Common bugs from reference sharing in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this code with shared list references?

Consider the following C# code where two variables reference the same list. What will be printed?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        List<int> listA = new List<int> {1, 2, 3};
        List<int> listB = listA;
        listB.Add(4);
        Console.WriteLine(string.Join(",", listA));
    }
}

A 1,2,3,4

B 1,2,3

C 4

D Compilation error

💡 Hint

Think about what happens when two variables point to the same list and one changes it.

Both listA and listB point to the same list in memory. Adding 4 via listB changes the original list, so printing listA shows the updated list with 4.

❓ Predict Output

intermediate

What happens when modifying a shared object property?

Look at this C# code where two variables reference the same object. What will be printed?

C Sharp (C#)

using System;

class Person {
    public string Name;
}

class Program {
    static void Main() {
        Person p1 = new Person { Name = "Alice" };
        Person p2 = p1;
        p2.Name = "Bob";
        Console.WriteLine(p1.Name);
    }
}

A Alice

B Bob

C null

D Compilation error

💡 Hint

Remember that objects are reference types in C#.

Both p1 and p2 point to the same Person object. Changing Name via p2 changes the same object, so p1.Name is also "Bob".

❓ Predict Output

advanced

What is the output when modifying a shared array inside a method?

What will this C# program print?

C Sharp (C#)

using System;

class Program {
    static void ModifyArray(int[] arr) {
        arr[0] = 100;
    }

    static void Main() {
        int[] numbers = {1, 2, 3};
        ModifyArray(numbers);
        Console.WriteLine(string.Join(",", numbers));
    }
}

A 1,2,3

B Compilation error

C 0,2,3

D 100,2,3

💡 Hint

Arrays are reference types in C#. What happens when you change an element inside a method?

The array numbers is passed by reference. Changing arr[0] inside ModifyArray changes the original array, so the first element becomes 100.

❓ Predict Output

advanced

What is the output when cloning a list incorrectly?

What will this C# code print?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        List<int> original = new List<int> {1, 2, 3};
        List<int> clone = original;
        clone.Add(4);
        Console.WriteLine(string.Join(",", original));
    }
}

A 1,2,3

B 4

C 1,2,3,4

D Compilation error

💡 Hint

Assigning one list to another copies the reference, not the list itself.

Both original and clone point to the same list. Adding 4 to clone changes the original list, so printing original shows 1,2,3,4.

❓ Predict Output

expert

What is the output when modifying a shared dictionary value?

Consider this C# code. What will be printed?

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program {
    static void Main() {
        var dict1 = new Dictionary<string, List<int>>();
        dict1["key"] = new List<int> {1, 2};
        var dict2 = dict1;
        dict2["key"].Add(3);
        Console.WriteLine(string.Join(",", dict1["key"]));
    }
}

A 1,2,3

B 1,2

C 3

D KeyNotFoundException

💡 Hint

Dictionaries and lists are reference types. What happens when you modify a list inside a shared dictionary?

Both dict1 and dict2 reference the same dictionary. The list at key "key" is also shared. Adding 3 modifies the list, so printing dict1["key"] shows 1,2,3.