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The yield keyword signals to the compiler that the method in which it appears is an iterator block. The compiler generates a class to implement the behavior that is expressed in the iterator block. In the iterator block, the yield keyword is used together with the return keyword to provide a value to the enumerator object. This is the value that is returned, for example, in each loop of a foreach statement. The yield keyword is also used with break to signal the end of iteration.

public class Generators
{
    public static IEnumerable<int> GetPrimeNumbers()
    {
        int counter = 1;

        while (true)
        {
            if (IsPrimeNumber(counter))
            {
                yield return counter;
            }
            
            counter++;
        }
    }

    private static bool IsPrimeNumber(int value)
    {
        bool output = true;

        for (int i = 2; i < value / 2; i++)
        {
            if (value % i == 0)
            {
                output = false;
                break;
            }
        }
        
        return output;
    }
}

• Do not put yield in an unsafe block.
• Do not use ref or out keywords with the parameters of the method, operator or accessor (getter / setter).
• Yield return can only be placed in the try block if it is followed by a finally block.
• Yield break can be put in the try block and catch, but not placed in the finally block.
• Do not use yield within the anonymous method.

One advantage of using yield is to help keep the code clean and concise. On top of that, because the yield is called within a method that uses IEnumerable there is no need to create or get a list of elements to browse. This applies in cases such as searching and browsing a number of elements required that will be reduce the dependency on the location of the element to find.