Stack vs heap mental model in C Sharp (C#) - Practice Questions

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this C# code involving stack and heap?

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

C Sharp (C#)

using System;
class Program {
    static void Main() {
        int x = 10;
        int y = x;
        y = 20;
        Console.WriteLine(x);
    }
}

A 0

B 20

C 10

D Compilation error

💡 Hint

Think about where value types like int are stored and how assignment works.

Variables x and y are value types stored on the stack. Assigning y = x copies the value. Changing y does not affect x. So x remains 10.

❓ Predict Output

intermediate

What is the output when modifying a reference type?

Look at this C# code. What will it print?

C Sharp (C#)

using System;
class Program {
    class Box { public int Value; }
    static void Main() {
        Box a = new Box();
        a.Value = 5;
        Box b = a;
        b.Value = 10;
        Console.WriteLine(a.Value);
    }
}

A 10

B Compilation error

C 0

D 5

💡 Hint

Remember that classes are reference types stored on the heap.

Both a and b point to the same object on the heap. Changing b.Value changes the object, so a.Value is also 10.

🧠 Conceptual

advanced

Which statement best describes stack and heap in C#?

Choose the statement that correctly explains the difference between stack and heap memory in C#.

A Stack and heap are the same memory area but used differently.

B Stack stores value types and method call info; heap stores reference type objects.

C Heap stores local variables and method parameters; stack stores objects created with <code>new</code>.

D Stack stores reference types and heap stores value types.

💡 Hint

Think about where value types and reference types are stored during execution.

In C#, the stack holds value types and method call info (like parameters and local variables). The heap stores objects created by reference types.

🔧 Debug

advanced

Why does this code cause a NullReferenceException?

Examine the code below. Why does it throw a NullReferenceException?

C Sharp (C#)

using System;
class Program {
    class Container { public string Text; }
    static void Main() {
        Container c = null;
        c.Text = "Hello";
        Console.WriteLine(c.Text);
    }
}

A Because <code>c</code> is null and accessing <code>Text</code> causes an error.

B Because <code>Text</code> is not initialized to a string.

C Because <code>Console.WriteLine</code> cannot print strings.

D Because <code>Container</code> is a value type and cannot be null.

💡 Hint

Think about what happens when you try to access a member of a null reference.

c is a reference type variable set to null. Trying to access c.Text causes a NullReferenceException because there is no actual object.

🚀 Application

expert

How many objects are created on the heap after this code runs?

Consider this C# code snippet. How many objects are created on the heap after Main finishes?

C Sharp (C#)

using System;
class Program {
    class Node {
        public int Value;
        public Node Next;
    }
    static void Main() {
        Node first = new Node();
        first.Value = 1;
        Node second = new Node();
        second.Value = 2;
        first.Next = second;
        Node third = second;
    }
}

A 0

B 1

C 3

D 2

💡 Hint

Count how many times new is used to create objects.

Two new Node() calls create two objects on the heap. The variable third just references the existing second object.

Practice

1. In C#, where are value types like int and bool typically stored during program execution?

easy

A. On the heap, because they are objects that need dynamic memory

B. In static memory, shared across all instances

C. In the CPU cache only, not in stack or heap

D. On the stack, because they are stored directly and accessed quickly

Solution

1. Step 1: Understand value type storage

   Value types like int and bool are stored on the stack because they hold data directly and have a short lifetime.

2. Step 2: Contrast with heap storage

   Heap stores objects (reference types), not simple value types, which makes stack the correct place for these.

3. Final Answer:

   On the stack, because they are stored directly and accessed quickly -> Option D

4. Quick Check:

   Value types = stack [OK]

Hint: Value types go on stack; objects go on heap [OK]

Common Mistakes:

• Thinking all data goes on heap
• Confusing value types with reference types
• Assuming stack is for objects only

2. Which of the following C# declarations correctly creates a reference type stored on the heap?

easy

A. string text = "hello";

B. double pi = 3.14;

C. bool flag = true;

D. int number = 5;

Solution

1. Step 1: Identify reference types

   In C#, string is a reference type stored on the heap, while int, bool, and double are value types stored on the stack.

2. Step 2: Match declaration to heap storage

   Only string text = "hello"; creates a reference type object on the heap.

3. Final Answer:

   string text = "hello"; -> Option A

4. Quick Check:

   Reference type = string [OK]

Hint: Strings are reference types stored on heap [OK]

Common Mistakes:

• Assuming all primitives are reference types
• Confusing string as value type
• Ignoring heap for object storage

3. Consider the following C# code:

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

What will be the output and why?

medium

A. "Bob" because p1 and p2 reference the same object on the heap

B. Compilation error due to missing constructor

C. "Alice" because p1 and p2 are separate objects on the heap

D. "Alice" because strings are immutable

Solution

1. Step 1: Understand reference assignment

   Both p1 and p2 point to the same Person object on the heap because p2 = p1; copies the reference.

2. Step 2: Trace the name change effect

   Changing p2.Name to "Bob" changes the same object that p1 references, so p1.Name is also "Bob".

3. Final Answer:

   "Bob" because p1 and p2 reference the same object on the heap -> Option A

4. Quick Check:

   Reference copy = shared object [OK]

Hint: Reference copy means both variables point to same heap object [OK]

Common Mistakes:

• Thinking p1 and p2 are independent copies
• Confusing value and reference assignment
• Ignoring that strings are immutable but object fields can change

4. Identify the error in this C# code snippet related to stack and heap usage:

int[] numbers;
numbers[0] = 10;
Console.WriteLine(numbers[0]);

medium

A. Array elements are stored on stack but accessed incorrectly

B. Array is not initialized before use, causing a runtime error

C. Syntax error: missing semicolon after array declaration

D. No error; code runs and prints 10

Solution

1. Step 1: Check array initialization

   The array numbers is declared but not initialized, so it points to null by default.

2. Step 2: Understand runtime behavior

   Accessing numbers[0] without initialization causes a NullReferenceException at runtime.

3. Final Answer:

   Array is not initialized before use, causing a runtime error -> Option B

4. Quick Check:

   Uninitialized reference = runtime error [OK]

Hint: Always initialize arrays before accessing elements [OK]

Common Mistakes:

• Assuming declaration allocates memory
• Confusing stack and heap for array storage
• Ignoring runtime null reference errors

5. You want to optimize a C# program that creates many small objects rapidly. Which approach best uses the stack and heap to improve performance?

hard

A. Avoid using the stack by allocating everything on the heap

B. Use only reference types to ensure all data is on the heap for consistency

C. Use value types (structs) for small data to store on stack and reduce heap allocations

D. Use global static variables to store all objects

Solution

1. Step 1: Understand stack vs heap performance

   The stack is faster for allocation and deallocation, so using value types (structs) reduces heap pressure.

2. Step 2: Apply optimization strategy

   Using structs for small data stores them on the stack, avoiding costly heap allocations and garbage collection.

3. Final Answer:

   Use value types (structs) for small data to store on stack and reduce heap allocations -> Option C

4. Quick Check:

   Value types on stack = better performance [OK]

Hint: Use structs for small data to leverage fast stack allocation [OK]

Common Mistakes:

• Thinking heap is always better for performance
• Ignoring garbage collection overhead
• Using static variables without reason