Question

• I have an IEnumerable<T> when I iterate through it and add it’s element to a list it becomes empty?

• Is there generally anything wrong about what I expect from the code?

  public class Apple
  {
      private ICollection<Fruit> _fruits = new List<Fruit>();
  
      public void AddFruits(IEnumerable<Fruit> fruits)
      {
          if (fruits == null) throw new ArgumentNullException("fruits");
          foreach (var fruit in fruits)
          {
              _fruits.Add(fruit);
          }
      }
  }
  

  The caller code:

  public void AddFruits(IEnumerable<Fruit> fruitsToAdd)
  {
      foreach (var apple in apples)
      {
          // Here fruitsToAdd has elements, fruitsToAdd.ToList() has two fruits.
          apple.AddFruits(fruitsToAdd);
          // Here fruitsToAdd has NO element!!, fruitsToAdd.ToList() is empty!
          // next iteration will not add any fruit to next apple since fruitsToAdd is empty.
      }
  }
  

Update

The ToList() solved the problem. The root of the problem was that the caller to AddFruits(IEnumerable fruitsToAdd) send fruitsToAdd that was like.

fruitsToAdd = obj.Fruits.Except(apples.Fruits);

Each time IEnumerable fruitsToAdd was Rest it run above statement. Which at next iteration run Except and thereby returned no fruits.

The right way is fruitsToAdd = obj.Fruits.Except(apples.Fruits).ToList(); Since we want one evaluation.

Answer 1

Ok, try this:

public void AddFruits(IEnumerable<Fruit> fruitsToAdd)
{
    var fruitsToAddCopy = fruitsToAdd.ToList();  // add just this line

    foreach (var apple in apples)
    {           
        apple.AddFruits(fruitsToAddCopy);    // and change this        
    }
}

Without knowing the origin of your fruitsToAdd it’s impossible to say more. Some IEnumerable<> can’t be re-used. Others can.