What causes this list to be passed by reference when called one way, but by value another?

Question

I was making a simple test for running a validation method and came across this strange situation.

public IEnumerable<int> ints (List<int> l)
{
 if(false)yield return 6;
 l.Add(4);
}


void Main()
{
 var a = new List<int>();
 var b = new List<int>();
 for( int i = 0; i < 4; i++ ){
  a.Add(i);
  b.Add(i);
 }
 a.AddRange(ints(a));
 ints(b);
 Console.WriteLine(a);
 Console.WriteLine(b);
}

Once this code runs, a will contain [0,1,2,3,4]. However, b will contain [0,1,2,3]. Why did calling the method as an argument in AddRange allow the list to be passed by reference? Or if that didn't happen, what did?

Answer 1

ints(b) call does not enumerate IEnumerable, so code never reaches l.Add(4) line, unlike in AddRange case which enumerates all items to add them to the list.

To see it called for b case enumerate result manually:

ints(b).ToList();

IEnumerable<T> implemented via functions are not executing body of the function before enumeration starts - the code is actually transformed by compiler into class with states to support true lazy evaluation of enumerable (details can be found in multiple articles, i.e. Iterator Pattern demystified - link provided by Tim Schmelter).