`IEnumerable<T, TEnumerator>` for more precise type propagation in C♯ generics.

Program.cs

/* See https://geelaw.blog/entries/csharp-generics-duck-typing/ for the context.

Copyright (c) 2020 by Gee Law
Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the “Software”),
to deal in the Software without restriction, including without limitation
the rights to use, copy, modify, merge, publish, distribute, sublicense,
and/or sell copies of the Software, and to permit persons to whom
the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included
in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED “AS IS”, WITHOUT WARRANTY OF ANY KIND, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.

*/

using System;
using System.Collections;
using System.Collections.Generic;

/* Expected output:

------------------------------------------------------------
UseIEnumerable
IEnumerable.GetEnumerator()
IEnumerator.MoveNext
IEnumerator.Current
------------------------------------------------------------
UseIEnumerableT
IEnumerable<int>.GetEnumerator()
IEnumerator.MoveNext
IEnumerator<int>.Current
------------------------------------------------------------
UseIEnumerableTTEnumerator
IEnumerable<int, MyEnumerator>.GetEnumerator()
IEnumerator.MoveNext
IEnumerator<int>.Current
------------------------------------------------------------
UsePreciseType
MyEnumerable.GetEnumerator()
MyEnumerator.MoveNext
MyEnumerator.Current

*/

namespace Example
{
  class Program
  {
    interface IEnumerable<out T, out TEnumerator> : IEnumerable<T>
      where TEnumerator : IEnumerator<T>
    {
      new TEnumerator GetEnumerator();
    }

    // Enumerates 1 to 9.
    class MyEnumerator : IEnumerator<int>
    {
      private int count, current;

      public MyEnumerator() { Reset(); }

      // IDisposable is not our focus.
      public void Dispose() { }

      // This method is not used in expanded foreach.
      // Generally, it should not be used.
      public void Reset() { count = 0; }

      public int Current
      {
        get
        {
          if (current == 1)
          {
            Console.WriteLine("MyEnumerator.Current");
          }
          return count;
        }
      }

      int IEnumerator<int>.Current
      {
        get
        {
          if (current == 1)
          {
            Console.WriteLine("IEnumerator<int>.Current");
          }
          return current;
        }
      }

      object IEnumerator.Current
      {
        get
        {
          if (current == 1)
          {
            Console.WriteLine("IEnumerator.Current");
          }
          return current;
        }
      }

      private bool MoveNextImpl()
      {
        if (count == 10) { return false; }
        current = ++count;
        return true;
      }

      public bool MoveNext()
      {
        if (count == 0)
        {
          Console.WriteLine("MyEnumerator.MoveNext");
        }
        return MoveNextImpl();
      }

      // IEnumerator<T> does not have a separate MoveNext method.
      bool IEnumerator.MoveNext()
      {
        if (count == 0)
        {
          Console.WriteLine("IEnumerator.MoveNext");
        }
        return MoveNextImpl();
      }
    }

    class MyEnumerable : IEnumerable<int, MyEnumerator>
    {
      public MyEnumerable() { }

      public MyEnumerator GetEnumerator()
      {
        Console.WriteLine("MyEnumerable.GetEnumerator()");
        return new MyEnumerator();
      }
      MyEnumerator IEnumerable<int, MyEnumerator>.GetEnumerator()
      {
        Console.WriteLine("IEnumerable<int, MyEnumerator>.GetEnumerator()");
        return new MyEnumerator();
      }
      IEnumerator<int> IEnumerable<int>.GetEnumerator()
      {
        Console.WriteLine("IEnumerable<int>.GetEnumerator()");
        return new MyEnumerator();
      }
      IEnumerator IEnumerable.GetEnumerator()
      {
        Console.WriteLine("IEnumerable.GetEnumerator()");
        return new MyEnumerator();
      }
    }

    static void UseIEnumerable(IEnumerable enumerable)
    {
      Console.WriteLine(new string('-', 60));
      Console.WriteLine("UseIEnumerable");
      foreach (var item in enumerable) { }
    }

    static void UseIEnumerableT<T>(IEnumerable<T> enumerable)
    {
      Console.WriteLine(new string('-', 60));
      Console.WriteLine("UseIEnumerableT");
      foreach (var item in enumerable) { }
    }

    // Less useful.
    static void UseTEnumerableT<TEnumerable, T>(TEnumerable enumerable)
      where TEnumerable : IEnumerable<T>
    {
      Console.WriteLine(new string('-', 60));
      Console.WriteLine("UseTEnumerableT");
      foreach (var item in enumerable) { }
    }

    static void UseIEnumerableTTEnumerator<T, TEnumerator>
      (IEnumerable<T, TEnumerator> enumerable)
      where TEnumerator : IEnumerator<T>
    {
      Console.WriteLine(new string('-', 60));
      Console.WriteLine("UseIEnumerableTTEnumerator");
      foreach (var item in enumerable) { }
    }

    // Less useful.
    static void UseTEnumerableTTEnumerator<TEnumerable, T, TEnumerator>
      (TEnumerable enumerable)
      where TEnumerable : IEnumerable<T, TEnumerator>
      where TEnumerator : IEnumerator<T>
    {
      Console.WriteLine(new string('-', 60));
      Console.WriteLine("UseTEnumerableTTEnumerator");
      foreach (var item in enumerable) { }
    }

    static void UsePreciseType(MyEnumerable enumerable)
    {
      Console.WriteLine(new string('-', 60));
      Console.WriteLine("UsePreciseType");
      foreach (var item in enumerable) { }
    }

    static void Main()
    {
      var enumerable = new MyEnumerable();
      UseIEnumerable(enumerable);
      UseIEnumerableT(enumerable);
      // UseTEnumerableT(enumerable);
      UseIEnumerableTTEnumerator(enumerable);
      // UseTEnumerableTTEnumerator(enumerable);
      UsePreciseType(enumerable);
    }
  }
}