C# Language Tutorial => Lazy Evaluation

Example

Only when the foreach statement moves to the next item does the iterator block evaluate up to the next yield statement.

Consider the following example:

private IEnumerable<int> Integers()
{
    var i = 0;
    while(true)
    {
        Console.WriteLine("Inside iterator: " + i);
        yield return i;
        i++;
    }
}

private void PrintNumbers()
{
    var numbers = Integers().Take(3);
    Console.WriteLine("Starting iteration");

    foreach(var number in numbers)
    {
        Console.WriteLine("Inside foreach: " + number);
    }
}

This will output:

Starting iteration
Inside iterator: 0
Inside foreach: 0
Inside iterator: 1
Inside foreach: 1
Inside iterator: 2
Inside foreach: 2

View Demo

As a consequence:

"Starting iteration" is printed first even though the iterator method was called before the line printing it because the line Integers().Take(3); does not actually starts iteration (no call to IEnumerator.MoveNext() was made)
The lines printing to console alternate between the one inside the iterator method and the one inside the foreach, rather than all the ones inside the iterator method evaluating first
This program terminates due to the .Take() method, even though the iterator method has a while true which it never breaks out of.

PDF - Download C# Language for free

Previous Next

C# Language

Fastest Entity Framework Extensions

Example

Got any C# Language Question?

C# Language

C# Language Yield Keyword Lazy Evaluation

Fastest Entity Framework Extensions

Example

Got any C# Language Question?