Yield

The C# yield-keyword interacts with the foreach-loop. It is a contextual keyword: yield is a keyword only in certain statements. It can make loops clearer and simpler to write.

Yield allows each iteration in a foreach-loop to be generated only when needed. In this way (by enabling lazy evaluation) it can improve performance.

Example

We use the yield-keyword in methods that return the type IEnumerable (without any angle brackets), or the type IEnumerable with a type parameter in the angle brackets.

using System;
using System.Collections.Generic;

public class Program
{
    static void Main()
    {
        // Part 1: compute powers of two with the exponent of 30.
        foreach (int value in ComputePower(2, 30))
        {
            Console.WriteLine(value);
        }
    }

    public static IEnumerable<int> ComputePower(int number, int exponent)
    {
        // Part 2: continue loop until the exponent count is reached.
        // ... Yield the current result.
        int exponentNum = 0;
        int numberResult = 1;
        while (exponentNum < exponent)
        {
            // Multiply the result.
            numberResult *= number;
            exponentNum++;
            // Return the result with yield.
            yield return numberResult;
        }
    }
}
2
4
8
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65536
131072
262144
524288
1048576
2097152
4194304
8388608
16777216
33554432
67108864
134217728
268435456
536870912
1073741824

Internals

The C# code you have that uses yield is not directly executed by the runtime. Instead, the C# compiler transforms that code before the runtime ever occurs.

// Nested Types
[CompilerGenerated]
private sealed class <ComputePower>d__0 : IEnumerable<int>,
    IEnumerable, IEnumerator<int> // ...
{
    // Fields
    private int <>1__state;
    private int <>2__current;
    public int <>3__exponent;
    public int <>3__number;
    private int <>l__initialThreadId;
    public int <exponentNum>5__1;
    public int <numberResult>5__2;
    public int exponent;
    public int number;
    // Methods [omitted]
}

Benchmark, yield

Is yield-return fast? Or does it incur a significant performance loss? This code does have some overhead, but if you need its advanced features it can be faster.

using System;
using System.Collections.Generic;
using System.Diagnostics;

class Program
{
    const int _max = 1000000;
    static void Main()
    {
        int[] numbers = { 10, 20, 30, 40, 50 };

        var s1 = Stopwatch.StartNew();
        // Version 1: use foreach with array.
        for (int i = 0; i < _max; i++)
        {
            if (Method1(numbers) != 300)
            {
                throw new Exception();
            }
        }
        s1.Stop();
        var s2 = Stopwatch.StartNew();
        // Version 2: use foreach with yield keyword.
        for (int i = 0; i < _max; i++)
        {
            if (Method2(numbers) != 300)
            {
                throw new Exception();
            }
        }
        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
    }

    static int Method1(int[] numbers)
    {
        // Sum with simple statements.
        int sum = 0;
        foreach (int number in numbers)
        {
            sum += number * 2;
        }
        return sum;
    }

    static int Method2(int[] numbers)
    {
        // Use yield return to get multiplied numbers and then sum those.
        int sum = 0;
        foreach (int number in GetNumbers(numbers))
        {
            sum += number;
        }
        return sum;
    }

    static IEnumerable<int> GetNumbers(int[] numbers)
    {
        // Return all numbers multiplied.
        foreach (int number in numbers)
        {
            yield return number * 2;
        }
    }
}
 3.84 ns    inline expression
50.68 ns    yield return

Notes, IEnumerable

IEnumerable is an interface. It describes a type that implements the internal methods required for using foreach-loops.

Return

Yield return is similar to a return statement, followed by a "goto" to the yield-statement in the next iteration of the foreach-loop.

Yield benefits

Suppose we have a large (or even infinite) set of items (like prime numbers, or species of beetles). Yield can provide a benefit here.

Summary

The yield-keyword is contextual—it only has special significance in some places. Often, the yield return pattern helps simplify foreach-loops.