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

Using Positional Patterns in C#

📖 Scenario: You work at a delivery company that handles different types of packages. Each package has a type and a weight. You want to write a program that checks the package type and weight to decide how to handle it.

🎯 Goal: Build a C# program that uses positional patterns to check package details and print the handling instructions.

📋 What You'll Learn

Create a record type Package with two properties: Type (string) and Weight (int).

Create a variable package with specific values.

Create a variable maxWeight to set the weight limit.

Use a switch expression with positional patterns to decide the handling message.

Print the handling message.

💡 Why This Matters

🌍 Real World

Delivery companies often need to decide how to handle packages based on their type and weight. Using positional patterns helps write clear and concise code for these decisions.

💼 Career

Understanding positional patterns and switch expressions is useful for writing clean, readable C# code in many business applications, especially those involving data classification and decision making.

1

Create the Package record and a package instance

Create a record called Package with two properties: string Type and int Weight. Then create a variable called package and set it to new Package("Fragile", 15).

C Sharp (C#)

// Your code here

Hint

Use the record keyword to create a simple data container with two properties.

2

Add a maxWeight variable

Add an integer variable called maxWeight and set it to 20.

C Sharp (C#)

record Package(string Type, int Weight);

Package package = new Package("Fragile", 15);

// Your code here

Hint

This variable will be used to compare the package weight later.

3

Use a switch expression with positional patterns

Create a string variable called handling and set it using a switch expression on package. Use positional patterns to match Type and Weight as follows: if Type is "Fragile" and Weight is less than or equal to maxWeight, set handling to "Handle with care"; if Type is "Heavy" and Weight is greater than maxWeight, set handling to "Use forklift"; otherwise, set handling to "Standard handling".

C Sharp (C#)

record Package(string Type, int Weight);

Package package = new Package("Fragile", 15);

int maxWeight = 20;

// Your code here

Hint

Use the switch expression with positional patterns and when clauses for conditions.

4

Print the handling message

Write a Console.WriteLine statement to print the handling variable.

C Sharp (C#)

record Package(string Type, int Weight);

Package package = new Package("Fragile", 15);

int maxWeight = 20;

string handling = package switch
{
    ("Fragile", var w) when w  "Handle with care",
    ("Heavy", var w) when w > maxWeight => "Use forklift",
    _ => "Standard handling"
};

// Your code here

Hint

Use Console.WriteLine(handling); to show the result.

Practice

1.

What is the main purpose of positional patterns in C#?

record Point(int X, int Y);
Using case Point(var x, var y): inside a switch statement helps to:

easy

A. Extract values of X and Y from a Point object easily

B. Create a new Point object with given X and Y

C. Change the values of X and Y in the Point object

D. Call a method named Deconstruct on the Point object

Solution

1. Step 1: Understand positional pattern usage

   Positional patterns allow matching an object and extracting its parts in one step, like case Point(var x, var y):.

2. Step 2: Identify what the pattern does with Point

   It extracts the X and Y values from the Point object into variables x and y for use inside the case block.

3. Final Answer:

   Extract values of X and Y from a Point object easily -> Option A

4. Quick Check:

   Positional pattern extracts values [OK]

Hint: Positional patterns extract parts of an object [OK]

Common Mistakes:

• Thinking positional patterns create or modify objects
• Confusing Deconstruct method call with pattern matching
• Assuming positional patterns only check types without extracting

2.

Which of the following is the correct syntax for using a positional pattern with a record Person(string Name, int Age) inside a switch expression?

switch(person) {
    case ???:
        // code
        break;
}

easy

A. Person(var name, var age)

B. Person[name, age]

C. Person{name, age}

D. Person(name: name, age: age)

Solution

1. Step 1: Recall positional pattern syntax

   Positional patterns use parentheses with variables: Type(var1, var2).

2. Step 2: Match syntax to options

   Person(var name, var age) uses parentheses and variables correctly: Person(var name, var age). Others use invalid syntax for positional patterns.

3. Final Answer:

   Person(var name, var age) -> Option A

4. Quick Check:

   Positional pattern syntax uses parentheses [OK]

Hint: Positional patterns always use parentheses, not brackets or braces [OK]

Common Mistakes:

• Using square brackets or curly braces instead of parentheses
• Trying to use named arguments syntax inside patterns
• Omitting var keyword or variable names

3.

What is the output of the following code?

record Point(int X, int Y);

var p = new Point(3, 4);
switch (p)
{
    case Point(0, 0):
        Console.WriteLine("Origin");
        break;
    case Point(var x, var y) when x == y:
        Console.WriteLine("On diagonal");
        break;
    case Point(var x, var y):
        Console.WriteLine($"Point at ({x}, {y})");
        break;
}

medium

A. Origin

B. Point at (3, 4)

C. On diagonal

D. No output

Solution

1. Step 1: Check first case Point(0, 0)

   p is (3,4), so it does not match (0,0).

2. Step 2: Check second case with condition x == y

   x=3, y=4, so x != y; condition fails.

3. Step 3: Third case matches any Point and prints coordinates

   It matches and prints "Point at (3, 4)".

4. Final Answer:

   Point at (3, 4) -> Option B

5. Quick Check:

   Pattern matches last case with output [OK]

Hint: Check cases top to bottom; first match runs [OK]

Common Mistakes:

• Assuming (3,4) matches (0,0)
• Ignoring the when condition in second case
• Thinking no case matches and no output

4.

Identify the error in this code snippet using positional patterns:

record Rectangle(int Width, int Height);

var rect = new Rectangle(5, 10);
switch(rect)
{
    case Rectangle(var w, var h):
        Console.WriteLine($"Width: {w}, Height: {h}");
        break;
    case Rectangle(w, h):
        Console.WriteLine("Invalid pattern");
        break;
}

medium

A. Switch expression cannot use records

B. First case should use named arguments instead of positional pattern

C. Second case uses variables without var keyword, causing a syntax error

D. No error, code runs fine

Solution

1. Step 1: Review positional pattern variable declaration

   Variables in positional patterns must be declared with var or a type.

2. Step 2: Identify error in second case

   Second case uses Rectangle(w, h) without var, which is invalid syntax.

3. Final Answer:

   Second case uses variables without var keyword, causing a syntax error -> Option C

4. Quick Check:

   Variables need var keyword in patterns [OK]

Hint: Always declare variables with var in positional patterns [OK]

Common Mistakes:

• Omitting var keyword for variables in patterns
• Thinking records can't be used in switch
• Confusing named arguments with pattern variables

5.

Given the record Person(string Name, int Age), write a switch expression using positional patterns that returns:

• "Child" if Age < 13
• "Teen" if Age is between 13 and 19 inclusive
• "Adult" otherwise

Which of the following switch expressions correctly implements this?

hard

A. person switch {
Person(_, var age) when age < 13 => "Child",
Person(_, var age) when age > 13 && age <= 19 => "Teen",
Person(_, _) => "Adult"
};

B. person switch {
Person(var name, var age) when age < 13 => "Child",
Person(var name, var age) when age > 13 && age < 19 => "Teen",
_ => "Adult"
};

C. person switch {
Person(var name, var age) when age <= 13 => "Child",
Person(var name, var age) when age >= 13 && age <= 19 => "Teen",
Person(var name, var age) => "Adult"
};

D. person switch {
Person(var name, var age) when age < 13 => "Child",
Person(var name, var age) when age >= 13 && age <= 19 => "Teen",
Person(var name, var age) => "Adult"
};

Solution

1. Step 1: Check age ranges and conditions

   Child: age < 13, Teen: 13 ≤ age ≤ 19, Adult: otherwise.

2. Step 2: Analyze each option's conditions

   person switch { Person(var name, var age) when age < 13 => "Child", Person(var name, var age) when age >= 13 && age <= 19 => "Teen", Person(var name, var age) => "Adult" }; correctly uses age < 13 for Child, age >= 13 && age <= 19 for Teen, and default Adult case. person switch { Person(_, var age) when age < 13 => "Child", Person(_, var age) when age > 13 && age <= 19 => "Teen", Person(_, _) => "Adult" }; misses age=13 (falls to Adult) because Teen condition uses > 13. person switch { Person(var name, var age) when age < 13 => "Child", Person(var name, var age) when age > 13 && age < 19 => "Teen", _ => "Adult" }; excludes age == 13 and 19 for Teen. person switch { Person(var name, var age) when age <= 13 => "Child", Person(var name, var age) when age >= 13 && age <= 19 => "Teen", Person(var name, var age) => "Adult" }; overlaps Child and Teen at age 13.

3. Step 3: Confirm the correct implementation

   person switch { Person(var name, var age) when age < 13 => "Child", Person(var name, var age) when age >= 13 && age <= 19 => "Teen", Person(var name, var age) => "Adult" }; matches all conditions correctly and clearly.

4. Final Answer:

   Option D code correctly implements the age groups -> Option D

5. Quick Check:

   Check age ranges carefully in when clauses [OK]

Hint: Check boundary conditions carefully in when clauses [OK]

Common Mistakes:

• Overlapping or missing age boundaries
• Using _ wildcard instead of variables when needed
• Incorrect logical operators in conditions