How does C# disambiguate between multiple interfaces in method calls?

Question

Assume I have a class C that inherits from 2 interfaces (I1 and I2). I also have two versions of a method (DoStuff), each taking one of the interfaces as a parameter. If I call DoStuff(C), which one gets called?

interface I1 { ... }
interface I2 { ... }
class C : I1, I2 { ... }

int DoStuff(I1 target) { ... }
int DoStuff(I2 target) { ... }

//What gets called here?
C target = new C()
DoStuff(target)

If I2 derives from I1, I think it's relatively simple - the I2 version gets called. I'm interested in the case where the interfaces are independent.

Assume I can't edit C, I1 or I2. And .NET 2.0 if that makes any difference.

-1 You obviously didn't test yourself before asking this question. — user703016, Sep 08 '11 at 10:34
A question about what happens at run time needs to be about code that actually compiles! (and I'm not talking about the ellipses!) — AakashM, Sep 08 '11 at 10:34
I find it curious that you decided to post a question instead of just testing it. — Stilgar, Sep 08 '11 at 10:34

BoltClock · Accepted Answer · 2011-09-08T10:44:00.740

15

Neither gets called. If your two methods are overloads in the same class, the compiler does not attempt to disambiguate at all. It'll simply not compile your code, saying it's ambiguous between your two overloads as you declared target to be a type that implements both interfaces.

If you declare target to be of one of the interface types, or cast it at call-time (as Jon shows), then there's no ambiguity.

edited Sep 08 '11 at 10:44

answered Sep 08 '11 at 10:31

BoltClock

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+1. I assume that the call to DoStuff is something that you can edit (otherwise it won't compile), so a cast to I1 or I2 will solve your problem. – Francesco Baruchelli Sep 08 '11 at 10:40

score 6 · Answer 2 · answered Sep 08 '11 at 10:34

As BoltClock says, it won't compile. However, it's easy to make it do what you want: just use an expression of the type that you want to use for the argument. For example:

C target = new C();
DoStuff((I1) target);

or

C target = new C();
I1 i1 = target;
DoStuff(i1);

Basically without any extra work, the overload resolution steps will find both methods in the candidate set, and determine that neither is "better" than the other, so overload resolution will fail.

score 3 · Answer 3 · edited Sep 08 '11 at 10:40

3

There will be an error when you try to compile it:

error CS0121: The call is ambiguous between the following methods or properties: 'DoStuff(I1)' and 'DoStuff(I2)'"

edited Sep 08 '11 at 10:40

AakashM

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answered Sep 08 '11 at 10:35

Dima

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How does C# disambiguate between multiple interfaces in method calls?

3 Answers3