C#, Object and multiple Interface implementations: how to use correctly?

Question

So I have two interfaces:

public interface ISomething
{
    public int A();
}


public interface ISomethingElse
{
    public int B();
}

And an object that implements both:

public class MyObject : ISomething, ISomethingElse
{      
}

Now I have this running code:

...
List<MyObject> objects = myObjectManager.SelectAll(); // now have say 10 MyObject

MyUtilityClass myUtilityClass = new MyUtilityClass();
MyOtherUtilityClass myOtherUtilityClass = new MyOtherUtilityClass();
myUtilityClass.MySpecialMethod(objects);                  // <- compile failure
myOtherUtilityClass.MySpecialMethod(objects);             // <- another failure
...

If I want to call A or B on all of them, how can I write code like this:

public class MyUtilityClass
{
    public void MySpecialMethod(List<ISomething> objects) // <- the problem
    {
        foreach (ISomething o in objects)
            o.A();   
    }
}

public class MyOtherUtilityClass
{
    public void MySpecialMethod(List<ISomethingElse> objects) // <- the problem
    {
        foreach (ISomethingElse o in objects)
            o.B();   
    }
}

How can I cleanly call MyUtilityClass.MySpecialMethod() on my List<MyObject> objects? Is it possible without all typecasting? The parameters of MyUtilityClass.MySpecialMethod() appear to be the issue (I want to define the parameter as a List of objects that implement ISomething).

are you getting a compile error? What is it? edit: Or is the problem that you do not want to have to do the cast in MySpecialMethod? — Jonathan Henson
– Jonathan Henson, Commented May 3, 2011 at 22:05

oxilumin · Accepted Answer · 2011-05-03 22:05:34Z

5

You can use IEnumerable<> interface instead of List<>. IEnumerable<> is covariant.

answered May 3, 2011 at 22:05

oxilumin

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1 Comment

oxilumin Over a year ago

You can read more about covariance/contravariance here

Aliostad · Accepted Answer · 2011-05-03 22:06:02Z

4

List does not support covariance.

You may change it to IEnumerable<ISomething> and pass a List<MyObject>.

answered May 3, 2011 at 22:06

Aliostad

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Comments

Femaref · Accepted Answer · 2011-05-03 22:06:29Z

4

Personally, I would use the following signature as IEnumerable<T> is covariant:

public void MySpecialMethod(this IEnumerable<ISomething> objects) // <- the problem
{
    foreach (ISomething o in objects)
        o.A();   
}

Calling it:

objects.MySpecialMethod();

answered May 3, 2011 at 22:06

Femaref

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Comments

Jonathan Henson · Accepted Answer · 2011-05-03 22:13:06Z

1

Shouldn't

public void MySpecialMethod(List<MyObject> objects)
{
    foreach (ISomethingElse o in objects)
        o.B();   
}

work?

answered May 3, 2011 at 22:13

Jonathan Henson

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1 Comment

Cymen Over a year ago

Yes but I want to accept any object that implements the interface not a concrete object type.

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