so that I could remove the need for the reflection code.

Public Interface IC

Inherits IA

Inherits IB

End Interface

Sub Situation(ByVal a As IA)

If TypeOf a Is IC Then

Situation(CType(a, IC))

End If

End Sub

Sub Situation(ByVal a As IC)

AandB(Of IC)(a)

End Sub

And then pushed some of the functionality back on the caller that if they want method AandB() to be called, they need an IC.

But it really does depend on the situation (no pun intended). I'd try and make it easier, but when you have a third party API that works this way, there's not much you can do except work with what you've got available.

# October 5, 2006 10:12 PM