Declaring a struct to be returned from an interface method

Question

I'm writing a method in C# (2.0) that has to return a collection of simple objects. Normally I would do something like this:

class MyWidget
{
    struct LittleThing
    {
        int foo;
        DateTime bar;
    }

    public IList<LittleThing> LookupThings()
    {
        // etc.
    }
}

However, I have to declare this method in an interface. The caller doesn't get to see MyWidget, only an IWidget interface. The above setup doesn't work in that situation, because C# does not allow defining types inside an interface. What is the proper or best way to do such a declaration?

The straighforward thing I thought of is to simply declare LittleThing outside of the interface. That doesn't seem great, for a couple of reasons. One: it is only ever used by that single method in that single class, so it doesn't seem that LittleThing should be an independent type just floating around by itself. Two: if similar methods wind up being written for other classes, they will be returning different kinds of data (for good design reasons), and I don't want to clutter the namepace with a ton of similar-named structs that differ only slightly from each other.

If we could upgrade our version of .Net, I would just return a Tuple<>, but that's not going to be an option for some time yet.

[Edited to add: The small object does need to contain more than two fields, so KeyValuePair<K,V> won't quite cut it.]

[Edited to add further: IWidget is implemented by only one class, Widget. I think it weird to have an interface for only one class, but this was done to satisfy an old coding policy that required the contract to always be in a separate assembly from the implementation. Said policy has now gone away, but we haven't the resources to refactor the entire application and remove all the unnecessary interfaces.]

What's the best practice?

Answer 1

If the "LittleThing" only has two values, you can return a KeyValuePair<TKey,TValue>.

If there are more than two, you could always make your own Tuple class, and replace it with .NET 4's when you finally do move to .NET 4.

Otherwise, I would just define the struct with the interface, and include it as part of your API. Namespaces take care of the naming concern...

Answer 2

1. Why use structs? Why not use classes instead?
2. Declare the class separately, as a public class.