Reference assignment and shared state in C Sharp (C#) - Mini Project: Build & Apply

Reference assignment and shared state

📖 Scenario: Imagine you have a simple program that keeps track of a person's name and age using a class. You want to see what happens when you assign one object to another variable and then change the data.

🎯 Goal: You will create a Person class, make two variables that refer to the same object, change the data through one variable, and see how it affects the other.

📋 What You'll Learn

Create a class called Person with two public fields: Name (string) and Age (int).

Create an object of Person with specific values.

Assign this object to another variable to share the reference.

Change the Age using the second variable.

Print the Name and Age from both variables to show shared state.

💡 Why This Matters

🌍 Real World

Understanding reference assignment helps avoid bugs when multiple parts of a program share and modify the same data.

💼 Career

Many programming jobs require knowledge of how objects and references work to write correct and efficient code.

1

Create the Person class and first object

Create a class called Person with public fields Name (string) and Age (int). Then create a variable person1 of type Person and set Name to "Alice" and Age to 30.

C Sharp (C#)

// Your code here

Hint

Define the class with public fields. Then create an object and set its fields.

2

Assign the first object to a second variable

Create a variable person2 and assign it to person1 so both variables refer to the same Person object.

C Sharp (C#)

public class Person
{
    public string Name;
    public int Age;
}

Person person1 = new Person();
person1.Name = "Alice";
person1.Age = 30;

// Your code here

Hint

Just assign person2 to person1 to share the same object.

3

Change the Age using the second variable

Change the Age field of person2 to 35.

C Sharp (C#)

public class Person
{
    public string Name;
    public int Age;
}

Person person1 = new Person();
person1.Name = "Alice";
person1.Age = 30;

Person person2 = person1;

// Your code here

Hint

Use person2.Age = 35; to change the age.

4

Print the Name and Age from both variables

Print the Name and Age of both person1 and person2 using Console.WriteLine to show that both variables reflect the same data.

C Sharp (C#)

public class Person
{
    public string Name;
    public int Age;
}

Person person1 = new Person();
person1.Name = "Alice";
person1.Age = 30;

Person person2 = person1;

person2.Age = 35;

// Your code here

Hint

Use Console.WriteLine with string interpolation to print both variables' fields.

Practice

1. What happens when you assign one reference type variable to another in C#?
ClassA a = new ClassA();
ClassA b = a;

easy

A. Both variables point to the same object in memory.

B. A new copy of the object is created for b.

C. The variables become independent and do not affect each other.

D. The program throws a compile-time error.

Solution

1. Step 1: Understand reference assignment

   Assigning one reference variable to another makes both variables point to the same object in memory.

2. Step 2: Effect of shared reference

   Changes made through either variable affect the same object, so both see the changes.

3. Final Answer:

   Both variables point to the same object in memory. -> Option A

4. Quick Check:

   Reference assignment = shared object [OK]

Hint: Reference variables share the same object after assignment [OK]

Common Mistakes:

• Thinking a new object is created on assignment
• Assuming variables become independent
• Confusing value types with reference types

2. Which of the following is the correct syntax to assign one reference variable to another in C#?

easy

A. int a = b;

B. ClassA a := b;

C. ClassA a == b;

D. ClassA a = new ClassA(); a = b;

Solution

1. Step 1: Identify correct assignment operator

   In C#, the single equals sign (=) is used for assignment, not double equals (==) or :=.

2. Step 2: Check variable types and initialization

   ClassA a = new ClassA(); a = b; correctly declares a reference variable and assigns another reference variable to it.

3. Final Answer:

   ClassA a = new ClassA(); a = b; -> Option D

4. Quick Check:

   Use = for assignment in C# [OK]

Hint: Use single = for assignment, not == or := [OK]

Common Mistakes:

• Using == instead of = for assignment
• Using := which is invalid in C#
• Assigning incompatible types without casting

3. What is the output of the following code?

class Box { public int Value; }
Box box1 = new Box();
box1.Value = 5;
Box box2 = box1;
box2.Value = 10;
Console.WriteLine(box1.Value);

medium

A. 5

B. 0

C. 10

D. Compilation error

Solution

1. Step 1: Understand reference assignment effect

   box2 is assigned to box1, so both point to the same Box object.

2. Step 2: Trace value changes

   Changing box2.Value to 10 changes the shared object, so box1.Value is also 10.

3. Final Answer:

   10 -> Option C

4. Quick Check:

   Shared reference means shared data [OK]

Hint: Changing one reference changes all pointing to same object [OK]

Common Mistakes:

• Assuming box1.Value stays 5 after box2 changes it
• Confusing reference and value types
• Expecting a compile error due to assignment

4. Identify the error in this code snippet:

class Person { public string Name; }
Person p1 = new Person();
p1.Name = "Alice";
Person p2;
p2.Name = "Bob";
p1 = p2;

medium

A. p2 is used before being assigned an object.

B. Cannot assign p2 to p1 because they are different types.

C. Cannot assign string to Name property.

D. No error, code runs fine.

Solution

1. Step 1: Check variable initialization

   p2 is declared but not assigned an object before accessing p2.Name.

2. Step 2: Understand runtime error

   Accessing p2.Name causes a NullReferenceException because p2 is null.

3. Final Answer:

   p2 is used before being assigned an object. -> Option A

4. Quick Check:

   Uninitialized reference causes runtime error [OK]

Hint: Always initialize reference variables before use [OK]

Common Mistakes:

• Assuming p2 has default object automatically
• Confusing compile-time and runtime errors
• Ignoring null reference exceptions

5. Given this code, what will be the output?

class Data { public int Number; }
Data d1 = new Data() { Number = 1 };
Data d2 = new Data() { Number = 2 };
d1 = d2;
d2.Number = 5;
Console.WriteLine(d1.Number);

hard

A. 2

B. 5

C. 1

D. Compilation error

Solution

1. Step 1: Analyze reference assignment

   d1 is assigned to d2, so both point to the same object with Number initially 2.

2. Step 2: Trace value change through d2

   Changing d2.Number to 5 changes the shared object, so d1.Number is also 5.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Reference assignment shares object state [OK]

Hint: After assignment, variables share the same object state [OK]

Common Mistakes:

• Thinking d1.Number remains 1 after assignment
• Confusing value and reference types
• Expecting compile errors on assignment