Type checking patterns in C Sharp (C#) - Mini Project: Build & Apply

Type Checking Patterns in C#

📖 Scenario: Imagine you are building a simple program that processes different shapes. Each shape can be a Circle or a Rectangle. You want to check the type of each shape and calculate its area accordingly.

🎯 Goal: You will create a list of shapes, add a configuration variable for a minimum area threshold, use type checking patterns to calculate areas only for shapes that meet the threshold, and finally print the areas.

📋 What You'll Learn

Create a list called shapes containing exactly two objects: a Circle with radius 3 and a Rectangle with width 4 and height 5.

Create a variable called minArea and set it to 10.

Use a foreach loop with type checking patterns to calculate the area of each shape only if it is a Circle or Rectangle and the area is greater than or equal to minArea. Store the results in a dictionary called areas with the shape type name as the key and the area as the value.

Print the areas dictionary.

💡 Why This Matters

🌍 Real World

Type checking patterns help you write clear and safe code when working with different object types, such as shapes, UI elements, or messages.

💼 Career

Understanding type checking patterns is important for C# developers to handle polymorphism, improve code readability, and avoid errors when processing mixed data.

1

Create the shapes list

Create a list called shapes containing exactly two objects: a Circle with Radius 3 and a Rectangle with Width 4 and Height 5. Use the provided Circle and Rectangle classes.

C Sharp (C#)

public class Circle
{
    public double Radius { get; set; }
    public Circle(double radius) => Radius = radius;
}

public class Rectangle
{
    public double Width { get; set; }
    public double Height { get; set; }
    public Rectangle(double width, double height) => (Width, Height) = (width, height);
}

using System.Collections.Generic;

// Create the list called shapes with the two objects
// Your code here

Need a hint?

Use List<object> to hold different shape objects. Add new Circle(3) and new Rectangle(4, 5) inside the list.

2

Add minimum area threshold

Create a variable called minArea and set it to 10.

C Sharp (C#)

public class Circle
{
    public double Radius { get; set; }
    public Circle(double radius) => Radius = radius;
}

public class Rectangle
{
    public double Width { get; set; }
    public double Height { get; set; }
    public Rectangle(double width, double height) => (Width, Height) = (width, height);
}

using System.Collections.Generic;

List shapes = new List
{
    new Circle(3),
    new Rectangle(4, 5)
};

// Create the minArea variable and set it to 10
// Your code here

Need a hint?

Just create a double variable named minArea and assign it the value 10.

3

Calculate areas using type checking patterns

Use a foreach loop with variables shape to iterate over shapes. Inside the loop, use type checking patterns with is to check if shape is a Circle or Rectangle. Calculate the area for each shape: for Circle, area = π x radius²; for Rectangle, area = width x height. Only add the shape type name and area to a dictionary called areas if the area is greater than or equal to minArea. Use Math.PI for π.

C Sharp (C#)

public class Circle
{
    public double Radius { get; set; }
    public Circle(double radius) => Radius = radius;
}

public class Rectangle
{
    public double Width { get; set; }
    public double Height { get; set; }
    public Rectangle(double width, double height) => (Width, Height) = (width, height);
}

using System.Collections.Generic;

List shapes = new List
{
    new Circle(3),
    new Rectangle(4, 5)
};

double minArea = 10;

// Create the dictionary areas
// Use foreach loop and type checking patterns to calculate areas
// Add to areas only if area >= minArea
// Your code here

Need a hint?

Use foreach (object shape in shapes). Inside, use if (shape is Circle c) and else if (shape is Rectangle r). Calculate area accordingly and add to areas dictionary only if area ≥ minArea.

4

Print the areas dictionary

Use a foreach loop with variables key and value to iterate over areas. Print each shape type and its area in the format: "{key}: {value:F2}" where {value:F2} formats the area to 2 decimal places.

C Sharp (C#)

public class Circle
{
    public double Radius { get; set; }
    public Circle(double radius) => Radius = radius;
}

public class Rectangle
{
    public double Width { get; set; }
    public double Height { get; set; }
    public Rectangle(double width, double height) => (Width, Height) = (width, height);
}

using System;
using System.Collections.Generic;

List shapes = new List
{
    new Circle(3),
    new Rectangle(4, 5)
};

double minArea = 10;

Dictionary areas = new Dictionary();

foreach (object shape in shapes)
{
    if (shape is Circle c)
    {
        double area = Math.PI * c.Radius * c.Radius;
        if (area >= minArea)
            areas["Circle"] = area;
    }
    else if (shape is Rectangle r)
    {
        double area = r.Width * r.Height;
        if (area >= minArea)
            areas["Rectangle"] = area;
    }
}

// Print each shape and area formatted to 2 decimals
// Your code here

Need a hint?

Use foreach (var (key, value) in areas) and Console.WriteLine($"{key}: {value:F2}") to print each shape and its area with two decimals.