C# Interfaces. Implicit implementation versus Explicit implementation

c# .net interface
Seb Nilsson picture Seb Nilsson · Sep 27, 2008 · Viewed 152.8k times · Source

What are the differences in implementing interfaces implicitly and explicitly in C#?

When should you use implicit and when should you use explicit?

Are there any pros and/or cons to one or the other?

Microsoft's official guidelines (from first edition Framework Design Guidelines) states that using explicit implementations are not recommended, since it gives the code unexpected behaviour.

I think this guideline is very valid in a pre-IoC-time, when you don't pass things around as interfaces.

Could anyone touch on that aspect as well?

Answer

mattlant picture mattlant · Sep 27, 2008

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.

Related questions

How can I implement static methods on an interface?

I have a 3rd party C++ DLL that I call from C#. The methods are static. I want to abstract it out to do some unit testing so I created an interface with the static methods in it but now …

Read More
Interfaces — What's the point?

The reason for interfaces truly eludes me. From what I understand, it is kind of a work around for the non-existent multi-inheritance which doesn't exist in C# (or so I was told). All I see is, you predefine some members …

Read More
Why are C# 4 optional parameters defined on interface not enforced on implementing class?

I noticed that with the optional parameters in C# 4 if you specify an optional parameter on an interface you don,t have to make that parameter optional on any implementing class: public interface MyInterface { void TestMethod(bool flag = false); } public …

Read More