Init-only setters in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an init-only setter in C#?

An init-only setter allows a property to be set only during object initialization, making the property immutable after the object is created.

beginner

How do you declare an init-only property in C#?

Use the init keyword instead of set in the property declaration, like:

public string Name { get; init; }

beginner

When can you assign a value to an init-only property?

You can assign a value only during object creation or in the object initializer block, but not after the object is fully constructed.

intermediate

What happens if you try to set an init-only property after object initialization?

The compiler will give an error because init-only properties cannot be changed after the object is created.

intermediate

Why are init-only setters useful?

They help create immutable objects with simpler syntax, improving safety by preventing accidental changes after creation.

Which keyword is used to declare an init-only setter in C#?

A readonly

B set

C init

D const

The init keyword declares a setter that can only be used during object initialization.

When can you assign a value to a property with an init-only setter?

A Anytime after object creation

B Only in constructors

C Only inside methods

D Only during object initialization

Init-only setters allow assignment only during object initialization or in the initializer block.

What will happen if you try to set an init-only property after the object is created?

A Compiler error

B Value changes successfully

C Runtime exception

D Property is ignored

The compiler prevents setting init-only properties after initialization, causing a compile-time error.

Which of these is a benefit of using init-only setters?

A Creating immutable objects easily

B Allowing properties to change anytime

C Making properties private

D Improving runtime performance

Init-only setters help create immutable objects by restricting property changes after initialization.

How do init-only setters improve code safety?

A By making properties static

B By preventing accidental property changes after creation

C By hiding properties from other classes

D By allowing properties to be changed anywhere

They prevent accidental changes to properties after the object is created, improving safety.

Explain what init-only setters are and how they differ from regular setters in C#.

Think about when you can assign values to properties.

Describe a real-life scenario where using init-only setters would be beneficial.

Consider objects that should not change after being set up.

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