c# - When is 'Yield' really needed?

What can be a real use case for C# yield?

Thanks.

Answer

When you want deferred execution.

This makes sense in most cases where the alternative is to construct a temporary collection.

Consider this scenario: I have a list of integers and I want to list their squares.

I could do this:

public static IEnumerable Squares(this IEnumerable numbers) {

    List squares = new List();
    foreach (int number in numbers)
        squares.Add(number * number);

    return squares;
}

Then I could sum the squares, take their average, find the greatest, etc.

But I really didn't need to populate a whole new List for that purpose. I could've used yield to enumerate over the initial list and return the squares one-by-one:

public static IEnumerable Squares(this IEnumerable numbers) {
    foreach (int number in numbers)
        yield return number * number;
}

The fact that this actually makes a difference might not be apparent until you start dealing with very large collections, where populating temporary collections proves to be quite wasteful.

For example suppose I wanted to find the first square above a certain threshold. I could do this:

IEnumerable numbers = GetLotsOfNumbers();
var squares = numbers.Squares();
int firstBigSquare = squares
    .Where(x => x >= 1000)
    .FirstOrDefault();

But if my Squares method populated an entire List before returning, the above code would be doing potentially far more work than necessary.