Why Property patterns in C Sharp (C#)? - Purpose & Use Cases

The Big Idea

What if you could describe exactly what you want from an object in one clean line of code?

The Scenario

Imagine you have a list of objects representing people, and you want to find those who live in a specific city and are above a certain age. Without property patterns, you must write multiple nested if-statements to check each property manually.

The Problem

This manual approach quickly becomes messy and hard to read. It's easy to make mistakes, like forgetting a condition or mixing up properties. Also, adding more conditions means more nested code, which is painful to maintain.

The Solution

Property patterns let you check multiple properties of an object in a clean, readable way. You can write conditions that look like natural sentences, making your code easier to understand and less error-prone.

Before vs After

✗ Before

if (person != null && person.Address != null && person.Address.City == "Seattle" && person.Age > 30) { /* do something */ }

✓ After

if (person is { Address: { City: "Seattle" }, Age: > 30 }) { /* do something */ }

What It Enables

It enables writing clear, concise checks on object properties that feel like describing what you want, not how to find it.

Real Life Example

Filtering a list of customers to find those who live in a certain city and meet age criteria becomes simple and readable, improving code quality in business applications.

Key Takeaways

Manual property checks get complicated and error-prone.

Property patterns simplify checking multiple properties at once.

Code becomes easier to read and maintain.

Practice

1. What is the main purpose of using property patterns in C#?

easy

A. To declare new properties in a class

B. To create new objects with default property values

C. To check multiple properties of an object in a clear and concise way

D. To override methods based on property values

Solution

1. Step 1: Understand what property patterns do

   Property patterns allow checking the values of properties inside an object directly in conditions.

2. Step 2: Identify the main use

   This helps write clearer and simpler code when you want to check multiple properties without nested ifs.

3. Final Answer:

   To check multiple properties of an object in a clear and concise way -> Option C

4. Quick Check:

   Property patterns simplify property checks = B [OK]

Hint: Property patterns simplify checking object properties [OK]

Common Mistakes:

• Confusing property patterns with property declaration
• Thinking property patterns create objects
• Assuming property patterns override methods

2. Which of the following is the correct syntax to use a property pattern in a switch expression for an object person with a property Age?

easy

A. case (Age > 18):

B. case { Age: > 18 }:

C. case Age > 18:

D. case person.Age > 18:

Solution

1. Step 1: Recall property pattern syntax

   Property patterns use curly braces with property name and condition inside, like { Age: > 18 }.

2. Step 2: Match with options

   case { Age: > 18 }: matches this syntax exactly, others use invalid or incorrect forms.

3. Final Answer:

   case { Age: > 18 }: -> Option B

4. Quick Check:

   Property pattern syntax uses braces = A [OK]

Hint: Use braces with property name and condition for property patterns [OK]

Common Mistakes:

• Using parentheses instead of braces
• Writing property condition without braces
• Referencing object name inside case

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

record Person(string Name, int Age);

Person p = new("Alice", 25);

string result = p switch
{
    { Age: < 18 } => "Minor",
    { Age: >= 18 and < 65 } => "Adult",
    _ => "Senior"
};

Console.WriteLine(result);

medium

A. Adult

B. Minor

C. Senior

D. Compilation error

Solution

1. Step 1: Check the Age property of the Person object

   The person has Age = 25.

2. Step 2: Match Age with switch cases

   25 is not less than 18, so first case fails. 25 is >= 18 and < 65, so second case matches and returns "Adult".

3. Final Answer:

   Adult -> Option A

4. Quick Check:

   Age 25 fits adult range = A [OK]

Hint: Match property values carefully in switch cases [OK]

Common Mistakes:

• Confusing range conditions
• Assuming default case runs first
• Ignoring property pattern syntax

4. Identify the error in the following code using property patterns:

record Product(string Name, decimal Price);

Product p = new("Book", 15.99m);

if (p is { Price > 10 })
{
    Console.WriteLine("Expensive");
}

medium

A. The if statement requires parentheses around the pattern

B. The decimal literal should not have 'm' suffix

C. The record declaration is missing a semicolon

D. The property pattern syntax is incorrect; it should be { Price: > 10 }

Solution

1. Step 1: Check property pattern syntax

   Property patterns require a colon between property name and condition, like { Price: > 10 }.

2. Step 2: Identify the error in the code

   The code uses { Price > 10 } which is invalid syntax.

3. Final Answer:

   The property pattern syntax is incorrect; it should be { Price: > 10 } -> Option D

4. Quick Check:

   Property patterns need colon between property and condition = D [OK]

Hint: Use colon between property and condition in patterns [OK]

Common Mistakes:

• Omitting colon in property pattern
• Misunderstanding decimal literal suffix
• Adding unnecessary parentheses

5. Given a list of Person objects, how can you use property patterns in a switch expression to classify each person as "Child" if Age < 13, "Teen" if Age between 13 and 19, "Adult" if Age between 20 and 64, and "Senior" otherwise? Which code snippet correctly implements this?

hard

A. persons.Select(p => p switch { { Age: < 13 } => "Child", { Age: >= 13 and <= 19 } => "Teen", { Age: >= 20 and <= 64 } => "Adult", _ => "Senior" })

B. persons.Select(p => p switch { Age < 13 => "Child", Age >= 13 and Age <= 19 => "Teen", Age >= 20 and Age <= 64 => "Adult", _ => "Senior" })

C. persons.Select(p => p switch { { Age < 13 } => "Child", { Age >= 13 and Age <= 19 } => "Teen", { Age >= 20 and Age <= 64 } => "Adult", _ => "Senior" })

D. persons.Select(p => p switch { { Age: < 13 } => "Child", { Age: > 13 and < 20 } => "Teen", { Age: >= 20 and < 65 } => "Adult", _ => "Senior" })

Solution

1. Step 1: Understand correct property pattern syntax with ranges

   Property patterns use colon and conditions like { Age: < 13 } and combine with and for ranges.

2. Step 2: Check each option for correct syntax and ranges

   persons.Select(p => p switch { { Age: < 13 } => "Child", { Age: >= 13 and <= 19 } => "Teen", { Age: >= 20 and <= 64 } => "Adult", _ => "Senior" }) uses correct syntax and inclusive ranges matching the problem. persons.Select(p => p switch { Age < 13 => "Child", Age >= 13 and Age <= 19 => "Teen", Age >= 20 and Age <= 64 => "Adult", _ => "Senior" }) lacks braces and colons. persons.Select(p => p switch { { Age < 13 } => "Child", { Age >= 13 and Age <= 19 } => "Teen", { Age >= 20 and Age <= 64 } => "Adult", _ => "Senior" }) misplaces comparison operators inside braces. persons.Select(p => p switch { { Age: < 13 } => "Child", { Age: > 13 and < 20 } => "Teen", { Age: >= 20 and < 65 } => "Adult", _ => "Senior" }) uses exclusive Teen range (should include 13 and 19).

3. Final Answer:

   persons.Select(p => p switch { { Age: < 13 } => "Child", { Age: >= 13 and <= 19 } => "Teen", { Age: >= 20 and <= 64 } => "Adult", _ => "Senior" }) -> Option A

4. Quick Check:

   Correct property pattern syntax with proper ranges = C [OK]

Hint: Use braces and colon with 'and' for combined property conditions [OK]

Common Mistakes:

• Omitting braces or colon in property patterns
• Using incorrect range operators or missing boundaries
• Mixing syntax from other languages