Making Sense of yield in C#

Rikam Palkar
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Article

Introduction

The yield keyword in C# allows you to create iterators that return values one at a time, instead of generating a full collection upfront. This is classic example of lazy loading, meaning elements are only processed when needed, leading to better performance and reduced memory usage. It's especially useful for handling large data sets.

The two primary uses of yield are

yield return: Which returns a value from an iterator and pauses execution.
yield break: Ends the iteration immediately.

How Yield Works?

Let’s consider a basic example demonstrating how yield returns elements lazily.

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        foreach (int num in GetNumbers())
        {
            Console.WriteLine(num);
        }
    }

    static IEnumerable<int> GetNumbers()
    {
        yield return 1;
        yield return 2;
        yield return 3;
    }
}

//Output
1
2
3

Code Snippet 1

How does it Work?

The foreach loop calls GetNumbers().
The method runs until it encounters the first yield return, then pauses and gives control back to the loop.
Each time the loop requests the next item, execution resumes right where it left off.
This continues until there are no more yield return statements.

Example 1. Real-World Example: Processing Large Files

A common use case for yield return is when dealing with large files where loading all data into memory at once would be inefficient.

Benefits

Reads-only: one line at a time instead of loading the whole file.
Memory-Saving: Prevents high memory consumption for large files.
Stateful: If interrupted, it can resume from where it left off instead of restarting.

using System;
using System.Collections.Generic;
using System.IO;

class Program
{
    static void Main()
    {
        foreach (string line in ReadLargeFile("bigfile.txt"))
        {
            Console.WriteLine(line); // Processes one line at a time
        }
    }

    static IEnumerable<string> ReadLargeFile(string filePath)
    {
        using (StreamReader reader = new(filePath))
        {
            string? line;
            while ((line = reader.ReadLine()) != null)
            {
                yield return line; // Returns one line at a time
            }
        }
    }
}

Code Snippet 2

Yield vs. Normal Return

If we remove yield return, we would have to store all results in a List and return it, leading to higher memory usage.

Feature	Using yield return	Using List<T>
Execution	Returns one item at a time	Processes everything at once
Memory Usage	Efficient (lazy loading)	Stores everything in memory
Performance	Faster for large datasets	Slower for large datasets
Complexity	Simple to implement	Requires manual list creation

Example 2. Filtering using Yield

Let’s now look at how yield return can simplify filtering operations.

The method does not create a full list of filtered items.
Instead, it yields each matching Product one by one as they are found.
If the calling code stops iterating early, unused elements aren't processed, saving resources.

public IEnumerable<Product> FilterBySize(IEnumerable<Product> products, Size size)
{
    foreach (var p in products)
    {
        if (p.Size == size)
            yield return p;
    }
}

Code Snippet 3

Filtering without Yield

public IEnumerable<Product> FilterBySize(IEnumerable<Product> products, Size size)
{
    List<Product> result = new List<Product>();
    foreach (var p in products)
    {
        if (p.Size == size)
            result.Add(p);
    }
    return result;
}

Code Snippet 4

Drawbacks