Base keyword behavior in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the base keyword do in C#?

The <code>base</code> keyword lets a derived class access members (methods, properties, constructors) of its base class. It helps call or refer to the base class version of a member.

beginner

How do you use <code>base</code> to call a base class constructor?

You use <code>base(arguments)</code> in the derived class constructor's initializer to call a specific base class constructor before running the derived constructor's body.

intermediate

Can base be used to access overridden methods?

Yes. Inside an overridden method, <code>base.MethodName()</code> calls the base class version of that method, bypassing the override.

beginner

What happens if you don't use base when overriding a method?

If you don't use <code>base</code>, the derived class method completely replaces the base class method. The base method is not called unless explicitly invoked with <code>base</code>.

intermediate

Can <code>base</code> be used to access base class fields directly?

No. <code>base</code> cannot be used to access base class fields directly if they are private. It works with accessible members like protected or public fields, properties, and methods.

What is the purpose of the base keyword in C#?

A To access members of the base class from a derived class

B To create a new instance of a class

C To declare a new class

D To override a method

The base keyword is used to access members of the base class from within a derived class.

How do you call a base class constructor with parameters from a derived class?

A Using <code>this(parameters)</code> in the derived constructor

B Using <code>base(parameters)</code> in the derived constructor initializer

C By calling <code>new BaseClass(parameters)</code> inside the derived constructor

D You cannot call base class constructors explicitly

You call a base class constructor by using base(parameters) in the derived class constructor's initializer.

Inside an overridden method, how do you call the base class version of that method?

A Using <code>super.MethodName()</code>

B Using <code>this.MethodName()</code>

C You cannot call the base method once overridden

D Using <code>base.MethodName()</code>

You use base.MethodName() to call the base class version of an overridden method.

Can base be used to access private fields of the base class?

A Only if the derived class is in the same namespace

B Yes, always

C No, private fields are not accessible even with <code>base</code>

D Only if the field is static

Private fields are not accessible from derived classes, even with base.

What happens if you override a method but do not call base.Method() inside it?

A The base method is not executed

B The base method is automatically called

C The program will not compile

D The base method is called twice

If you don't call base.Method(), the base method is not executed when the override runs.

Explain how the base keyword helps when overriding methods in C#.

Think about how to run the original method from the base class when you have a new version in the derived class.

Describe how to use base to call a base class constructor from a derived class constructor.

Remember the syntax after the colon in the derived constructor.

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