Why implement interface explicitly?

Question

If you implement two interfaces, both with the same method and different implementations, then you have to implement explicitly.

public interface IDoItFast
{
    void Go();
}
public interface IDoItSlow
{
    void Go();
}
public class JustDoIt : IDoItFast, IDoItSlow
{
    void IDoItFast.Go()
    {
    }

    void IDoItSlow.Go()
    {
    }
}

It's useful to hide the non-preferred member. For instance, if you implement both IComparable<T> and IComparable it is usually nicer to hide the IComparable overload to not give people the impression that you can compare objects of different types. Similarly, some interfaces are not CLS-compliant, like IConvertible, so if you don't explicitly implement the interface, end users of languages that require CLS compliance cannot use your object. (Which would be very disastrous if the BCL implementers did not hide the IConvertible members of the primitives :))

Another interesting note is that normally using such a construct means that struct that explicitly implement an interface can only invoke them by boxing to the interface type. You can get around this by using generic constraints::

void SomeMethod<T>(T obj) where T:IConvertible

Will not box an int when you pass one to it.

Some additional reasons to implement an interface explicitly:

backwards compatibility: In case the ICloneable interface changes, implementing method class members don't have to change their method signatures.

cleaner code: there will be a compiler error if the Clone method is removed from ICloneable, however if you implement the method implicitly you can end up with unused 'orphaned' public methods

strong typing: To illustrate supercat's story with an example, this would be my preferred sample code, implementing ICloneable explicitly allows Clone() to be strongly typed when you call it directly as a MyObject instance member:

public class MyObject : ICloneable
{
  public MyObject Clone()
  {
    // my cloning logic;  
  }

  object ICloneable.Clone()
  {
    return this.Clone();
  }
}

Another useful technique is to have a function's public implementation of a method return a value which is more specific than specified in an interface.

For example, an object can implement ICloneable, but still have its publicly-visible Clone method return its own type.

Likewise, an IAutomobileFactory might have a Manufacture method which returns an Automobile, but a FordExplorerFactory, which implements IAutomobileFactory, might have its Manufacture method return a FordExplorer (which derives from Automobile). Code which knows that it has a FordExplorerFactory could use FordExplorer-specific properties on an object returned by a FordExplorerFactory without having to typecast, while code which merely knew that it had some type of IAutomobileFactory would simply deal with its return as an Automobile.

It's also useful when you have two interfaces with the same member name and signature, but want to change the behavior of it depending how it's used. (I don't recommend writing code like this):

interface Cat
{
    string Name {get;}
}

interface Dog
{
    string Name{get;}
}

public class Animal : Cat, Dog
{
    string Cat.Name
    {
        get
        {
            return "Cat";
        }
    }

    string Dog.Name
    {
        get
        {
            return "Dog";
        }
    }
}
static void Main(string[] args)
{
    Animal animal = new Animal();
    Cat cat = animal; //Note the use of the same instance of Animal. All we are doing is picking which interface implementation we want to use.
    Dog dog = animal;
    Console.WriteLine(cat.Name); //Prints Cat
    Console.WriteLine(dog.Name); //Prints Dog
}

It can keep the public interface cleaner to explicitly implement an interface, i.e. your File class might implement IDisposable explicitly and provide a public method Close() which might make more sense to a consumer than Dispose().

F# only offers explicit interface implementation so you always have to cast to the particular interface to access its functionality, which makes for a very explicit (no pun intended) use of the interface.