object oriented programming

Question

5.00/5 (1 vote)

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see below c# code

public interface Itest1
{
     int Add();
}

public interface Itest2
{
    int Add();
}

public class test : Itest1,Itest2
{
    public int Add()
    {
        return 10 ;
    }
}

which Add method is implemented in test class

is it from Itest1 or Itest2 or both

many thanks...

Posted 4-Jul-12 7:07am

Rakesh S S

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Richard MacCutchan 4-Jul-12 13:18pm

Neither, it's the one in class test. Before asking this question you could easily have tested it for yourself.

Sergey Alexandrovich Kryukov 4-Jul-12 15:55pm

That is true, but I nevertheless answered and even up-voted the question.
I am not quite sure if OP deserves my vote, but I never saw such pathological case (and, frankly, could not even answer this question immediately); and trying to test pathological cases is an important skill; if you understand what do I mean...
--SA

Sergey Alexandrovich Kryukov 4-Jul-12 13:27pm

Vote 5 for the question -- pretty funny one.
--SA

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Sergey Alexandrovich Kryukov · Accepted Answer · 2012-07-04T07:22:00

You need explicit interface implementation. Here is the syntax:

C#

interface IFirstTest {int Add(); }

interface ISecondTest {int Add(); }

class DoubleTest : IFirstTest, ISecondTest {
    int IFirstTest.Add() { return ++value; } //pay attention: no access modifiers!
    int ISecondTest.Add() { value += 2; return value; }
    int value;
}

Even though your problem hardly can have practical sense, it illustrates the syntax which has benefits over implicit implementations and can solve real problems.

One of such problems is: how two write two indexed properties with the same number of indices of different types in one class? This will not compile:

C#

class MyClass {
    public int this[int index] { /* ... */ } 
    public double this[string index] { /* ... */ } //second one will not compile 
}

The solution is:

C#

class MyClass {
    public int this[int index] { /* ... */ } 
    double MyInterface.this[string index] { /* ... */ } //should not have access modifiers 
}

//where
interface MyInterface {
    double this[string index] { /* ... */ }
    // in the comment above, it should be "get;" or "get; set;" or "set;"
}

In general case, I would rather advise explicit implementation (even if there are no problems illustrated above); it helps to promote using classes purely through interfaces.

[EDIT]

To answer the original question, you should have tried to work with your class through both interfaces:

C#

Itest2 first = new test();
Itest1 second = new test();
first.Add();
second.Add();

You would see that in both cases, the same function is called; so, in this sense, you have implemented both interfaces at once. The case of implementing of two identical unrelated interfaces is pathological; I don't have it ever makes any practical sense, but you can see it's not prohibited by the compiler.

—SA