C# (almost) has implicit interfaces

Background: I move between Go and C# a lot, and like to compare them

Go has implicit interfaces, which means that a type implements an interface iff it satisfies that interface. It’s not declared. It’s simply something that the type system determines to be true or false, as needed.

I though it was clever, and I was a bit enamored when I first heard it (having come from a C# background). Now, I think implicit interfaces are simply, obviously right. An interface is a characteristic, not an assertion.

There is an idiom in Go for making such declarations explcit, if only to signal intent and avoid regressions.

C#, on the other hand, has explicit interfaces, by design. It’s a fine choice, but I have found myself wishing to classify types that have a shared method signature, but do not share an interface.

For example, C#’s Stream and StreamReader have identical Read methods – but no Reader interface declared. I have a type that I wish to accept Stream or StreamReader. What to do?

Well, I realized I don’t care about the types, per se, I care about their Read methods.

Turns out, if one wants “implicit interfaces” in C#, one can use generic delegates. Here’s one called Read<T>.

Stream.Read<byte> matches that delegate, as does StreamReader.Read<char>. So my type can accept both.

A delegate is just a function signature, and a method “implicitly implements” the delegate or doesn’t, regardless of whether one declares it.