Why Init-only setters in C Sharp (C#)? - Purpose & Use Cases

The Big Idea

What if you could set a property once and be sure it never changes by mistake?

The Scenario

Imagine you want to create a person record in your program and set their name and age. You write code that sets these values one by one after creating the person object.

Later, you realize you want to make sure these values never change once set, but your code allows changes anywhere, causing bugs.

The Problem

Manually preventing changes after setting values means extra code everywhere to check if a value can be changed.

This is slow, error-prone, and makes your code messy and hard to maintain.

The Solution

Init-only setters let you set properties only once during object creation.

After that, the properties become read-only automatically, so you don't need extra checks.

This keeps your code clean and safe from accidental changes.

Before vs After

✗ Before

var person = new Person();
person.Name = "Alice";
person.Age = 30;
// Later someone changes person.Name = "Bob"; // no restriction

✓ After

var person = new Person { Name = "Alice", Age = 30 };
// person.Name = "Bob"; // error: cannot change after init

What It Enables

You can create objects with properties that are set once and then stay fixed, making your programs more reliable and easier to understand.

Real Life Example

When creating a configuration object for an app, you want to set values once at startup and never change them accidentally during runtime.

Key Takeaways

Init-only setters allow setting properties only during object creation.

This prevents accidental changes later, reducing bugs.

It makes your code cleaner and safer without extra checks.

Practice

1. What is the main purpose of init-only setters in C#?

easy

A. To prevent object creation

B. To allow setting properties only during object creation

C. To make methods run faster

D. To allow properties to be changed anytime

Solution

1. Step 1: Understand init-only setters

   Init-only setters allow properties to be set only when the object is created, not after.

2. Step 2: Compare options

   Only To allow setting properties only during object creation correctly describes this behavior; others are unrelated or incorrect.

3. Final Answer:

   To allow setting properties only during object creation -> Option B

4. Quick Check:

   Init-only setters = set only at creation [OK]

Hint: Init-only means set once during creation only [OK]

Common Mistakes:

• Thinking init-only allows changes anytime
• Confusing init-only with readonly fields
• Assuming init-only affects methods

2. Which of the following is the correct syntax to declare an init-only property in C#?

easy

A. public string Name { get; init; }

B. public string Name { get; set; }

C. public string Name { get; readonly; }

D. public string Name { get; private set; }

Solution

1. Step 1: Recall init-only syntax

   Init-only properties use init instead of set to allow setting only during initialization.

2. Step 2: Check options

   public string Name { get; init; } uses { get; init; }, which is correct syntax. Others use set, readonly (invalid here), or private set.

3. Final Answer:

   public string Name { get; init; } -> Option A

4. Quick Check:

   Init-only syntax = get + init [OK]

Hint: Init-only uses 'init' keyword, not 'set' [OK]

Common Mistakes:

• Using 'set' instead of 'init'
• Trying to use 'readonly' in property accessor
• Confusing 'private set' with init-only

3. What will happen when you run this code?

public class Person {
    public string Name { get; init; }
}

var p = new Person { Name = "Alice" };
p.Name = "Bob";
Console.WriteLine(p.Name);

medium

A. Prints 'Alice'

B. Runtime exception

C. Prints 'Bob'

D. Compile-time error

Solution

1. Step 1: Understand init-only property behavior

   The property Name can only be set during object initialization (inside the braces). Setting it later causes a compile error.

2. Step 2: Analyze the code

   After creating p with Name = "Alice", the code tries to assign p.Name = "Bob" outside initialization, which is invalid.

3. Final Answer:

   Compile-time error -> Option D

4. Quick Check:

   Changing init-only after creation = compile error [OK]

Hint: Init-only properties can't be changed after creation [OK]

Common Mistakes:

• Assuming property can be changed anytime
• Expecting runtime error instead of compile error
• Confusing init-only with readonly fields

4. Identify the error in this code snippet:

public class Car {
    public string Model { get; init; }
}

var car = new Car();
car.Model = "Sedan";

medium

A. Missing constructor call

B. Property Model must have a setter, not init

C. Cannot assign init-only property outside object initializer

D. No error, code is correct

Solution

1. Step 1: Check how init-only properties are set

   Init-only properties must be set during object creation using an initializer, not after.

2. Step 2: Analyze the code

   The code creates car with default constructor, then tries to set Model property outside initialization, which is invalid.

3. Final Answer:

   Cannot assign init-only property outside object initializer -> Option C

4. Quick Check:

   Init-only set only in initializer, not later [OK]

Hint: Set init-only properties inside braces only [OK]

Common Mistakes:

• Trying to set init-only property after creation
• Thinking init-only means readonly
• Ignoring object initializer syntax

5. Given this record declaration:

public record Book {
    public string Title { get; init; }
    public string Author { get; init; }
}

var book1 = new Book { Title = "C# Guide", Author = "Jane" };
var book2 = book1 with { Author = "John" };

Console.WriteLine(book2.Title + ", " + book2.Author);

What is the output?

hard

A. C# Guide, John

B. C# Guide, Jane

C. Compile-time error due to init-only setter

D. Runtime exception

Solution

1. Step 1: Understand 'with' expression on records

   The 'with' expression creates a copy of book1 but allows changing init-only properties during creation of the new object.

2. Step 2: Analyze the output

   book2 copies Title as "C# Guide" and sets Author to "John". Printing shows "C# Guide, John".

3. Final Answer:

   C# Guide, John -> Option A

4. Quick Check:

   'with' allows changing init-only during copy [OK]

Hint: 'with' can set init-only properties on new record copies [OK]

Common Mistakes:

• Thinking 'with' cannot change init-only properties
• Expecting compile error on 'with' usage
• Confusing init-only with immutable fields