Base keyword behavior in C Sharp (C#)

Introduction

The base keyword helps you use features from a parent class inside a child class. It lets you reuse and extend code easily.

When you want to call a method from the parent class inside a child class method.

When you want to access a parent class constructor from a child class constructor.

When you want to use a property or field defined in the parent class from the child class.

When you want to override a method but still use the original behavior from the parent.

Syntax

C Sharp (C#)

class ChildClass : ParentClass
{
    public ChildClass() : base() { }

    public override void SomeMethod()
    {
        base.SomeMethod();
        // additional code
    }
}

base() calls the parent class constructor.

base.MethodName() calls a method from the parent class.

Examples

Here, base.Show() calls the Show method from the parent class inside the child class.

C Sharp (C#)

class Parent
{
    public void Show() => Console.WriteLine("Parent Show");
}

class Child : Parent
{
    public void ShowChild()
    {
        base.Show();
        Console.WriteLine("Child Show");
    }
}

The child constructor calls the parent constructor using base() before running its own code.

C Sharp (C#)

class Parent
{
    public Parent()
    {
        Console.WriteLine("Parent constructor");
    }
}

class Child : Parent
{
    public Child() : base()
    {
        Console.WriteLine("Child constructor");
    }
}

The child overrides Greet but still calls the parent's version first using base.Greet().

C Sharp (C#)

class Parent
{
    public virtual void Greet() => Console.WriteLine("Hello from Parent");
}

class Child : Parent
{
    public override void Greet()
    {
        base.Greet();
        Console.WriteLine("Hello from Child");
    }
}

Sample Program

This program shows how the base keyword calls the parent constructor and method from the child class.

C Sharp (C#)

using System;

class Parent
{
    public Parent()
    {
        Console.WriteLine("Parent constructor called");
    }

    public virtual void Display()
    {
        Console.WriteLine("Display method in Parent");
    }
}

class Child : Parent
{
    public Child() : base()
    {
        Console.WriteLine("Child constructor called");
    }

    public override void Display()
    {
        base.Display();
        Console.WriteLine("Display method in Child");
    }
}

class Program
{
    static void Main()
    {
        Child c = new Child();
        c.Display();
    }
}

Important Notes

You cannot use base in static methods because base refers to instance members.

If the parent class method is not marked virtual or abstract, you cannot override it but can still call it with base.

Summary

The base keyword lets child classes access parent class members.

Use base() to call the parent constructor.

Use base.Method() to call a parent method inside an overridden method.

Practice

1. What does the base keyword do in C#?

easy

A. It creates a new instance of a class.

B. It allows a child class to access members of its parent class.

C. It defines a new class.

D. It deletes an object from memory.

Solution

1. Step 1: Understand the role of base

   The base keyword is used in a child class to refer to its parent class members.

2. Step 2: Identify what base allows

   It allows access to parent class methods, properties, or constructors from the child class.

3. Final Answer:

   It allows a child class to access members of its parent class. -> Option B

4. Quick Check:

   base keyword = access parent members [OK]

Hint: Remember: base = parent class access [OK]

Common Mistakes:

• Confusing base with new instance creation
• Thinking base deletes objects
• Assuming base defines a class

2. Which of the following is the correct way to call a parent class constructor using base in C#?

easy

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

B. public Child() base() { }

C. public Child() call base() { }

D. public Child() : parent() { }

Solution

1. Step 1: Recall syntax for calling parent constructor

   In C#, to call a parent constructor, use : base() after the child constructor signature.

2. Step 2: Match correct syntax

   public Child() : base() { } uses public Child() : base() { }, which is the correct syntax.

3. Final Answer:

   public Child() : base() { } -> Option A

4. Quick Check:

   Parent constructor call = : base() [OK]

Hint: Use colon and base() after constructor name [OK]

Common Mistakes:

• Omitting colon before base()
• Using base() inside constructor body incorrectly
• Using wrong keyword like parent()

3. What will be the output of the following code?

class Parent {
    public virtual void Show() {
        Console.WriteLine("Parent Show");
    }
}
class Child : Parent {
    public override void Show() {
        base.Show();
        Console.WriteLine("Child Show");
    }
}

var obj = new Child();
obj.Show();

medium

A. Child Show

B. Parent Show

C. Compilation error

D. Parent Show\nChild Show

Solution

1. Step 1: Understand method overriding and base call

   The Child class overrides Show() and calls base.Show() which runs the Parent version first.

2. Step 2: Trace the output

   First, "Parent Show" is printed from base.Show(), then "Child Show" is printed from the child method.

3. Final Answer:

   Parent Show Child Show -> Option D

4. Quick Check:

   base.Method() runs parent method first [OK]

Hint: base.Method() runs parent method inside override [OK]

Common Mistakes:

• Ignoring base.Show() call
• Expecting only child output
• Thinking code causes error

4. Identify the error in this code snippet:

class Parent {
    public void Display() {
        Console.WriteLine("Parent Display");
    }
}
class Child : Parent {
    public override void Display() {
        base.Display();
        Console.WriteLine("Child Display");
    }
}

medium

A. Cannot override non-virtual method Display() in Parent.

B. base.Display() is invalid syntax.

C. Child class must not call base.Display().

D. No error, code runs fine.

Solution

1. Step 1: Check method overriding rules

   In C#, only methods marked virtual or abstract in the parent can be overridden.

2. Step 2: Analyze the Parent class method

   The Display() method in Parent is not virtual, so override in Child causes a compile error.

3. Final Answer:

   Cannot override non-virtual method Display() in Parent. -> Option A

4. Quick Check:

   Override requires virtual method [OK]

Hint: Only virtual methods can be overridden [OK]

Common Mistakes:

• Trying to override non-virtual method
• Confusing base call syntax
• Assuming override works without virtual

5. Given the classes below, what will be the output?

class A {
    public virtual string GetName() => "A";
}
class B : A {
    public override string GetName() => "B";
}
class C : B {
    public override string GetName() => base.GetName() + "C";
}

var obj = new C();
Console.WriteLine(obj.GetName());

hard

A. AC

B. Compilation error

C. BC

D. C

Solution

1. Step 1: Trace method calls through inheritance

   Class C overrides GetName() and calls base.GetName(), which refers to B's override returning "B".

2. Step 2: Combine returned strings

   C appends "C" to the result from B, so the final string is "B" + "C" = "BC".

3. Final Answer:

   BC -> Option C

4. Quick Check:

   base.GetName() calls immediate parent method [OK]

Hint: base calls immediate parent method, not grandparent [OK]

Common Mistakes:

• Assuming base calls grandparent method
• Ignoring string concatenation
• Expecting only "C" output