C# Interfaces. Implicit implementation versus Explicit implementation

Implicit is when you define your interface via a member on your class. Explicit is when you define methods within your class on the interface. I know that sounds confusing but here is what I mean: IList.CopyTo would be implicitly implemented as:

public void CopyTo(Array array, int index)
{
    throw new NotImplementedException();
}

and explicitly as:

void ICollection.CopyTo(Array array, int index)
{
    throw new NotImplementedException();
}

The difference is that implicit implementation allows you to access the interface through the class you created by casting the interface as that class and as the interface itself. Explicit implementation allows you to access the interface only by casting it as the interface itself.

MyClass myClass = new MyClass(); // Declared as concrete class
myclass.CopyTo //invalid with explicit
((IList)myClass).CopyTo //valid with explicit.

I use explicit primarily to keep the implementation clean, or when I need two implementations. Regardless, I rarely use it.

I am sure there are more reasons to use/not use explicit that others will post.

See the next post in this thread for excellent reasoning behind each.

Implicit definition would be to just add the methods / properties, etc. demanded by the interface directly to the class as public methods.

Explicit definition forces the members to be exposed only when you are working with the interface directly, and not the underlying implementation. This is preferred in most cases.

1. By working directly with the interface, you are not acknowledging, and coupling your code to the underlying implementation.
2. In the event that you already have, say, a public property Name in your code and you want to implement an interface that also has a Name property, doing it explicitly will keep the two separate. Even if they were doing the same thing I'd still delegate the explicit call to the Name property. You never know, you may want to change how Name works for the normal class and how Name, the interface property works later on.
3. If you implement an interface implicitly then your class now exposes new behaviours that might only be relevant to a client of the interface and it means you aren't keeping your classes succinct enough (my opinion).

In addition to excellent answers already provided, there are some cases where explicit implementation is REQUIRED for the compiler to be able to figure out what is required. Take a look at IEnumerable<T> as a prime example that will likely come up fairly often.

Here's an example:

public abstract class StringList : IEnumerable<string>
{
    private string[] _list = new string[] {"foo", "bar", "baz"};

    // ...

    #region IEnumerable<string> Members
    public IEnumerator<string> GetEnumerator()
    {
        foreach (string s in _list)
        { yield return s; }
    }
    #endregion

    #region IEnumerable Members
    IEnumerator IEnumerable.GetEnumerator()
    {
        return this.GetEnumerator();
    }
    #endregion
}

Here, IEnumerable<string> implements IEnumerable, hence we need to too. But hang on, both the generic and the normal version both implement functions with the same method signature (C# ignores return type for this). This is completely legal and fine. How does the compiler resolve which to use? It forces you to only have, at most, one implicit definition, then it can resolve whatever it needs to.

ie.

StringList sl = new StringList();

// uses the implicit definition.
IEnumerator<string> enumerableString = sl.GetEnumerator();
// same as above, only a little more explicit.
IEnumerator<string> enumerableString2 = ((IEnumerable<string>)sl).GetEnumerator();
// returns the same as above, but via the explicit definition
IEnumerator enumerableStuff = ((IEnumerable)sl).GetEnumerator();

PS: The little piece of indirection in the explicit definition for IEnumerable works because inside the function the compiler knows that the actual type of the variable is a StringList, and that's how it resolves the function call. Nifty little fact for implementing some of the layers of abstraction some of the .NET core interfaces seem to have accumulated.

To quote Jeffrey Richter from CLR via C#
(EIMI means Explicit Interface Method Implementation)

It is critically important for you to understand some ramifications that exist when using EIMIs. And because of these ramifications, you should try to avoid EIMIs as much as possible. Fortunately, generic interfaces help you avoid EIMIs quite a bit. But there may still be times when you will need to use them (such as implementing two interface methods with the same name and signature). Here are the big problems with EIMIs:

• There is no documentation explaining how a type specifically implements an EIMI method, and there is no Microsoft Visual Studio IntelliSense support.
• Value type instances are boxed when cast to an interface.
• An EIMI cannot be called by a derived type.

If you use an interface reference ANY virtual chain can be explicitly replaced with EIMI on any derived class and when an object of such type is cast to the interface, your virtual chain is ignored and the explicit implementation is called. That's anything but polymorphism.

EIMIs can also be used to hide non-strongly typed interface members from basic Framework Interfaces' implementations such as IEnumerable<T> so your class doesn't expose a non strongly typed method directly, but is syntactical correct.

In addition to the other reasons already stated, this is the situation in which a class is implementing two different interfaces that have a property/method with the same name and signature.

/// <summary>
/// This is a Book
/// </summary>
interface IBook
{
    string Title { get; }
    string ISBN { get; }
}

/// <summary>
/// This is a Person
/// </summary>
interface IPerson
{
    string Title { get; }
    string Forename { get; }
    string Surname { get; }
}

/// <summary>
/// This is some freaky book-person.
/// </summary>
class Class1 : IBook, IPerson
{
    /// <summary>
    /// This method is shared by both Book and Person
    /// </summary>
    public string Title
    {
        get
        {
            string personTitle = "Mr";
            string bookTitle = "The Hitchhikers Guide to the Galaxy";

            // What do we do here?
            return null;
        }
    }

    #region IPerson Members

    public string Forename
    {
        get { return "Lee"; }
    }

    public string Surname
    {
        get { return "Oades"; }
    }

    #endregion

    #region IBook Members

    public string ISBN
    {
        get { return "1-904048-46-3"; }
    }

    #endregion
}

This code compiles and runs OK, but the Title property is shared.

Clearly, we'd want the value of Title returned to depend on whether we were treating Class1 as a Book or a Person. This is when we can use the explicit interface.

string IBook.Title
{
    get
    {
        return "The Hitchhikers Guide to the Galaxy";
    }
}

string IPerson.Title
{
    get
    {
        return "Mr";
    }
}

public string Title
{
    get { return "Still shared"; }
}

Notice that the explicit interface definitions are inferred to be Public - and hence you can't declare them to be public (or otherwise) explicitly.

Note also that you can still have a "shared" version (as shown above), but whilst this is possible, the existence of such a property is questionable. Perhaps it could be used as a default implementation of Title - so that existing code would not have to be modified to cast Class1 to IBook or IPerson.

If you do not define the "shared" (implicit) Title, consumers of Class1 must explicitly cast instances of Class1 to IBook or IPerson first - otherwise the code will not compile.

If you implement explicitly, you will only be able to reference the interface members through a reference that is of the type of the interface. A reference that is the type of the implementing class will not expose those interface members.

If your implementing class is not public, except for the method used to create the class (which could be a factory or IoC container), and except for the interface methods (of course), then I don't see any advantage to explicitly implementing interfaces.

Otherwise, explicitly implementing interfaces makes sure that references to your concrete implementing class are not used, allowing you to change that implementation at a later time. "Makes sure", I suppose, is the "advantage". A well-factored implementation can accomplish this without explicit implementation.

The disadvantage, in my opinion, is that you will find yourself casting types to/from the interface in the implementation code that does have access to non-public members.

Like many things, the advantage is the disadvantage (and vice-versa). Explicitly implementing interfaces will ensure that your concrete class implementation code is not exposed.

The previous answers explain why implementing an interface explicitly in C# may be preferrable (for mostly formal reasons). However, there is one situation where explicit implementation is mandatory: In order to avoid leaking the encapsulation when the interface is non-public, but the implementing class is public.

// Given:
internal interface I { void M(); }

// Then explicit implementation correctly observes encapsulation of I:
// Both ((I)CExplicit).M and CExplicit.M are accessible only internally.
public class CExplicit: I { void I.M() { } }

// However, implicit implementation breaks encapsulation of I, because
// ((I)CImplicit).M is only accessible internally, while CImplicit.M is accessible publicly.
public class CImplicit: I { public void M() { } }

The above leakage is unavoidable because, according to the C# specification, "All interface members implicitly have public access." As a consequence, implicit implementations must also give public access, even if the interface itself is e.g. internal.

Implicit interface implementation in C# is a great convenience. In practice, many programmers use it all the time/everywhere without further consideration. This leads to messy type surfaces at best and leaked encapsulation at worst. Other languages, such as F#, don't even allow it.

I've found myself using explicit implementations more often recently, for the following practical reasons:

• Always using explicit from the starts prevents having any naming collisions, in which explicit implementation would be required anyways

• Consumers are "forced" to use the interface (which they should/must do anyways when you're using DI)

• No "zombie" members in the implementations - removing any member from the interface declaration will result in compiler errors if not removed from the implementation too

• Default values for optional parameters, as well constraints on generic arguments are automatically adopted - no need to write them twice and keep them in sync