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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
Richard MacCutchan at 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 at 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...
Sergey Alexandrovich Kryukov at 4-Jul-12 13:27pm
Vote 5 for the question -- pretty funny one.

1 solution

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Solution 1

You need explicit interface implementation. Here is the syntax:
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:
class MyClass {
    public int this[int index] { /* ... */ } 
    public double this[string index] { /* ... */ } //second one will not compile 
The solution is:
class MyClass {
    public int this[int index] { /* ... */ } 
    double MyInterface.this[string index] { /* ... */ } //should not have access modifiers 
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.
To answer the original question, you should have tried to work with your class through both interfaces:
Itest2 first = new test();
Itest1 second = new test();
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.
Sandeep Mewara at 4-Jul-12 14:54pm
5! for full detailed explanation.
Sergey Alexandrovich Kryukov at 4-Jul-12 15:22pm
Thank you, Sandeep.
Amol_Joshi at 5-Jul-12 0:28am
great explanation!!!
Sergey Alexandrovich Kryukov at 5-Jul-12 0:38am
Thank you, Amol.

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