How constructor chaining works in C Sharp (C#) - Practice Exercises

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of constructor chaining with base and this

What is the output of this C# program that uses constructor chaining with this and base?

C Sharp (C#)

using System;
class BaseClass {
    public BaseClass() {
        Console.WriteLine("BaseClass default constructor");
    }
    public BaseClass(string msg) {
        Console.WriteLine("BaseClass says: " + msg);
    }
}
class DerivedClass : BaseClass {
    public DerivedClass() : this("Hello") {
        Console.WriteLine("DerivedClass default constructor");
    }
    public DerivedClass(string msg) : base(msg) {
        Console.WriteLine("DerivedClass says: " + msg);
    }
}
class Program {
    static void Main() {
        var obj = new DerivedClass();
    }
}

A

BaseClass default constructor
DerivedClass says: Hello
DerivedClass default constructor

B

BaseClass says: Hello
DerivedClass says: Hello
DerivedClass default constructor

C

BaseClass says: Hello
DerivedClass default constructor
DerivedClass says: Hello

D

BaseClass default constructor
DerivedClass default constructor
DerivedClass says: Hello

💡 Hint

Remember that this calls another constructor in the same class before executing the body, and base calls the base class constructor.

The DerivedClass() constructor calls this("Hello"), which calls DerivedClass(string msg). That constructor calls base(msg), which prints "BaseClass says: Hello". Then it prints "DerivedClass says: Hello". After returning, the DerivedClass() constructor prints "DerivedClass default constructor".

❓ Predict Output

intermediate

Constructor chaining order in inheritance

What will be printed when this C# program runs?

C Sharp (C#)

using System;
class A {
    public A() {
        Console.WriteLine("A()");
    }
    public A(int x) {
        Console.WriteLine("A(int)");
    }
}
class B : A {
    public B() : base(5) {
        Console.WriteLine("B()");
    }
    public B(int x) : this() {
        Console.WriteLine("B(int)");
    }
}
class Program {
    static void Main() {
        var b = new B(10);
    }
}

A

A(int)
B()
B(int)

B

A()
B()
B(int)

C

A(int)
B(int)
B()

D

A()
B(int)
B()

💡 Hint

Check which constructors call which and in what order base constructors run.

The B(int x) constructor calls this(), so B() runs first. B() calls base(5), so A(int) runs first. Then B() prints, then B(int) prints.

🔧 Debug

advanced

Identify the error in constructor chaining

What error will this C# code produce when compiled?

C Sharp (C#)

class Test {
    public Test() : this(5) {
    }
    public Test(int x) : this() {
    }
}

A No error, compiles and runs fine

B Compile-time error: Missing base constructor call

C Compile-time error: Constructor chaining causes a cycle

D Runtime error: Stack overflow

💡 Hint

Check if constructors call each other in a loop.

The Test() constructor calls this(5), and Test(int x) calls this(). This creates an infinite loop of constructor calls, which is detected at compile time as a cycle.

❓ Predict Output

advanced

Output with multiple constructor chaining levels

What is the output of this C# program?

C Sharp (C#)

using System;
class X {
    public X() {
        Console.WriteLine("X()");
    }
    public X(string s) : this() {
        Console.WriteLine("X(string): " + s);
    }
}
class Y : X {
    public Y() : base("hello") {
        Console.WriteLine("Y()");
    }
    public Y(int n) : this() {
        Console.WriteLine("Y(int): " + n);
    }
}
class Program {
    static void Main() {
        var y = new Y(10);
    }
}

A

X()
X(string): hello
Y()
Y(int): 10

B

X(string): hello
X()
Y()
Y(int): 10

C

X()
Y()
X(string): hello
Y(int): 10

D

X()
Y(int): 10
X(string): hello
Y()

💡 Hint

Follow the constructor calls from Y(int) down to base.

Y(int) calls this(), so Y() runs first. Y() calls base("hello"), so X(string) runs, which calls X() first. So order is X(), X(string), Y(), Y(int).

🧠 Conceptual

expert

Why constructor chaining is useful

Which of the following best explains why constructor chaining is useful in C#?

A It prevents any constructor from calling base class constructors explicitly.

B It allows multiple constructors to run in parallel threads for faster object creation.

C It automatically generates default values for all fields without writing code.

D It avoids code duplication by letting one constructor reuse another's initialization logic.

💡 Hint

Think about how constructors can share code.

Constructor chaining lets one constructor call another, so common initialization code is written once and reused. This reduces duplication and errors.

Practice

1. What does constructor chaining in C# allow you to do?

easy

A. Override a constructor in a derived class

B. Call a method from a constructor

C. Create multiple instances of a class at once

D. Call one constructor from another constructor in the same class

Solution

1. Step 1: Understand constructor chaining concept

   Constructor chaining means one constructor calls another constructor within the same class to reuse code.

2. Step 2: Identify what constructor chaining does

   It helps avoid repeating initialization code by calling another constructor using : this(...) syntax.

3. Final Answer:

   Call one constructor from another constructor in the same class -> Option D

4. Quick Check:

   Constructor chaining = calling another constructor [OK]

Hint: Constructor chaining calls another constructor in the same class [OK]

Common Mistakes:

• Confusing constructor chaining with method calls
• Thinking it creates multiple objects
• Mixing it up with inheritance or overriding

2. Which of the following is the correct syntax to chain constructors in C#?

easy

A. public MyClass() : base() { }

B. public MyClass() : this() { }

C. public MyClass() : this(5) { }

D. public MyClass() { this(); }

Solution

1. Step 1: Recall constructor chaining syntax

   Constructor chaining uses : this(parameters) after the constructor signature to call another constructor in the same class.

2. Step 2: Analyze options

   public MyClass() : this(5) { } uses : this(5) which correctly calls another constructor with an int parameter. public MyClass() : this() { } calls itself recursively causing error. public MyClass() : base() { } calls base class constructor, not chaining. public MyClass() { this(); } tries to call constructor inside body, which is invalid.

3. Final Answer:

   public MyClass() : this(5) { } -> Option C

4. Quick Check:

   Constructor chaining syntax = : this(...) [OK]

Hint: Use ': this(...)' after constructor signature to chain [OK]

Common Mistakes:

• Using 'this()' inside constructor body instead of after signature
• Confusing base() with this()
• Calling the same constructor recursively

3. What will be the output of this C# code?

class Test {
  public Test() : this(10) {
    Console.WriteLine("Default constructor");
  }
  public Test(int x) {
    Console.WriteLine($"Constructor with {x}");
  }
}

class Program {
  static void Main() {
    Test t = new Test();
  }
}

medium

A. Constructor with 10
Default constructor

B. Constructor with 10

C. Default constructor
Constructor with 10

D. Default constructor

Solution

1. Step 1: Understand constructor chaining call order

   The parameterless constructor calls this(10), so the constructor with int parameter runs first.

2. Step 2: Trace output order

   First, "Constructor with 10" is printed from the int constructor. Then control returns to parameterless constructor which prints "Default constructor".

3. Final Answer:

   Constructor with 10 Default constructor -> Option A

4. Quick Check:

   Chained constructor runs first, then caller prints [OK]

Hint: Chained constructor runs before the calling constructor body [OK]

Common Mistakes:

• Assuming calling constructor runs first
• Ignoring constructor chaining order
• Mixing output order

4. Identify the error in this constructor chaining code:

class Sample {
  public Sample() : this() {
    Console.WriteLine("Hello");
  }
}

medium

A. Missing base keyword for chaining

B. Recursive constructor call causing infinite loop

C. Constructor chaining syntax is correct

D. Constructor must have a return type

Solution

1. Step 1: Analyze constructor chaining call

   The constructor calls itself with : this(), causing infinite recursion.

2. Step 2: Identify error type

   This recursive call leads to a runtime stack overflow error because no termination occurs.

3. Final Answer:

   Recursive constructor call causing infinite loop -> Option B

4. Quick Check:

   Constructor calling itself = infinite recursion [OK]

Hint: Avoid chaining a constructor to itself directly [OK]

Common Mistakes:

• Thinking chaining to self is allowed
• Confusing base() and this() calls
• Expecting constructor to have return type

5. Given this class, what will be the output when creating new Box()?

class Box {
  public int Width, Height;
  public Box() : this(5, 10) {
    Console.WriteLine("Default Box");
  }
  public Box(int w, int h) {
    Width = w;
    Height = h;
    Console.WriteLine($"Box: {Width}x{Height}");
  }
}

hard

A. Box: 5x10
Default Box

B. Default Box
Box: 5x10

C. Box: 0x0
Default Box

D. Default Box

Solution

1. Step 1: Understand constructor chaining and initialization

   The parameterless constructor calls this(5, 10), so the two-parameter constructor runs first, setting Width and Height and printing their values.

2. Step 2: Trace output order

   First, "Box: 5x10" is printed from the two-parameter constructor. Then control returns to the parameterless constructor which prints "Default Box".

3. Final Answer:

   Box: 5x10 Default Box -> Option A

4. Quick Check:

   Chained constructor runs first, then caller prints [OK]

Hint: Chained constructor runs before caller's body executes [OK]

Common Mistakes:

• Assuming default values 0 for Width and Height
• Thinking default constructor runs first
• Ignoring chaining call order