Iterators (C#)

An iterator can be used to step through collections such as lists and arrays.

An iterator method or get accessor performs a custom iteration over a collection. An iterator method uses the yield return statement to return each element one at a time. When a yield return statement is reached, the current location in code is remembered. Execution is restarted from that location the next time the iterator function is called.

You consume an iterator from client code by using a foreach statement or by using a LINQ query.

In the following example, the first iteration of the foreach loop causes execution to proceed in the SomeNumbers iterator method until the first yield return statement is reached. This iteration returns a value of 3, and the current location in the iterator method is retained. On the next iteration of the loop, execution in the iterator method continues from where it left off, again stopping when it reaches a yield return statement. This iteration returns a value of 5, and the current location in the iterator method is again retained. The loop completes when the end of the iterator method is reached.

static void Main()  
{  
    foreach (int number in SomeNumbers())  
    {  
        Console.Write(number.ToString() + " ");  
    }  
    // Output: 3 5 8  
    Console.ReadKey();  
}  

public static System.Collections.IEnumerable SomeNumbers()  
{  
    yield return 3;  
    yield return 5;  
    yield return 8;  
}  

The return type of an iterator method or get accessor can be IEnumerable, IEnumerable<T>, IEnumerator, or IEnumerator<T>.

You can use a yield break statement to end the iteration.

Iterators were introduced in C# in Visual Studio 2005.

In this topic

Note

For all examples in this topic except the Simple Iterator example, include using directives for the System.Collections and System.Collections.Generic namespaces.

Simple Iterator

The following example has a single yield return statement that is inside a for loop. In Main, each iteration of the foreach statement body creates a call to the iterator function, which proceeds to the next yield return statement.

static void Main()  
{  
    foreach (int number in EvenSequence(5, 18))  
    {  
        Console.Write(number.ToString() + " ");  
    }  
    // Output: 6 8 10 12 14 16 18  
    Console.ReadKey();  
}  

public static System.Collections.Generic.IEnumerable<int>  
    EvenSequence(int firstNumber, int lastNumber)  
{  
    // Yield even numbers in the range.  
    for (int number = firstNumber; number <= lastNumber; number++)  
    {  
        if (number % 2 == 0)  
        {  
            yield return number;  
        }  
    }  
}  

Creating a Collection Class

In the following example, the DaysOfTheWeek class implements the IEnumerable interface, which requires a GetEnumerator method. The compiler implicitly calls the GetEnumerator method, which returns an IEnumerator.

The GetEnumerator method returns each string one at a time by using the yield return statement.

static void Main()  
{  
    DaysOfTheWeek days = new DaysOfTheWeek();  

    foreach (string day in days)  
    {  
        Console.Write(day + " ");  
    }  
    // Output: Sun Mon Tue Wed Thu Fri Sat  
    Console.ReadKey();  
}  

public class DaysOfTheWeek : IEnumerable  
{  
    private string[] days = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };  

    public IEnumerator GetEnumerator()  
    {  
        for (int index = 0; index < days.Length; index++)  
        {  
            // Yield each day of the week.  
            yield return days[index];  
        }  
    }  
}  

The following example creates a Zoo class that contains a collection of animals.

The foreach statement that refers to the class instance (theZoo) implicitly calls the GetEnumerator method. The foreach statements that refer to the Birds and Mammals properties use the AnimalsForType named iterator method.

static void Main()  
{  
    Zoo theZoo = new Zoo();  

    theZoo.AddMammal("Whale");  
    theZoo.AddMammal("Rhinoceros");  
    theZoo.AddBird("Penguin");  
    theZoo.AddBird("Warbler");  

    foreach (string name in theZoo)  
    {  
        Console.Write(name + " ");  
    }  
    Console.WriteLine();  
    // Output: Whale Rhinoceros Penguin Warbler  

    foreach (string name in theZoo.Birds)  
    {  
        Console.Write(name + " ");  
    }  
    Console.WriteLine();  
    // Output: Penguin Warbler  

    foreach (string name in theZoo.Mammals)  
    {  
        Console.Write(name + " ");  
    }  
    Console.WriteLine();  
    // Output: Whale Rhinoceros  

    Console.ReadKey();  
}  

public class Zoo : IEnumerable  
{  
    // Private members.  
    private List<Animal> animals = new List<Animal>();  

    // Public methods.  
    public void AddMammal(string name)  
    {  
        animals.Add(new Animal { Name = name, Type = Animal.TypeEnum.Mammal });  
    }  

    public void AddBird(string name)  
    {  
        animals.Add(new Animal { Name = name, Type = Animal.TypeEnum.Bird });  
    }  

    public IEnumerator GetEnumerator()  
    {  
        foreach (Animal theAnimal in animals)  
        {  
            yield return theAnimal.Name;  
        }  
    }  

    // Public members.  
    public IEnumerable Mammals  
    {  
        get { return AnimalsForType(Animal.TypeEnum.Mammal); }  
    }  

    public IEnumerable Birds  
    {  
        get { return AnimalsForType(Animal.TypeEnum.Bird); }  
    }  

    // Private methods.  
    private IEnumerable AnimalsForType(Animal.TypeEnum type)  
    {  
        foreach (Animal theAnimal in animals)  
        {  
            if (theAnimal.Type == type)  
            {  
                yield return theAnimal.Name;  
            }  
        }  
    }  

    // Private class.  
    private class Animal  
    {  
        public enum TypeEnum { Bird, Mammal }  

        public string Name { get; set; }  
        public TypeEnum Type { get; set; }  
    }  
}  

Using Iterators with a Generic List

In the following example, the Stack(Of T) generic class implements the IEnumerable<T> generic interface. The Push method assigns values to an array of type T. The GetEnumerator method returns the array values by using the yield return statement.

In addition to the generic GetEnumerator method, the non-generic GetEnumerator method must also be implemented. This is because IEnumerable<T> inherits from IEnumerable. The non-generic implementation defers to the generic implementation.

The example uses named iterators to support various ways of iterating through the same collection of data. These named iterators are the TopToBottom and BottomToTop properties, and the TopN method.

The BottomToTop property uses an iterator in a get accessor.

static void Main()  
{  
    Stack<int> theStack = new Stack<int>();  

    //  Add items to the stack.  
    for (int number = 0; number <= 9; number++)  
    {  
        theStack.Push(number);  
    }  

    // Retrieve items from the stack.  
    // foreach is allowed because theStack implements  
    // IEnumerable<int>.  
    foreach (int number in theStack)  
    {  
        Console.Write("{0} ", number);  
    }  
    Console.WriteLine();  
    // Output: 9 8 7 6 5 4 3 2 1 0  

    // foreach is allowed, because theStack.TopToBottom  
    // returns IEnumerable(Of Integer).  
    foreach (int number in theStack.TopToBottom)  
    {  
        Console.Write("{0} ", number);  
    }  
    Console.WriteLine();  
    // Output: 9 8 7 6 5 4 3 2 1 0  

    foreach (int number in theStack.BottomToTop)  
    {  
        Console.Write("{0} ", number);  
    }  
    Console.WriteLine();  
    // Output: 0 1 2 3 4 5 6 7 8 9  

    foreach (int number in theStack.TopN(7))  
    {  
        Console.Write("{0} ", number);  
    }  
    Console.WriteLine();  
    // Output: 9 8 7 6 5 4 3  

    Console.ReadKey();  
}  

public class Stack<T> : IEnumerable<T>  
{  
    private T[] values = new T[100];  
    private int top = 0;  

    public void Push(T t)  
    {  
        values[top] = t;  
        top++;  
    }  
    public T Pop()  
    {  
        top--;  
        return values[top];  
    }  

    // This method implements the GetEnumerator method. It allows  
    // an instance of the class to be used in a foreach statement.  
    public IEnumerator<T> GetEnumerator()  
    {  
        for (int index = top - 1; index >= 0; index--)  
        {  
            yield return values[index];  
        }  
    }  

    IEnumerator IEnumerable.GetEnumerator()  
    {  
        return GetEnumerator();  
    }  

    public IEnumerable<T> TopToBottom  
    {  
        get { return this; }  
    }  

    public IEnumerable<T> BottomToTop  
    {  
        get  
        {  
            for (int index = 0; index <= top - 1; index++)  
            {  
                yield return values[index];  
            }  
        }  
    }  

    public IEnumerable<T> TopN(int itemsFromTop)  
    {  
        // Return less than itemsFromTop if necessary.  
        int startIndex = itemsFromTop >= top ? 0 : top - itemsFromTop;  

        for (int index = top - 1; index >= startIndex; index--)  
        {  
            yield return values[index];  
        }  
    }  

}  

Syntax Information

An iterator can occur as a method or get accessor. An iterator cannot occur in an event, instance constructor, static constructor, or static finalizer.

An implicit conversion must exist from the expression type in the yield return statement to the return type of the iterator.

In C#, an iterator method cannot have any ref or out parameters.

In C#, "yield" is not a reserved word and has special meaning only when it is used before a return or break keyword.

Technical Implementation

Although you write an iterator as a method, the compiler translates it into a nested class that is, in effect, a state machine. This class keeps track of the position of the iterator as long the foreach loop in the client code continues.

To see what the compiler does, you can use the Ildasm.exe tool to view the Microsoft intermediate language code that is generated for an iterator method.

When you create an iterator for a class or struct, you don't have to implement the whole IEnumerator interface. When the compiler detects the iterator, it automatically generates the Current, MoveNext, and Dispose methods of the IEnumerator or IEnumerator<T> interface.

On each successive iteration of the foreach loop (or the direct call to IEnumerator.MoveNext), the next iterator code body resumes after the previous yield return statement. It then continues to the next yield return statement until the end of the iterator body is reached, or until a yield break statement is encountered.

Iterators don't support the IEnumerator.Reset method. To re-iterate from the start, you must obtain a new iterator.

For additional information, see the C# Language Specification.

Use of Iterators

Iterators enable you to maintain the simplicity of a foreach loop when you need to use complex code to populate a list sequence. This can be useful when you want to do the following:

  • Modify the list sequence after the first foreach loop iteration.

  • Avoid fully loading a large list before the first iteration of a foreach loop. An example is a paged fetch to load a batch of table rows. Another example is the EnumerateFiles method, which implements iterators within the .NET Framework.

  • Encapsulate building the list in the iterator. In the iterator method, you can build the list and then yield each result in a loop.

See Also

System.Collections.Generic
IEnumerable<T>
foreach, in
yield
Using foreach with Arrays
Generics