The yield keyword in C# is used to create iterators. An iterator is a method that can be used to iterate over a collection of data. The yield keyword allows you to create iterators that can be used to iterate over collections that are not fully loaded into memory.

For example, the following code shows how to create an iterator that can be used to iterate over a list of numbers:

C#

public IEnumerable<int> GetNumbers()
{
    int i = 0;

    while (true)
    {
        yield return i;

        i++;
    }
}

This code creates a method called GetNumbers() that returns an iterator. The GetNumbers() method uses the yield keyword to return the next number in the list. The yield keyword also allows the GetNumbers() method to continue iterating even if the list of numbers is not fully loaded into memory.

The yield keyword can be used to create iterators for a variety of different collections, including lists, arrays, and sets. The yield keyword can also be used to create iterators for custom collections.