Record structs in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a record struct in C#?

A record struct is a value type that provides built-in features like immutability, value equality, and concise syntax for defining data containers, introduced in C# 10.

intermediate

How does a record struct differ from a regular struct?

A record struct automatically provides value-based equality, a concise syntax for defining properties, and supports with-expressions for copying with modifications, unlike regular structs which require manual implementation.

intermediate

Can a record struct be mutable or immutable?

By default, record structs are mutable, but you can make them immutable by declaring properties with only getters or using the readonly modifier.

beginner

What feature allows you to create a copy of a record struct with some properties changed?

The with expression lets you create a new record struct by copying an existing one and changing specified properties.

beginner

Write a simple example of a record struct with two properties: int X and int Y.

public record struct Point(int X, int Y);

Which of the following is true about record structs in C#?

A They are reference types with value equality.

B They are value types with built-in value equality.

C They cannot have properties.

D They do not support the with-expression.

Record structs are value types that provide built-in value equality and support with-expressions.

How do you declare a record struct named 'Point' with properties 'X' and 'Y'?

A struct Point record(int X, int Y);

B struct record Point { int X; int Y; }

C record Point struct(int X, int Y);

D record struct Point(int X, int Y);

The correct syntax is 'record struct Point(int X, int Y);' which declares a record struct with two properties.

What does the 'with' expression do for record structs?

A Creates a copy with some properties changed.

B Deletes the record struct instance.

C Converts the record struct to a class.

D Makes the record struct mutable.

The 'with' expression creates a new copy of the record struct with specified properties changed.

Are record structs mutable by default?

A No, they are immutable by default.

B They cannot be mutable.

C Yes, they are mutable by default.

D They are always readonly.

Record structs are mutable by default unless marked readonly or with immutable properties.

Which C# version introduced record structs?

A C# 10

B C# 8

C C# 9

D C# 7

Record structs were introduced in C# 10.

Explain what a record struct is and how it differs from a regular struct in C#.

Think about how record structs help with comparing and copying data.

Describe how you can create a copy of a record struct with one property changed.

Remember the keyword that starts with 'w' used for copying with changes.

Practice

1. What is a key feature of record struct in C#?

easy

A. It provides value-based equality and immutability by default.

B. It allows inheritance from other structs.

C. It can only be used for reference types.

D. It does not support property declarations.

Solution

1. Step 1: Understand record struct purpose

   Record structs combine struct's lightweight nature with record's value-based equality and immutability.

2. Step 2: Analyze options

   It provides value-based equality and immutability by default. correctly states value-based equality and immutability. Options B, C, and D are incorrect because structs cannot inherit, record structs are value types, and they support properties.

3. Final Answer:

   It provides value-based equality and immutability by default. -> Option A

4. Quick Check:

   record struct = value equality + immutability [OK]

Hint: Remember: record structs behave like immutable value containers [OK]

Common Mistakes:

• Thinking record structs support inheritance
• Confusing record structs with classes
• Assuming record structs are reference types

2. Which of the following is the correct syntax to declare a record struct named Point with two properties X and Y of type int?

easy

A. struct record Point { int X; int Y; }

B. record Point struct { int X; int Y; }

C. record struct Point(int X, int Y);

D. record struct Point { int X; int Y; }

Solution

1. Step 1: Recall record struct declaration syntax

   The concise syntax for record structs uses positional parameters: record struct Name(type prop1, type prop2);

2. Step 2: Compare options

   record struct Point(int X, int Y); matches the correct syntax. record struct Point { int X; int Y; } is close but missing parentheses for positional parameters. Options A and C have incorrect order or keywords.

3. Final Answer:

   record struct Point(int X, int Y); -> Option C

4. Quick Check:

   record struct with positional params = record struct Point(int X, int Y); [OK]

Hint: Use parentheses for positional properties in record structs [OK]

Common Mistakes:

• Swapping 'record' and 'struct' keywords
• Omitting parentheses for positional parameters
• Using braces instead of parentheses for positional syntax

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

record struct Point(int X, int Y);
var p1 = new Point(3, 4);
var p2 = new Point(3, 4);
Console.WriteLine(p1 == p2);

medium

A. True

B. False

C. Compilation error

D. Runtime exception

Solution

1. Step 1: Understand equality in record structs

   Record structs provide value-based equality by default, so two instances with same values are equal.

2. Step 2: Analyze the code output

   Both p1 and p2 have X=3 and Y=4, so p1 == p2 evaluates to true.

3. Final Answer:

   True -> Option A

4. Quick Check:

   record struct equality = value equality [OK]

Hint: Record structs compare values, not references [OK]

Common Mistakes:

• Assuming == compares references for structs
• Expecting compilation error for == operator
• Confusing record structs with classes for equality

4. Identify the error in this code snippet:

record struct Rectangle(int Width, int Height);
var rect = new Rectangle(5, 10);
rect.Width = 7;
Console.WriteLine(rect);

medium

A. Rectangle must be a class to allow property assignment.

B. Missing semicolon after record struct declaration.

C. Console.WriteLine cannot print record structs.

D. Cannot assign to 'Width' because it is readonly.

Solution

1. Step 1: Understand immutability of record structs

   Record structs with positional parameters generate readonly properties by default.

2. Step 2: Analyze the assignment line

   Trying to assign rect.Width = 7; causes a compile-time error because Width is readonly.

3. Final Answer:

   Cannot assign to 'Width' because it is readonly. -> Option D

4. Quick Check:

   Positional record struct properties are readonly [OK]

Hint: Positional record struct properties are readonly by default [OK]

Common Mistakes:

• Trying to modify readonly properties
• Confusing record structs with mutable classes
• Ignoring compiler error messages

5. You want to create a lightweight data type to store a 3D point with coordinates X, Y, and Z. You want it to be immutable, support value equality, and print nicely. Which is the best way to declare it using record structs?

hard

A. struct Point3D { public int X; public int Y; public int Z; }

B. record struct Point3D(int X, int Y, int Z);

C. class Point3D { public int X { get; set; } public int Y { get; set; } public int Z { get; set; } }

D. record class Point3D(int X, int Y, int Z);

Solution

1. Step 1: Identify requirements

   Needs lightweight, immutable, value equality, and nice printing.

2. Step 2: Match requirements to options

   record struct Point3D(int X, int Y, int Z); uses record struct with positional parameters, providing all requirements. struct Point3D { public int X; public int Y; public int Z; } is mutable struct without value equality or nice printing. class Point3D { public int X { get; set; } public int Y { get; set; } public int Z { get; set; } } is mutable class, heavier and reference type. record class Point3D(int X, int Y, int Z); is record class, reference type, not lightweight.

3. Final Answer:

   record struct Point3D(int X, int Y, int Z); -> Option B

4. Quick Check:

   record struct = lightweight + immutable + value equality [OK]

Hint: Use record struct with positional params for immutable lightweight data [OK]

Common Mistakes:

• Choosing class instead of struct for lightweight data
• Using mutable fields instead of readonly properties
• Confusing record class with record struct