Understanding LINQ to Objects (8) The Design Of IEnumerable<T>

[LINQ via C# series

Currently in .NET, iterator pattern is implemented via IEnumerable<T> and IEnumerator<T> (or IEnumerable and IEnumerator):

namespace System.Collections
{
    // Represents a collection which can be iterated.
    public interface IEnumerable
    {
        IEnumerator GetEnumerator();
    }

    // Represents an iterator which is used to iterate that collection.
    public interface IEnumerator
    {
        object Current { get; }

        bool MoveNext();

        void Reset();
    }
}

namespace System.Collections.Generic
{
    // T is covariant.
    public interface IEnumerable<out T> : IEnumerable
    {
        IEnumerator<T> GetEnumerator();
    }

    // T is covariant.
    public interface IEnumerator<out T> : IDisposable, IEnumerator
    {
        T Current { get; }
    }
}

The meaning of out keyword is explained in another post Understanding C# Covariance And Contravariance (2) Interfaces.

For years I have different ideas on the design:

  • The first problem is, why they are called enumerable and enumerator? Iteratable and iterator sounds natural enough;
  • The second problem is, why IEnumerable IEnumerable<T> have Current properties? According to Framework Design Guidelines, they should be designed as methods, because they returns different values for each invocation (similar with Guid.NewGuid()).

In my opinion, the following design should be more perfect:

namespace System.Collections
{
    public interface IIteratable
    {
        IIterator GetEnumerator();
    }

    public interface IIterator
    {
        object GetCurrent();

        bool MoveNext();

        void Reset();
    }
}

namespace System.Collections.Generic
{
    public interface IIteratable<out T> : IIteratable
    {
        IIterator<T> GetEnumerator();
    }

    public interface IIterator<out T> : IDisposable, IIterator
    {
        T GetCurrent();
    }
}

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