Casting with as and is operators in C Sharp (C#) - Mini Project: Build & Apply

Casting with as and is operators

📖 Scenario: Imagine you have a list of different shapes, but you want to work only with circles. You need to check if each shape is a circle and then get its radius.

🎯 Goal: You will create a list of shapes, check each shape's type using is and as operators, and print the radius of circles.

📋 What You'll Learn

Create a list called shapes with different shape objects

Create a variable called circleCount to count circles

Use a foreach loop with is operator to count circles

Use a foreach loop with as operator to print circle radii

💡 Why This Matters

🌍 Real World

In real programs, you often have collections of different objects and need to work with only some types safely.

💼 Career

Understanding type checking and casting is important for writing safe and clear code in many C# applications.

1

Create the shapes list

Create a list called shapes containing these objects: a Circle with radius 5, a Rectangle with width 4 and height 6, and another Circle with radius 3.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Shape {}

class Circle : Shape {
    public int Radius { get; }
    public Circle(int radius) { Radius = radius; }
}

class Rectangle : Shape {
    public int Width { get; }
    public int Height { get; }
    public Rectangle(int width, int height) { Width = width; Height = height; }
}

class Program {
    static void Main() {
        // Create the shapes list here
        
    }
}

Hint

Use List<Shape> and add the objects inside curly braces.

2

Create a circle counter

Create an integer variable called circleCount and set it to 0 to count how many circles are in the list.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Shape {}

class Circle : Shape {
    public int Radius { get; }
    public Circle(int radius) { Radius = radius; }
}

class Rectangle : Shape {
    public int Width { get; }
    public int Height { get; }
    public Rectangle(int width, int height) { Width = width; Height = height; }
}

class Program {
    static void Main() {
        List shapes = new List {
            new Circle(5),
            new Rectangle(4, 6),
            new Circle(3)
        };
        // Create circleCount variable here
        
    }
}

Hint

Use int circleCount = 0; to start counting circles.

3

Count circles using is operator

Use a foreach loop with variables shape to go through shapes. Inside the loop, use the is operator to check if shape is a Circle. If yes, increase circleCount by 1.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Shape {}

class Circle : Shape {
    public int Radius { get; }
    public Circle(int radius) { Radius = radius; }
}

class Rectangle : Shape {
    public int Width { get; }
    public int Height { get; }
    public Rectangle(int width, int height) { Width = width; Height = height; }
}

class Program {
    static void Main() {
        List shapes = new List {
            new Circle(5),
            new Rectangle(4, 6),
            new Circle(3)
        };
        int circleCount = 0;
        // Count circles using is operator
        
    }
}

Hint

Use if (shape is Circle) inside the loop to check the type.

4

Print circle radii using as operator

Use a foreach loop with variable shape to go through shapes. Inside the loop, use the as operator to cast shape to Circle and store it in a variable called circle. If circle is not null, print "Circle radius: " followed by circle.Radius. Finally, print "Total circles: " followed by circleCount.

C Sharp (C#)

using System;
using System.Collections.Generic;

class Shape {}

class Circle : Shape {
    public int Radius { get; }
    public Circle(int radius) { Radius = radius; }
}

class Rectangle : Shape {
    public int Width { get; }
    public int Height { get; }
    public Rectangle(int width, int height) { Width = width; Height = height; }
}

class Program {
    static void Main() {
        List shapes = new List {
            new Circle(5),
            new Rectangle(4, 6),
            new Circle(3)
        };
        int circleCount = 0;
        foreach (Shape shape in shapes) {
            if (shape is Circle) {
                circleCount++;
            }
        }
        // Print circle radii using as operator
        
    }
}

Hint

Use Circle circle = shape as Circle; and check if circle != null before printing.

Practice

1. What does the as operator do in C#?

easy

A. It tries to cast an object to a type and returns null if it fails.

B. It checks if an object is exactly a certain type and returns true or false.

C. It converts a value type to a string representation.

D. It throws an exception if the cast is invalid.

Solution

1. Step 1: Understand the as operator behavior

   The as operator attempts to cast an object to a specified type but returns null instead of throwing an exception if the cast fails.

2. Step 2: Compare with other options

   It checks if an object is exactly a certain type and returns true or false. describes the is operator, It converts a value type to a string representation. is unrelated, and It throws an exception if the cast is invalid. is incorrect because as does not throw exceptions.

3. Final Answer:

   It tries to cast an object to a type and returns null if it fails. -> Option A

4. Quick Check:

   as returns null on failure [OK]

Hint: Remember: as returns null, no exceptions [OK]

Common Mistakes:

• Confusing as with is
• Thinking as throws exceptions
• Assuming as works with value types

2. Which of the following is the correct syntax to check if an object obj is of type string using the is operator?

easy

A. if (obj == string) { /* code */ }

B. if (obj as string) { /* code */ }

C. if (obj is (string)) { /* code */ }

D. if (obj is string) { /* code */ }

Solution

1. Step 1: Recall is operator syntax

   The correct syntax to check type is if (obj is Type), so if (obj is string) is valid.

2. Step 2: Analyze other options

   if (obj as string) { /* code */ } uses as incorrectly in an if condition, if (obj == string) { /* code */ } compares object to type which is invalid, if (obj is (string)) { /* code */ } has unnecessary parentheses.

3. Final Answer:

   if (obj is string) { /* code */ } -> Option D

4. Quick Check:

   is syntax: obj is Type [OK]

Hint: Use is like: if (obj is Type) [OK]

Common Mistakes:

• Using as in if condition directly
• Comparing object to type with ==
• Adding unnecessary parentheses in is check

3. What is the output of the following code?

object obj = "hello";
string s = obj as string;
Console.WriteLine(s != null ? s.ToUpper() : "null");

medium

A. HELLO

B. null

C. hello

D. Runtime error

Solution

1. Step 1: Analyze the as cast

   The object obj holds a string "hello". Using as string casts it successfully, so s is "hello".

2. Step 2: Evaluate the conditional output

   Since s is not null, s.ToUpper() is called, producing "HELLO".

3. Final Answer:

   HELLO -> Option A

4. Quick Check:

   as cast success means uppercase output [OK]

Hint: If as cast succeeds, use the object; else null [OK]

Common Mistakes:

• Assuming as throws exception on failure
• Forgetting to check for null after as
• Confusing output case sensitivity

4. Identify the error in this code snippet:

object obj = 123;
string s = obj as string;
Console.WriteLine(s.Length);

medium

A. The is operator is used incorrectly.

B. The as cast will fail and s will be null, causing a null reference exception.

C. You cannot use as with value types like int.

D. The code will compile but print 3.

Solution

1. Step 1: Understand as cast with incompatible types

   Since obj holds an int (123), casting it as string returns null.

2. Step 2: Analyze the usage

   Since s is null, calling s.Length causes a null reference exception.

3. Final Answer:

   The as cast will fail and s will be null, causing a null reference exception. -> Option B

4. Quick Check:

   as returns null on failure; check before use [OK]

Hint: Always check for null after as cast before use [OK]

Common Mistakes:

• Assuming is fixes null issues
• Not checking s for null before accessing members
• Thinking as works with value types

5. Given the classes:

class Animal { }
class Dog : Animal { public string Bark() => "Woof!"; }

What is the safest way to call Bark() on an Animal reference a that might be a Dog?

hard

A. if (a is Dog) { Console.WriteLine(a.Bark()); }

B. Console.WriteLine(((Dog)a).Bark());

C. Dog d = a as Dog; if (d != null) Console.WriteLine(d.Bark());

D. Console.WriteLine(a.Bark());

Solution

1. Step 1: Understand safe casting with as

   Using as casts a to Dog safely, returning null if a is not a Dog.

2. Step 2: Check for null before calling Bark()

   Checking d != null ensures Bark() is called only if a is a Dog, avoiding exceptions.

3. Step 3: Compare with other options

   if (a is Dog) { Console.WriteLine(a.Bark()); } checks type but doesn't cast, so a.Bark() is invalid because Animal has no Bark(). Console.WriteLine(((Dog)a).Bark()); casts without check, risking exceptions. Console.WriteLine(a.Bark()); is invalid because Animal has no Bark().

4. Final Answer:

   Dog d = a as Dog; if (d != null) Console.WriteLine(d.Bark()); -> Option C

5. Quick Check:

   Use as + null check for safe cast [OK]

Hint: Use as then check null before method call [OK]

Common Mistakes:

• Casting without checking type first
• Calling methods on base type without override
• Using is then casting again unnecessarily