Why I use explicit interface implementation as a default implementation technique

Introduction

Implementing interfaces in C# is an everyday programmer task. Most interfaces are implemented using implicit technique without any doubt. Explicit implementation not only is unfamiliar, but is considered to be a some awkward solution to a very rare problem of implementing two interfaces with same members.

In this article I'm going to discuss, why, in my opinion, explicit interface implementation - EIMI - should be considered as the default implementation technique. And how using EIMIs keeps interfaces and their implementations in a coherent state.

Basics

For the beginning, let's recall what implicit and explicit interface implementation means.
Given an interface ILogger

interface ILogger
{
    void Trace(string format, params object[] args);
}

We usually implement it in the following way

class Logger : ILogger
{
    public void Trace(string format, params object[] args) { }
}

This is an implicit interface implementation.

The meaning of implicit here is - the compiler decides on its own that a public method Logger.Trace(...) will implement the corresponding ILogger.Trace(...) interface member.

Let's put the above explicitly

class Logger : ILogger
{
    void ILogger.Trace(string format, params object[] args) { }
}

This is an explicit interface implementation - EIMI

What MSDN tells us about the need of EIMI?

Given two interfaces with the same members

interface IControl
{
    void Paint()
}

interface ISurface
{
    void Paint()
}

We need a way to distinguish the same members if we are going to implement both of the interfaces in a single class

public class SampleClass : IControl, ISurface
{
    void IControl.Paint() { }
    void ISurface.Paint() { }
}

The Key Difference of Explicit Implementation

Explicitly implemented methods and properties are private and inaccessible even to the implementing class

interface ILogger
{
    void Trace(string format, params object[] args);
}

class Logger : ILogger
{
    void ILogger.Trace(string format, params object[] args) { }
}

void main()
{
    Logger log = new Logger();
    log.Trace("");    // Compilation error, Trace() is not accessible
}

Explicitly implemented methods can be accessed through interface only

void main()
{
    ILogger log = new Logger();
    log.Trace("");    // Accessing through interface is OK
}

The Glory of the Explicit Implementation

Refactoring

Let's implement some simple interface ICalculator.
Then, let's trace revisions of the interface as software is being developed.

interface ICalculator
{
    void Solve(int startPoint);
}

class BasicCalculator : ICalculator
{
    public void Solve(int startPoint) { }
}

Half a year later, obviously, the interface is enriched with an additional method

interface ICalculator
{
    void Solve(int startPoint);
    void Solve(int startPoint, int endPoint);
}

In this half a year other stuff was added to BasicCalculator, as well. And here is what we might get after implementing the additional Solve(int startPoint, int endPoint) method

class BasicCalculator : ICalculator
{
    public void Solve(int startPoint);
    
    // other very useful stuff
    
    public void Solve(int startPoint, int endPoint);
}

In another half a year a major refactoring takes place, and the first Solve(int startPoint) method is removed from the ICalculator interface.

interface ICalculator
{
    // calculation with start point only is not good enough, method is removed
    
    void Solve(int startPoint, int endPoint);
}

Now, I will ask a question: "Do you know many programmers there who bother to go over all implementations and check if a particular change in interface leaves ghost public functions?"

class BasicCalculator : ICalculator
{
    public void Solve(int startPoint);      // this one is left here forever
    
    // other very useful stuff
    
    public void Solve(int startPoint, int endPoint);
}

I doubt, that the answer is yes. So, the first version of Solve(int startPoint) method is left inside all implementations of ICalculator interface forever.

With explicit implementation this will not going to happen !

interface ICalculator
{
    void Solve(int startPoint, int endPoint);
}

class BasicCalculator : ICalculator
{
    void ICalculator.Solve(int startPoint);    // Compilation error!
                                               // ICalculator has no such a method!    
    
    void ICalculator.Solve(int startPoint, int endPoint);
}

EIMI forces the compiler to look all over the code and tells us that we no longer need to implement the Solve(int) method.

In my opinion this "feature" is a killer and is a "must-to-use" in any large code base with many maintainers, especially if some of them are less experienced.

Decoupling From a Specific Implementation

There are other benefits of EIMIs as well.

Consider the following code

interface ICalculator
{
    void Solve(int startPoint);
}

class BasicCalculator : ICalculator
{
    public void Solve(int startPoint);

    // unfortunately, methods like this one seem to appear by their self in any enterprise code base
    public void UseSinToSolveTheProblemIfStartPointIsZero(int startPoint);
}

void main()
{
    var calculator = new BasicCalculator();    

    // bad! dependency on a specific implementation
    calculator.Solve(123);
}

The var keyword is very convenient and commonly used. As a result, in the example above the calculator variable becomes an instance of the BasicCalculator class. This is obviously bad, since the code becomes coupled to a specific implementation of a general ICalculator interface.

Later an inexperienced maintainer, will be tempted to use the calculator.UseSinToSolveTheProblemIfStartPointIsZero() method.

With EIMI the main() above won't compile since Solve() method is not accessible within the BasicCalculator class. We are forced to use the explicit ICalculator declaration instead of just the var keyword

void main()
{
    ICalculator calculator = new BasicCalculator();    

    // good! decoupled from a specific implementation
    calculator.Solve(123);
}

Decoupling Public Interface From Implementation Details

And the last (for this article) benefit of EIMI.
Consider this code

interface ILogger
{
    string Name { set; }
}

class Logger : ILogger
{
    public string Name { get; set; }
    
    private void GenerateAutoName()
    {
        Name = "MySophisticatedLogger";
    }
}

Here a public property Name is used to implement a private implementation detail inside the GenerateAutoName() method.

Half a year later we will probably add a validation code for the Name property

class Logger : ILogger
{
    private string _name;

    public string Name
    {
        get { return _name; }
        set 
        {
            if (String.IsNullOrEmpty(value))
                throw new BadArgumentException();
                
            _name = value;
        }
    }
}

We no longer use the auto-implemented property feature of C#. But what happens inside our private GenerateAutoName() method? The validation code is "injected" into the method as well.

    private void GenerateAutoName()
    {
        Name = "MySophisticatedLogger";
    }

Do we need the validation code of external data inside internal implementation? I guess that the answer is no.

Avoiding use of own public methods in private implementation details is a good practice, IMHO.

interface ILogger
{
    string Name { set; }
}

class Logger : ILogger
{
    public string ILogger.Name { get; set; }
    
    private void GenerateAutoName()
    {
        Name = "MySophisticatedLogger";        // compilation error!
    }
}

EIMI forces us to add private field _name as soon as we are going to use it in a private implementation.

Conclusion

In my every day programming I implement interfaces. The explicit way of implementing is the default for me. I use an implicit implementation only when I can not avoid it. Each time I'm refactoring, I see the actual benefits of EIMIs.

Here is a nifty implementation of a template method design pattern using EIMI

interface ICalculator
{
    void Solve();
}

abstract class CalculatorBase : ICalculator
{
    protected abstract void CalculateSolution();
    
    void ICalculator.Solve()
    {
        CalculateSolution();
    }
}

I like the way the design is expressed here. The interface Solve() method delegates the implementation details to a protected virtual method overridden by descendants.

EIMI keeps code cleaner and more maintainable.

Farther Reading

What is EIMI?

• .NET Basics : Explicit vs Implicit Interface Implementation
• C#: Interfaces - Implicit and Explicit implementation

EIMIs are good

• C# : Why implement interface explicitly?

EIMIs are bad

• Implicit Vs. Explicit Interface Method Implementation?