C# Academy · Lesson

Immutability with init & with

Use init-only setters to create immutable objects and with-expressions to produce non-destructively modified copies.

Lesson 2 of 412 steps

Immutability with init & with is a free C# Academy lesson on CoddyKit. This is lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the C# Academy learning path, and your progress syncs across the web and the CoddyKit app. The C# Academy course includes 4 lessons in total.

What Is Immutability?

An immutable object cannot be changed after creation. Immutability prevents accidental mutation, makes objects thread-safe by default, and simplifies reasoning about code. C# provides init setters and with expressions to make immutability ergonomic.

init-Only Setters

An init accessor allows a property to be set during object initialization (constructor or object initializer) but never after. It's the immutable counterpart to set.

public class Point
{
    public double X { get; init; }
    public double Y { get; init; }
}

var p = new Point { X = 3.0, Y = 4.0 }; // OK — init phase
p.X = 5.0; // COMPILE ERROR — cannot set after init

// All of these are valid init-phase assignments:
var p2 = new Point(X: 1.0, Y: 2.0);
var p3 = new Point { X = 0, Y = 0 };

init in Record Positional Properties

Positional record properties are init-only by default — that's what makes records immutable. The compiler generates get; init; for each positional parameter.

// This record:
public record Person(string Name, int Age);

// Is equivalent to:
public record Person
{
    public string Name { get; init; }
    public int Age    { get; init; }
    public Person(string Name, int Age) { this.Name = Name; this.Age = Age; }
    // + Deconstruct, Equals, GetHashCode, ToString
}

var alice = new Person("Alice", 30);
alice.Name = "Bob"; // COMPILE ERROR

with Expressions: Non-Destructive Mutation

A with expression creates a copy of a record with specific properties changed. The original is unchanged — true immutability.

public record Person(string Name, int Age, string Email);

var alice = new Person("Alice", 30, "alice@example.com");

// Create a modified copy — alice is unchanged
var olderAlice = alice with { Age = 31 };
var renamed    = alice with { Name = "Alicia", Email = "alicia@example.com" };

Console.WriteLine(alice.Name);      // Alice (unchanged)
Console.WriteLine(olderAlice.Age);  // 31
Console.WriteLine(renamed.Name);    // Alicia

with on Non-Record Types

C# 10 allows with on any struct or class that has copy constructor semantics — but it's most natural with records. For classes you need to implement it manually.

// struct with with-expression:
public struct Temperature
{
    public double Celsius { get; init; }
    public double Fahrenheit => Celsius * 9 / 5 + 32;
}

var t1 = new Temperature { Celsius = 20 };
var t2 = t1 with { Celsius = 25 }; // copy with change
Console.WriteLine(t1.Celsius); // 20 — unchanged
Console.WriteLine(t2.Celsius); // 25

Chaining with Expressions

Chain multiple with expressions to build up complex transformations, each step creating a new immutable value from the previous one.

public record Order(int Id, string Status, decimal Total, DateTime UpdatedAt);

var order = new Order(42, "Pending", 99.99m, DateTime.UtcNow);

// Apply a promotion discount then mark as confirmed
var finalOrder = order
    with { Total    = order.Total * 0.9m }    // 10% off
    with { Status   = "Confirmed" }
    with { UpdatedAt = DateTime.UtcNow };

Console.WriteLine(order.Status);     // Pending (original unchanged)
Console.WriteLine(finalOrder.Status); // Confirmed

Computed Properties on Immutable Types

Derived properties on immutable types are naturally pure — they compute from the fixed property values and always return the same result for the same input.

public record Money(decimal Amount, string Currency)
{
    public Money Add(Money other)
    {
        if (Currency != other.Currency)
            throw new InvalidOperationException("Currency mismatch");
        return this with { Amount = Amount + other.Amount };
    }

    public Money Multiply(decimal factor) =>
        this with { Amount = Amount * factor };

    public override string ToString() =>
        $"{Amount:F2} {Currency}";
}

var price = new Money(10.00m, "USD");
var tax   = price.Multiply(0.08m);
var total = price.Add(tax);
Console.WriteLine(total); // 10.80 USD

Immutable Collections

Combine records with ImmutableList<T> and other types from System.Collections.Immutable for fully immutable data structures.

using System.Collections.Immutable;

public record ShoppingCart(
    string UserId,
    ImmutableList<CartItem> Items)
{
    public ShoppingCart AddItem(CartItem item) =>
        this with { Items = Items.Add(item) };

    public ShoppingCart RemoveItem(int productId) =>
        this with { Items = Items.RemoveAll(i => i.ProductId == productId) };

    public decimal Total => Items.Sum(i => i.Price * i.Quantity);
}

Thread Safety from Immutability

Immutable objects are inherently thread-safe — no synchronization needed when sharing them across threads because their state cannot change after construction.

// Immutable config record shared across all threads safely
public record AppConfig(
    string ConnectionString,
    int MaxRetries,
    TimeSpan Timeout);

// Register as singleton — safe because record is immutable
builder.Services.AddSingleton(
    new AppConfig(
        ConnectionString: config["DB"]!,
        MaxRetries: 3,
        Timeout: TimeSpan.FromSeconds(30)));

// Any thread can read this simultaneously without locks

Real-World: Functional Event Sourcing

Immutable records pair naturally with event sourcing: each domain event is immutable, and state is derived by folding events — no mutation, no surprises.

public record OrderState(
    int Id,
    string Status = "Draft",
    decimal Total = 0m);

public static OrderState Apply(OrderState state, object evt) => evt switch
{
    OrderPlaced   e => state with { Status = "Pending", Total = e.Total },
    OrderShipped  _ => state with { Status = "Shipped" },
    OrderCancelled _ => state with { Status = "Cancelled" },
    _ => state
};

// Fold events to get current state:
var state = events.Aggregate(
    new OrderState(id),
    Apply);

Quick Check

What does a 'with' expression do to the original record?

Answer it in the interactive lesson

Recap: Immutability with init & with

Key takeaways:

init accessor: settable only during initialization, never after
Record positional properties are init-only by default
with expression: creates a modified copy — original is unchanged
Chain with expressions for multi-step transformations
Immutable types are thread-safe without synchronization
Combine with ImmutableList<T> for fully immutable object graphs

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Use init-only setters to create immutable objects and with-expressions to produce non-destructively modified copies. You practise C# Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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