Foreach loop over collections in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of foreach loop over array

What is the output of the following C# code?

C Sharp (C#)

string[] fruits = {"apple", "banana", "cherry"};
foreach (var fruit in fruits)
{
    Console.Write(fruit[0]);
}

A abc

B applebananacherry

C Syntax error

D a b c

💡 Hint

Look at what character of each string is printed inside the loop.

The loop prints the first character of each fruit string without spaces, so output is 'abc'.

❓ Predict Output

intermediate

Foreach loop over Dictionary keys

What will be printed by this C# code?

C Sharp (C#)

var dict = new Dictionary<int, string> {{1, "one"}, {2, "two"}};
foreach (var key in dict.Keys)
{
    Console.Write(key + " ");
}

A KeyValuePair<int,string> KeyValuePair<int,string>

B one two

C Compilation error

D 1 2

💡 Hint

The loop iterates over the keys, not the values.

dict.Keys returns the keys 1 and 2, which are printed with a space.

🔧 Debug

advanced

Identify the runtime error in foreach loop

What error will this code cause when run?

C Sharp (C#)

List<int> numbers = null;
foreach (var num in numbers)
{
    Console.Write(num);
}

A InvalidOperationException

B NullReferenceException

C No error, prints nothing

D Syntax error

💡 Hint

Consider what happens if you try to loop over a null list.

The variable 'numbers' is null, so foreach throws NullReferenceException at runtime.

❓ Predict Output

advanced

Foreach loop with modification inside loop

What is the output of this C# code?

C Sharp (C#)

var list = new List<int> {1, 2, 3};
foreach (var item in list)
{
    Console.Write(item);
    list.Add(item + 3);
    if (list.Count > 6) break;
}

A 1234

B 123456

C InvalidOperationException

D 123

💡 Hint

Modifying a collection while iterating it causes an error.

Adding items to the list during foreach causes InvalidOperationException at runtime.

🧠 Conceptual

expert

Foreach loop behavior with custom collection

Given a custom collection implementing IEnumerable that yields numbers 1 to 3, what will this code print?

foreach (var x in customCollection)
{
    Console.Write(x);
}

C Sharp (C#)

using System.Collections;
using System.Collections.Generic;

public class CustomCollection : IEnumerable<int>
{
    public IEnumerator<int> GetEnumerator()
    {
        yield return 1;
        yield return 2;
        yield return 3;
    }
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

var customCollection = new CustomCollection();
foreach (var x in customCollection)
{
    Console.Write(x);
}

A 123

B 1 2 3

C Compilation error

D Runtime error

💡 Hint

The custom collection yields numbers 1, 2, and 3 sequentially.

The foreach prints each yielded number without spaces, so output is '123'.