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yield statement - provide the next element

  • Grein

You use the yield statement in an iterator to provide the next value or signal the end of an iteration. The yield statement has the two following forms:

  • yield return: to provide the next value in iteration, as the following example shows:

    foreach (int i in ProduceEvenNumbers(9))
{
    Console.Write(i);
    Console.Write(" ");
}
// Output: 0 2 4 6 8

IEnumerable<int> ProduceEvenNumbers(int upto)
{
    for (int i = 0; i <= upto; i += 2)
    {
        yield return i;
    }
}

  • yield break: to explicitly signal the end of iteration, as the following example shows:

    Console.WriteLine(string.Join(" ", TakeWhilePositive(new int[] {2, 3, 4, 5, -1, 3, 4})));
// Output: 2 3 4 5

Console.WriteLine(string.Join(" ", TakeWhilePositive(new int[] {9, 8, 7})));
// Output: 9 8 7

IEnumerable<int> TakeWhilePositive(IEnumerable<int> numbers)
{
    foreach (int n in numbers)
    {
        if (n > 0)
        {
            yield return n;
        }
        else
        {
            yield break;
        }
    }
}

    Iteration also finishes when control reaches the end of an iterator.

In the preceding examples, the return type of iterators is IEnumerable<T> (in nongeneric cases, use IEnumerable as the return type of an iterator). You can also use IAsyncEnumerable<T> as the return type of an iterator. That makes an iterator async. Use the await foreach statement to iterate over iterator's result, as the following example shows:

await foreach (int n in GenerateNumbersAsync(5))
{
    Console.Write(n);
    Console.Write(" ");
}
// Output: 0 2 4 6 8

async IAsyncEnumerable<int> GenerateNumbersAsync(int count)
{
    for (int i = 0; i < count; i++)
    {
        yield return await ProduceNumberAsync(i);
    }
}

async Task<int> ProduceNumberAsync(int seed)
{
    await Task.Delay(1000);
    return 2 * seed;
}

IEnumerator<T> or IEnumerator can also be the return type of an iterator. Use those return types when you implement the GetEnumerator method in the following scenarios:

  • You design the type that implements IEnumerable<T> or IEnumerable interface.

  • You add an instance or extension GetEnumerator method to enable iteration over the type's instance with the foreach statement, as the following example shows:

    public static void Example()
{
    var point = new Point(1, 2, 3);
    foreach (int coordinate in point)
    {
        Console.Write(coordinate);
        Console.Write(" ");
    }
    // Output: 1 2 3
}

public readonly record struct Point(int X, int Y, int Z)
{
    public IEnumerator<int> GetEnumerator()
    {
        yield return X;
        yield return Y;
        yield return Z;
    }
}

You can't use the yield statements in:

Execution of an iterator

The call of an iterator doesn't execute it immediately, as the following example shows:

var numbers = ProduceEvenNumbers(5);
Console.WriteLine("Caller: about to iterate.");
foreach (int i in numbers)
{
    Console.WriteLine($"Caller: {i}");
}

IEnumerable<int> ProduceEvenNumbers(int upto)
{
    Console.WriteLine("Iterator: start.");
    for (int i = 0; i <= upto; i += 2)
    {
        Console.WriteLine($"Iterator: about to yield {i}");
        yield return i;
        Console.WriteLine($"Iterator: yielded {i}");
    }
    Console.WriteLine("Iterator: end.");
}
// Output:
// Caller: about to iterate.
// Iterator: start.
// Iterator: about to yield 0
// Caller: 0
// Iterator: yielded 0
// Iterator: about to yield 2
// Caller: 2
// Iterator: yielded 2
// Iterator: about to yield 4
// Caller: 4
// Iterator: yielded 4
// Iterator: end.

As the preceding example shows, when you start to iterate over an iterator's result, an iterator is executed until the first yield return statement is reached. Then, the execution of an iterator is suspended and the caller gets the first iteration value and processes it. On each subsequent iteration, the execution of an iterator resumes after the yield return statement that caused the previous suspension and continues until the next yield return statement is reached. The iteration completes when control reaches the end of an iterator or a yield break statement.

C# language specification

For more information, see The yield statement section of the C# language specification.

See also