C#’s Yield operator allows you to define enumerable sets which can be dynamically generated and are lazily evaluated. This enables a number of fun things that would be difficult otherwise such as infinite lists. Some other frameworks have also taken advantage of yield to power different features like Unity 3D’s Coroutines.

When used in a function or property which returns an IEnumerable, yield will provide two features. You may return the next value in the set or break out of the loop - effectively terminating the set. Values returned from a yield keyword are composed into the enumerable set as your code evaluates. Yield statements are evaluated lazily (meaning the function generating the enumerable object is not generated completely before the first element is returned), this means that after returning with yield the code in the function will continue from the point it returned from including preserving the state (such as local variable values).

Using these concepts we can create a function which returns all positive integers between 1 and a max value.

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// Note: you'll need to replace the IEnumerable<int> with correct syntax as well as the >= symbol
public static IEnumerable<int> PositiveInts(int max) {
    int i = 1;
    while(true) {
        yield return i++;
        if (i >= max) {
            yield break;
        }
    }
}

• yield return: returns the next value in an IEnumerable.
• yield break: marks the end of the IEnumerable.

More information about using yield with some other examples is available here: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/yield

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