Why With expressions for immutable copies in C Sharp (C#)? - Purpose & Use Cases

The Big Idea

What if you could update data without rewriting everything or risking mistakes?

The Scenario

Imagine you have a person object with many details, and you want to create a new person just like the first but with one small change, like a new address.

Doing this manually means copying every detail by hand, which is tiring and easy to mess up.

The Problem

Manually copying all properties is slow and error-prone.

You might forget to copy some fields or accidentally change the original object.

This makes your code messy and hard to maintain.

The Solution

With expressions let you create a new object by copying an existing one and changing only what you want.

This keeps your objects immutable (unchanged) and your code clean and safe.

Before vs After

✗ Before

var newPerson = new Person { Name = oldPerson.Name, Age = oldPerson.Age, Address = "New Address" };

✓ After

var newPerson = oldPerson with { Address = "New Address" };

What It Enables

It enables easy, safe updates to data without changing the original, making your programs more reliable and easier to understand.

Real Life Example

Think of a contact list app where you want to update a person's phone number without losing their other info or accidentally changing the original contact.

Key Takeaways

Manual copying is slow and risky.

With expressions create new objects by changing only what you want.

This keeps data safe and code simple.

Practice

1. What does the with expression do in C# when used with records?

easy

A. Creates a new record by copying an existing one and changing specified properties

B. Modifies the original record directly

C. Deletes the original record and creates a new one

D. Copies the record without allowing any changes

Solution

1. Step 1: Understand the purpose of with expression

   The with expression is designed to create a new object by copying an existing one but allows changing some properties.

2. Step 2: Compare behavior with original object

   The original object remains unchanged; only the new object has the updated properties.

3. Final Answer:

   Creates a new record by copying an existing one and changing specified properties -> Option A

4. Quick Check:

   With expression = new copy with changes [OK]

Hint: Remember: with creates a new object, original stays same [OK]

Common Mistakes:

• Thinking with modifies the original object
• Assuming with deletes the original object
• Believing with copies without allowing changes

2. Which of the following is the correct syntax to create a new record person2 from person1 with a changed Age property using a with expression?

easy

A. var person2 = person1.Age = 30 with;

B. var person2 = with person1 { Age = 30 };

C. var person2 = person1 { with Age = 30 };

D. var person2 = person1 with { Age = 30 };

Solution

1. Step 1: Recall the correct syntax for with expressions

   The correct syntax is: var newRecord = oldRecord with { Property = value };

2. Step 2: Match the syntax with options

   var person2 = person1 with { Age = 30 }; matches the correct syntax exactly, others have misplaced keywords or incorrect order.

3. Final Answer:

   var person2 = person1 with { Age = 30 }; -> Option D

4. Quick Check:

   Correct syntax = var person2 = person1 with { Age = 30 }; [OK]

Hint: with comes after the variable, then braces with changes [OK]

Common Mistakes:

• Placing 'with' before the variable
• Using incorrect braces or order
• Trying to assign property outside braces

3. Given the record declaration and code below, what is the output?

public record Person(string Name, int Age);

var person1 = new Person("Alice", 25);
var person2 = person1 with { Age = 30 };
Console.WriteLine(person1.Age);
Console.WriteLine(person2.Age);

medium

A. 25
25

B. 30
30

C. 25
30

D. 30
25

Solution

1. Step 1: Understand the with expression effect on Age

   person2 is a new record copied from person1 but with Age changed to 30. person1 remains unchanged with Age 25.

2. Step 2: Determine the printed values

   Printing person1.Age outputs 25, printing person2.Age outputs 30.

3. Final Answer:

   25 30 -> Option C

4. Quick Check:

   Original Age = 25, New Age = 30 [OK]

Hint: Original stays same, new copy has changed property [OK]

Common Mistakes:

• Assuming original object changes
• Mixing up which object's property is printed first
• Expecting both to have same Age

4. Identify the error in the following code using with expressions:

public record Point(int X, int Y);

var p1 = new Point(5, 10);
var p2 = p1 with (X = 7);

medium

A. Using parentheses instead of braces in with expression

B. Cannot use with expression on records

C. Missing semicolon after record declaration

D. Cannot change properties in with expression

Solution

1. Step 1: Check syntax of with expression

   The with expression requires braces { } around property assignments, not parentheses ( ).

2. Step 2: Identify the error in code

   Code uses with (X = 7) which is invalid syntax; it should be with { X = 7 }.

3. Final Answer:

   Using parentheses instead of braces in with expression -> Option A

4. Quick Check:

   with uses braces, not parentheses [OK]

Hint: Use braces { } after with, not parentheses ( ) [OK]

Common Mistakes:

• Using parentheses instead of braces
• Thinking with can't change properties
• Assuming with is not allowed on records

5. Consider the following records and code. What will be the output?

public record Address(string City, string Country);
public record Person(string Name, int Age, Address Address);

var addr1 = new Address("Paris", "France");
var person1 = new Person("Bob", 40, addr1);
var person2 = person1 with { Address = person1.Address with { City = "Lyon" } };

Console.WriteLine(person1.Address.City);
Console.WriteLine(person2.Address.City);

hard

A. Lyon
Lyon

B. Paris
Lyon

C. Paris
Paris

D. Lyon
Paris

Solution

1. Step 1: Understand nested with expressions

   person2 is a new Person copied from person1, but its Address property is also copied and changed with a new City value "Lyon".

2. Step 2: Determine original and new city values

   person1.Address.City remains "Paris" because original objects are immutable. person2.Address.City is "Lyon" due to the nested with expression.

3. Final Answer:

   Paris Lyon -> Option B

4. Quick Check:

   Nested with copies inner object with changes [OK]

Hint: Use nested with to copy and change inner objects immutably [OK]

Common Mistakes:

• Assuming inner object changes original
• Expecting both cities to be same
• Not using nested with for inner property change