Init-only setters in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an init-only property named Name.

C Sharp (C#)

public string Name { get; [1]; }

Drag options to blanks, or click blank then click option'

Aprivate set

Binit

Cset

Dreadonly

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' allows changing the property anytime, not just during initialization.

Using 'readonly' is not valid for properties.

The init accessor allows setting the property only during object initialization.

2fill in blank

medium

Complete the code to create a record with an init-only property Age.

C Sharp (C#)

public record Person { public int Age { get; [1]; } }

Drag options to blanks, or click blank then click option'

Ainit

Bset

Cprivate set

Dreadonly

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' allows changing the property after creation.

Using 'private set' restricts setting even during initialization.

Records often use init to make properties immutable after creation.

3fill in blank

hard

Fix the error in the property declaration to make it init-only.

C Sharp (C#)

public string Title { get; [1]; }

Drag options to blanks, or click blank then click option'

Aset

Breadonly

Cprivate set

Dinit

Attempts:

3 left

💡 Hint

Common Mistakes

Leaving 'set' allows modification anytime.

Using 'readonly' is invalid for properties.

Changing set to init makes the property settable only during initialization.

4fill in blank

hard

Fill both blanks to define an init-only property with a private getter.

C Sharp (C#)

public string Description { [1] get; [2] }

Drag options to blanks, or click blank then click option'

Aprivate

Binit

Cpublic

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' instead of 'init' allows modification after initialization.

Making getter public when it should be private.

The property has a private getter and an init-only setter.

5fill in blank

hard

Fill all three blanks to create a record with an init-only property and a constructor.

C Sharp (C#)

public record Book [1] { public string Author { get; [2]; } = [3]; }

Drag options to blanks, or click blank then click option'

A()

Binit

C"Unknown"

Dset

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'set' instead of 'init' allows property changes after creation.

Omitting the constructor parentheses.

Not providing a default value.

The record has a parameterless constructor, an init-only property, and a default value.

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