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Return IList <IList <T>>

I have a method that builds lists of lists. I would like the return type to use the generic IList <> interface to reduce communication with a specific List <> downstream type. However, the compiler struggles with type conversion.

public IList<IList<T>> Foo<T>()
    {
        return new List<List<T>>();
    }

Why it doesn't work when it works:

public IList<T> Foo<T>()
    {
        return new List<T>();
    }

What is the most elegant way out of this mess?

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2 answers

Just do the following:

return new List<IList<T>>();

Since it List<T>implements IList<T>, you can add any type to the result IList<T>. For instance:

Foo<int>().Add(new List<int>());

, , / ( ). , , IList<IList<T>>, - IList<T> :

Foo<int>().Add(new []{1}); // arrays implement `IList`

, List<List<T>>, : T[] List<T>. , List<T> IList<T>, a List<List<T>> IList<IList<T>>.

. .NET 4 , .

, , , , - , , IEnumerable<IList<T>>, a List<List<T>>. ? IEnumerable<T> :

public interface IEnumerable<out T>

"out" , , , T (, GetEnumerator), , -, T, (, Add).

+12

# . .

T, T, T. IEnumerable T.

+1

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