The yield keyword in C# is used to define iterator methods or get accessors of custom iterators. It provides a concise and efficient way to implement custom iteration logic for collections without the need to explicitly create a separate class that implements IEnumerable or IEnumerator.

 

Creating Iterators:

public static IEnumerable<int> GetEvenNumbers(int limit){
   for (int i = 0; i <= limit; i++)
   {
       if (i % 2 == 0)
       {
           yield return i;
       }
   }
}

public static void Main(string[] args){
   foreach (int number in GetEvenNumbers(10))
   {
       Console.Write($"{number} "); // Output: 0 2 4 6 8 10
   }
   Console.WriteLine();
}

• GetEvenNumbers is an iterator method because it uses yield return.
• When the foreach loop starts, GetEvenNumbers begins execution.
• For each even number, yield return i; is executed, returning the number to the foreach loop.
• The state of the for loop is saved between iterations.
• Once the loop finishes, the iteration ends.

 

 

Creating Custom Iterators

yield can also be used within the get accessor of an indexer or a property that returns a collection type. This allows you to implement custom iteration logic directly within the property or indexer.

public class MyCollection
{
   private string[] items = { "apple", "banana", "cherry" };

   public IEnumerable<string> Fruits
   {
       get
       {
           foreach (string item in items)
           {
               yield return item.ToUpper();
           }
       }
   }
}

public static void Main(string[] args)
{
   MyCollection collection = new MyCollection();
   foreach (string fruit in collection.Fruits)
   {
       Console.WriteLine(fruit);
       // Output:
       // APPLE
       // BANANA
       // CHERRY
   }
}

 

the get accessor of the Fruits property uses yield return to iterate over the internal items array and return an uppercase version of each fruit.