when implementing two interfaces

ratheesh_param · November 7th, 2006, 06:19 AM

hi all

Can anyone tell me .. when we implement a class with two interfaces which have same methods.. which reference the class will take?

public interface Interface
function fun() as string
end function
end interface

public interface Interface1
function fun() as string
end function
end interface

public class : implements interface,interface1

public function classInter()As String Implements interface1.fun,interface.fun
/* implementation */
end function
end class

This is the case.... here the ClassInter implements the fun() from which interface?
regards
Ratheesh

Bob Bedell · November 8th, 2006, 11:40 PM

In both VB.NET and C# you need to implement two functions/methods in the type implementing the interfaces with the duplicate method name, one funtion/method for each version of the duplicate method:

The C# compiler supports "interface-qualified names" for methods, but the VB.NET compiler doesn't. So the VB.NET version would need to look something like this:

Code:

Public Interface InterfaceA
   Function ReturnAString() As String
End Interface

Public Interface InterfaceB
   Function ReturnAString() As String
End Interface

Class Class1
   Implements InterfaceA
   Implements InterfaceB

   Public Function InterfaceAReturnAString() As String _
         Implements InterfaceA.ReturnAString
      Return "InterfaceA implementation"
   End Function

   Public Function InterfaceBReturnAString() As String _
         Implements InterfaceB.ReturnAString
      Return "InterfaceB implementation"
   End Function
End Class

Module Module1

   Sub Main()
      Dim TheClass As Class1 = New Class1
      Dim TheString As String
      TheString = TheClass.InterfaceAReturnAString()
      TheString = TheClass.InterfaceBReturnAString()
   End Sub

End Module

In C# you would use interface-qualified method names in the type implementing the interfaces, then cast the type to the interface whose version of the method you want to execute. So the C# version would need to look something like this:

Code:

using System;

public interface InterfaceA {
    string ReturnAString();
}

public interface InterfaceB {
    string ReturnAString();
}    

class Class1 : InterfaceA, InterfaceB      {

    // Notice no access modifier. Method considered private.
    // Can't be called by a variable referencing the type
    // because the interface method is implemented using
    // an "Ã¯nterface-qualified name".
    string InterfaceA.ReturnAString() {
        return "InterfaceA implementation";
    }

    string InterfaceB.ReturnAString() {
        return "InterfaceB implementation";
    }

    static void Main() {
        Class1 TheClass = new Class1();

        string TheString;
 
        // Return InterfaceA's implementation. Cast the reference
        // to TheClass to an InterfaceA, and only the method defined
        // by interfaceA is callable.
        TheString = ((InterfaceA)TheClass).ReturnAString();

        // Return InterfaceB's implementation.
        TheString = ((InterfaceB)TheClass).ReturnAString();        
    }
}



HTH,

Bob

Bob Bedell · November 9th, 2006, 12:05 AM

In VB.NET you might want to use method names like:

Public Function InterfaceA_ReturnAString() As String _
Implements InterfaceA.ReturnAString
Return "InterfaceA implementation"
End Function

The underscore character helps clarify the type.member relationship in Class View, Object Browser, Intellisense, etc. by mimicing an interface-qualified name.

Bob

woodyz · November 9th, 2006, 11:48 AM

Quote:

quote:Originally posted by ratheesh_param
hi all

Can anyone tell me .. when we implement a class with two interfaces which have same methods.. which reference the class will take?

public interface Interface
function fun() as string
end function
end interface

public interface Interface1
function fun() as string
end function
end interface

public class : implements interface,interface1

public function classInter()As String Implements interface1.fun,interface.fun
/* implementation */
end function
end class

This is the case.... here the ClassInter implements the fun() from which interface?
regards
Ratheesh

Your classInter method is implementing both methods. If you were to call this from an object that is declared as Interface1 or Interface2 you would get the same results. If you need different results you can follow the code that Bob Bedell provided.

However, there is one aspect to his code that should be clarified. In VB.NET you are still able to call either method using its interface name (just like C#) as long as the instance is cast or declared as the type of the interface you are wanting to use, which of course is one of the main typical uses of implementing interfaces.

Dim a As New Class1
Debug.WriteLine(CType(a, InterfaceA).ReturnAString)
Debug.WriteLine(CType(a, InterfaceB).ReturnAString)

Woody Z http://www.learntoprogramnow.com

Bob Bedell · November 9th, 2006, 10:58 PM

Thanks Woody Z. And to carry your point a little further, I've declared InterfaceAReturnAString and InterfaceBReturnAString with Public scope, which allows calling them through either a variable referencing the type, or a variable referencing the type cast to the interface.

Using (CType(a, InterfaceA).ReturnAString) allows the two methods to be declared with Private scope. The remaining difference then between the VB.NET version and the C# version is that C#'s interface-qualified method names automatically enforce private scope on the method declarations.

Bob

dparsons · November 10th, 2006, 01:52 PM

It would also be good to point out that you can not make Interface members static

Allowed:
string ReturnAString();

Disallowed:
static string ReturnAString();

And, if memory serves me, they must be declared with AT LEAST protected scope.

-------------------------
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Unless, of course, you want to hire me to do work for you.

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