Init-only setters in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of init-only property assignment

What will be the output of this C# code snippet using init-only setters?

C Sharp (C#)

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

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

A Alice

B null

C Compilation error

D Runtime exception

💡 Hint

Init-only properties can be set during object initialization.

The init-only setter allows setting the property only during object initialization. Here, Name is set to "Alice" when creating the object, so printing p.Name outputs "Alice".

❓ Predict Output

intermediate

Error when setting init-only property after initialization

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

C Sharp (C#)

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

var car = new Car { Model = "Sedan" };
car.Model = "Coupe";
Console.WriteLine(car.Model);

A Outputs "Sedan"

B Compilation error

C Outputs "Coupe"

D Runtime exception

💡 Hint

Init-only properties cannot be assigned outside initialization.

The line 'car.Model = "Coupe";' is invalid because init-only properties can only be set during initialization. This causes a compilation error.

🔧 Debug

advanced

Why does this init-only setter code fail to compile?

Identify the reason for the compilation error in this code using init-only setters.

C Sharp (C#)

public class Book {
    public string Title { get; init; }
}

var b = new Book();
b.Title = "C# Guide";
Console.WriteLine(b.Title);

A Title property must be readonly to use init-only

B Property Title is missing a setter

C Init-only property Title cannot be assigned outside object initialization

D Missing semicolon after object creation

💡 Hint

Check when init-only properties can be assigned.

Init-only properties can only be assigned during object initialization, not after. Here, 'b.Title = "C# Guide";' is outside initialization, causing a compile error.

🧠 Conceptual

advanced

Understanding init-only setters with inheritance

Given these classes, what is the output of the program?

C Sharp (C#)

public class Animal {
    public string Species { get; init; }
}

public class Dog : Animal {
    public string Breed { get; init; }
}

var dog = new Dog { Species = "Canine", Breed = "Beagle" };
Console.WriteLine($"{dog.Species} - {dog.Breed}");

A Canine - null

B Compilation error due to init-only in base class

C Runtime exception

D Canine - Beagle

💡 Hint

Init-only setters work with inheritance and can be set during initialization.

Both Species and Breed are init-only properties. They are set during object initialization, so the output is "Canine - Beagle".

📝 Syntax

expert

Which option causes a compilation error with init-only setters?

Which of the following code snippets will NOT compile due to incorrect use of init-only setters?

A var p = new Person { Name = "John" }; p.Name = "Jane";

B var p = new Person { Name = "John" }; Console.WriteLine(p.Name);

C Person p = new() { Name = "John" };

D var p = new Person(); var q = p with { Name = "Jane" };

💡 Hint

Remember when init-only properties can be assigned.

Option A tries to assign to the init-only property Name after initialization, causing a compilation error. Options A, C, and D are valid uses.

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