Immutability: Creating Custom Immutable Types in C#


In the ever-evolving world of software development, immutability has emerged as a powerful principle, offering numerous benefits such as improved thread safety, data consistency, and simplified reasoning. C#, with its versatile features, allows you to craft elegant and effective custom immutable types. This article dives into the key aspects of building such types and explores the advantages they bring to your applications.

What is an immutable type?

An immutable type, unlike its mutable counterpart, cannot change its state after creation. Think of it as a snapshot of a specific condition frozen in time. This unwavering characteristic grants it several advantages.

  • Thread safety: Immutable types are inherently thread-safe, as multiple threads can access and use the same instance without risking corruption.
  • Data consistency: Since values can't be altered, you're guaranteed data consistency throughout your application, minimizing unexpected state changes.
  • Simplified reasoning: With guaranteed immutability, reasoning about code becomes easier, as you can confidently predict the behavior of immutable objects.

Crafting your own immutable type

C# offers various ways to build custom immutable types. Here are some key principles to keep in mind.

  • Read-only fields and properties: Make sure all fields and properties are defined as readonly or utilize expression-bodied functions for getter-only access. This prevents any unintentional mutations.
  • Defensive copying: If your type contains mutable dependencies like collections, always create deep copies during initialization and expose read-only versions through properties. This shields your immutable type from external modifications.
  • Immutable methods: Methods should either return a new instance with the desired changes or throw exceptions if mutations are attempted. Avoid modifying the internal state within methods.
  • Equality comparisons: Implement proper equality checks based on the data contained within the object, leveraging value-based comparison techniques.

Example of a custom immutable type in C#

public class Point
    private readonly int x;
    private readonly int y;

    public Point(int x, int y)
        this.x = x;
        this.y = y;

    public int X => x;  // Read-only property
    public int Y => y;  // Read-only property

    // Immutable method to create a new Point with a different X value
    public Point WithX(int newX)
        return new Point(newX, y);

    // Immutable method to create a new Point with a different Y value
    public Point WithY(int newY)
        return new Point(x, newY);


  1. Read-only fields: The x and y fields are declared as readonly, ensuring their values cannot be changed after the object is created.
  2. Read-only properties: The X and Y properties provide getter-only access to the field values, preventing external modification.
  3. Immutable constructor: The constructor initializes the fields with the provided values, setting the initial state of the object.
  4. Immutable methods: The WithX and WithY methods demonstrate how to create new instances with modified values while preserving immutability. They return new Point objects with the desired changes instead of modifying the existing object.
  5. No setters: The absence of setters for the properties reinforces the immutability of the type.

Usage example

var point1 = new Point(5, 10);
var point2 = point1.WithX(8);  // Creates a new Point with X = 8, Y = 10
var point3 = point2.WithY(15); // Creates a new Point with X = 8, Y = 15

// point1 remains unchanged (X = 5, Y = 10)
// point2 and point3 are new instances with their own distinct values

Benefits of using custom immutable types

By embracing immutability, you can reap numerous benefits for your C# code.

  • Enhanced concurrency: As mentioned earlier, thread safety becomes much easier to achieve with immutable types, simplifying multithreaded scenarios.
  • Improved caching: Immutable objects can be freely cached without concerns about state inconsistency, potentially boosting performance.
  • Simplified testing: With predictable behavior, testing immutable types becomes more straightforward and reliable.
  • Cleaner code: By avoiding mutation-prone constructs, your code becomes cleaner, easier to read, and less prone to errors.


Building custom immutable types in C# provides a powerful tool to improve your code's quality, reliability, and maintainability. By embracing immutability, you can contribute to more robust and predictable applications, allowing you to focus on the bigger picture instead of battling mutable complexities.

This article provides a foundational understanding of creating custom immutable types in C#. However, there's a lot more to explore. Consider delving deeper into topics like immutability frameworks in C#, functional programming principles, and advanced techniques for handling complex scenarios.

I hope you will find this article helpful. If you have any suggestions, then please feel free to ask in the comment section.

Thank you.

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