Property patterns in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a property pattern in C#?

A property pattern lets you check if an object has certain properties with specific values or conditions, making it easier to match complex data shapes in a clear way.

beginner

How do you write a simple property pattern to check if a Point object has X equal to 0?

You write: point is { X: 0 }. This means the object 'point' has a property 'X' with value 0.

intermediate

Can property patterns check nested properties? Give an example.

Yes. For example: person is { Address: { City: "Paris" } } checks if 'person' has an 'Address' property whose 'City' is "Paris".

intermediate

What happens if a property in a property pattern is null or missing?

The pattern does not match. Property patterns require the property to exist and meet the condition; otherwise, the match fails.

intermediate

How can you combine multiple property patterns in one check?

You can list multiple properties inside braces, like obj is { Prop1: 5, Prop2: "hello" }, which means both conditions must be true.

What does the pattern obj is { Name: "Alice" } check?

A If obj has any property named Name

B If obj is a string "Alice"

C If obj is null

D If obj has a property Name equal to "Alice"

The pattern checks if the object has a property called Name with the value "Alice".

Can property patterns be used to check nested properties?

A Only for properties of type int

B No, only top-level properties

C Yes, by nesting braces for each property level

D Only if the object is a struct

Property patterns support nested properties by using nested braces, like { Address: { City: "Paris" } }.

What happens if a property in the pattern is missing on the object?

A The pattern does not match

B The pattern matches anyway

C The pattern throws an exception

D The property is ignored

If the property is missing, the pattern does not match because the condition cannot be verified.

How do you check multiple properties in one property pattern?

A List properties separated by commas inside braces

B Use multiple 'is' keywords

C Use && operator between patterns

D Use a switch statement

You list multiple properties inside one set of braces separated by commas, like { Prop1: 1, Prop2: 2 }.

Which C# version introduced property patterns?

A C# 6

B C# 9

C C# 8

D C# 7

Property patterns were introduced in C# 9 to improve pattern matching capabilities.

Explain what a property pattern is and how it helps in checking object properties.

Think about how you check if an object has certain values inside it.

Describe how to write a property pattern that checks multiple properties and nested properties.

Imagine checking a person's address city inside the person object.

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