Passing reference types to methods in C Sharp (C#) - Mini Project: Build & Apply

Passing reference types to methods

📖 Scenario: Imagine you have a simple program that manages a book's details. You want to change the book's title by passing the book object to a method.

🎯 Goal: Learn how to pass a reference type (an object) to a method and modify its properties inside that method.

📋 What You'll Learn

Create a class called Book with a public string property Title

Create an instance of Book with the title "Old Title"

Create a method called ChangeTitle that takes a Book object and changes its Title to "New Title"

Call the ChangeTitle method passing the book instance

Print the book's title after calling the method

💡 Why This Matters

🌍 Real World

In real programs, you often pass objects to methods to update their data, like updating a user's profile or changing settings.

💼 Career

Understanding how reference types work when passed to methods is essential for writing correct and efficient C# code in software development.

1

Create the Book class and a book instance

Create a class called Book with a public string property Title. Then create a variable called myBook and set it to a new Book object with Title set to "Old Title".

C Sharp (C#)

// Define the Book class and create myBook with Title "Old Title"
// Your code here

Hint

Use public string Title { get; set; } inside the class. Create myBook with new Book { Title = "Old Title" }.

2

Create the ChangeTitle method

Create a method called ChangeTitle that takes a Book object parameter named book and sets its Title property to "New Title".

C Sharp (C#)

public class Book
{
    public string Title { get; set; }
}

Book myBook = new Book { Title = "Old Title" };

// Create ChangeTitle method below
// Your code here

Hint

Define void ChangeTitle(Book book) and inside set book.Title = "New Title";.

3

Call the ChangeTitle method with myBook

Call the ChangeTitle method and pass the myBook object as an argument.

C Sharp (C#)

public class Book
{
    public string Title { get; set; }
}

Book myBook = new Book { Title = "Old Title" };

void ChangeTitle(Book book)
{
    book.Title = "New Title";
}

// Call ChangeTitle with myBook
// Your code here

Hint

Simply write ChangeTitle(myBook); to call the method.

4

Print the updated book title

Write a Console.WriteLine statement to print the Title property of myBook.

C Sharp (C#)

public class Book
{
    public string Title { get; set; }
}

Book myBook = new Book { Title = "Old Title" };

void ChangeTitle(Book book)
{
    book.Title = "New Title";
}

ChangeTitle(myBook);

// Print the title of myBook
// Your code here

Hint

Use Console.WriteLine(myBook.Title); to print the updated title.

Practice

1. What happens when you pass a reference type object to a method in C# without using ref keyword?

easy

A. The method can modify the object's properties, affecting the original object.

B. The method cannot modify the object's properties at all.

C. The method receives a copy of the object and changes do not affect the original.

D. The method can reassign the original reference to a new object automatically.

Solution

1. Step 1: Understand passing reference types

   When a reference type is passed to a method, the method receives a copy of the reference, pointing to the same object.

2. Step 2: Effect on object properties

   Modifying the object's properties inside the method changes the original object because both references point to the same memory.

3. Final Answer:

   The method can modify the object's properties, affecting the original object. -> Option A

4. Quick Check:

   Reference type properties change original [OK]

Hint: Reference types allow property changes to affect original object [OK]

Common Mistakes:

• Thinking the object is copied entirely
• Confusing reference and value types
• Assuming reassignment changes original reference

2. Which of the following method signatures correctly allows a method to reassign a reference type parameter so the caller sees the new object?

easy

A. void UpdateObject(MyClass obj) { obj = new MyClass(); }

B. void UpdateObject(ref MyClass obj) { obj = new MyClass(); }

C. void UpdateObject(out MyClass obj) { obj = new MyClass(); }

D. void UpdateObject(in MyClass obj) { obj = new MyClass(); }

Solution

1. Step 1: Understand parameter passing keywords

   The ref keyword allows the method to reassign the caller's reference variable.

2. Step 2: Analyze each signature

   void UpdateObject(ref MyClass obj) { obj = new MyClass(); } uses ref and assigns a new object, so the caller's reference changes. void UpdateObject(MyClass obj) { obj = new MyClass(); } reassigns local copy only. void UpdateObject(out MyClass obj) { obj = new MyClass(); } requires assignment but is for output parameters. void UpdateObject(in MyClass obj) { obj = new MyClass(); } is readonly and cannot reassign.

3. Final Answer:

   void UpdateObject(ref MyClass obj) { obj = new MyClass(); } -> Option B

4. Quick Check:

   Use ref to reassign reference type parameter [OK]

Hint: Use ref keyword to let method reassign caller's reference [OK]

Common Mistakes:

• Using no keyword and expecting reassignment to affect caller
• Confusing out and ref keywords
• Trying to reassign with in parameter

3. What is the output of the following C# code?

class Person {
  public string Name;
}

void ChangeName(Person p) {
  p.Name = "Alice";
  p = new Person();
  p.Name = "Bob";
}

Person person = new Person();
person.Name = "John";
ChangeName(person);
Console.WriteLine(person.Name);

medium

A. John

B. null

C. Bob

D. Alice

Solution

1. Step 1: Trace property change before reassignment

   The method sets p.Name = "Alice", changing the original object's Name to "Alice".

2. Step 2: Reassignment inside method does not affect caller

   Then p is assigned a new Person object with Name "Bob", but this does not affect the original reference outside the method.

3. Final Answer:

   Alice -> Option D

4. Quick Check:

   Property change affects original; reassignment does not [OK]

Hint: Property changes affect original; reassignment inside method does not [OK]

Common Mistakes:

• Thinking reassignment changes original reference
• Expecting output to be 'Bob'
• Ignoring property change before reassignment

4. Identify the error in the following code that tries to update a reference type parameter so the caller sees the new object:

class Data {
  public int Value;
}

void ResetData(Data d) {
  d = new Data();
  d.Value = 0;
}

Data data = new Data();
data.Value = 5;
ResetData(data);
Console.WriteLine(data.Value);

medium

A. The method should use ref keyword to reassign the caller's reference.

B. The method should not create a new object inside.

C. The method should assign d.Value = 5 instead of 0.

D. The method should return the new object instead of void.

Solution

1. Step 1: Understand reassignment effect without ref

   Assigning d = new Data() inside the method only changes the local copy of the reference, not the caller's variable.

2. Step 2: Fix by using ref keyword

   Using ref in the method parameter and call allows the method to reassign the caller's reference to the new object.

3. Final Answer:

   The method should use ref keyword to reassign the caller's reference. -> Option A

4. Quick Check:

   Use ref to reassign reference type parameter [OK]

Hint: Use ref keyword to let method reassign caller's reference [OK]

Common Mistakes:

• Expecting reassignment without ref to affect caller
• Thinking returning new object is required
• Confusing property assignment with reference reassignment

5. You want a method to update a reference type parameter's properties and also optionally replace the whole object with a new one, so the caller sees all changes. Which approach correctly achieves this in C#?

hard

A. Pass the object with out keyword; assign new object inside method but do not modify properties.

B. Pass the object normally and modify properties; reassigning inside method updates caller automatically.

C. Pass the object with ref keyword; modify properties and reassign new object inside method.

D. Pass the object with in keyword; modify properties but cannot reassign the object.

Solution

1. Step 1: Understand property modification vs reassignment

   Modifying properties affects original object without ref, but reassignment does not affect caller unless ref is used.

2. Step 2: Use ref to allow both property changes and reassignment

   Passing with ref lets method modify properties and replace the object so caller sees all changes.

3. Final Answer:

   Pass the object with ref keyword; modify properties and reassign new object inside method. -> Option C

4. Quick Check:

   Use ref to modify and reassign reference type parameter [OK]

Hint: Use ref to modify properties and reassign object in method [OK]

Common Mistakes:

• Assuming normal pass allows reassignment to affect caller
• Using out when input object is needed
• Using in which forbids reassignment