Reference assignment and shared state in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code?

Consider the following C# code snippet. What will be printed to the console?

C Sharp (C#)

class Box {
    public int Value;
}

Box a = new Box();
a.Value = 5;
Box b = a;
b.Value = 10;
Console.WriteLine(a.Value);

A 10

B 5

C 0

D Compilation error

💡 Hint

Remember that classes are reference types in C#.

Both a and b refer to the same Box object. Changing b.Value changes a.Value as well.

❓ Predict Output

intermediate

What is the output of this code with structs?

What will this C# code print to the console?

C Sharp (C#)

struct Point {
    public int X;
    public int Y;
}

Point p1 = new Point { X = 1, Y = 2 };
Point p2 = p1;
p2.X = 5;
Console.WriteLine(p1.X);

A 5

B 1

C 0

D Compilation error

💡 Hint

Structs are value types in C#.

Assigning p1 to p2 copies the data. Changing p2.X does not affect p1.X.

🔧 Debug

advanced

Why does this code print 0 instead of 10?

Examine the code below. It is intended to print 10, but it prints 0 instead. What is the reason?

C Sharp (C#)

class Counter {
    public int Count;
}

void Increment(Counter c) {
    c = new Counter();
    c.Count = 10;
}

Counter myCounter = new Counter();
Increment(myCounter);
Console.WriteLine(myCounter.Count);

A The Counter class is a struct and is copied by value.

B The Count field is private and cannot be accessed.

C The method assigns a new object to the parameter, which does not affect the original reference.

D The method Increment is never called.

💡 Hint

Think about how reference parameters work in C#.

The parameter c is a copy of the reference. Assigning a new object to c does not change the original myCounter reference.

❓ Predict Output

advanced

What is the output of this code using ref keyword?

What will this C# code print to the console?

C Sharp (C#)

class Number {
    public int Value;
}

void Change(ref Number n) {
    n = new Number();
    n.Value = 20;
}

Number num = new Number();
num.Value = 5;
Change(ref num);
Console.WriteLine(num.Value);

A 20

B 5

C 0

D Compilation error

💡 Hint

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

Using ref means the method can change the caller's variable to point to a new object.

🧠 Conceptual

expert

How many distinct objects exist after this code runs?

Consider this C# code snippet. How many distinct Person objects exist in memory after execution?

C Sharp (C#)

class Person {
    public string Name;
}

Person p1 = new Person { Name = "Alice" };
Person p2 = p1;
p2.Name = "Bob";
p2 = new Person { Name = "Charlie" };

A 1

B 3

C 0

D 2

💡 Hint

Count how many times new objects are created.

First, p1 and p2 point to the same object (Alice). Then p2 is assigned a new object (Charlie). So there are two objects: one with Name "Bob" (original) and one with Name "Charlie".

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