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'.

Practice

1. What does a foreach loop do in C#?

easy

A. It visits each item in a collection one by one.

B. It creates a new collection from scratch.

C. It only runs once regardless of collection size.

D. It deletes items from a collection while looping.

Solution

1. Step 1: Understand the purpose of foreach

   The foreach loop is designed to go through each element in a collection without manual index handling.

2. Step 2: Compare options to foreach behavior

   Only It visits each item in a collection one by one. correctly describes visiting each item one by one. Other options describe actions foreach does not perform.

3. Final Answer:

   It visits each item in a collection one by one. -> Option A

4. Quick Check:

   foreach loops = visit items one by one [OK]

Hint: Foreach always processes every item in the collection [OK]

Common Mistakes:

• Thinking foreach modifies the collection
• Confusing foreach with creating collections
• Assuming foreach runs only once

2. Which of the following is the correct syntax for a foreach loop over a list of strings named names?

easy

A. foreach (name string in names) { Console.WriteLine(name); }

B. foreach (string name in names) { Console.WriteLine(name); }

C. foreach string name in names { Console.WriteLine(name); }

D. foreach (string name : names) { Console.WriteLine(name); }

Solution

1. Step 1: Recall correct foreach syntax

   The correct syntax uses parentheses with type and variable, then 'in' keyword before collection.

2. Step 2: Check each option

   foreach (string name in names) { Console.WriteLine(name); } matches correct syntax. foreach string name in names { Console.WriteLine(name); } misses parentheses. foreach (name string in names) { Console.WriteLine(name); } swaps type and variable. foreach (string name : names) { Console.WriteLine(name); } uses ':' which is invalid in C# foreach.

3. Final Answer:

   foreach (string name in names) { Console.WriteLine(name); } -> Option B

4. Quick Check:

   foreach syntax = (type variable in collection) [OK]

Hint: Remember: foreach uses (type variable in collection) [OK]

Common Mistakes:

• Omitting parentheses
• Using ':' instead of 'in'
• Swapping variable and type order

3. What will be the output of this code?

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

medium

A. apple banana cherry

B. applebanana cherry

C. apple, banana, cherry

D. apple
banana
cherry

Solution

1. Step 1: Understand the loop behavior

   The foreach loop prints each fruit followed by a space, all on the same line.

2. Step 2: Analyze output format

   Since Console.Write (not WriteLine) is used, output is continuous with spaces between fruits.

3. Final Answer:

   apple banana cherry -> Option A

4. Quick Check:

   foreach prints items with space = apple banana cherry [OK]

Hint: Console.Write prints inline; spaces separate items [OK]

Common Mistakes:

• Assuming Console.WriteLine adds spaces
• Expecting commas between items
• Thinking output is on separate lines

4. Identify the error in this code snippet:

List<int> numbers = new List<int> {1, 2, 3};
foreach (int num in numbers)
{
    numbers.Remove(num);
}

medium

A. Variable 'num' is not declared.

B. Missing semicolon after foreach statement.

C. Cannot modify collection while iterating with foreach.

D. List initialization syntax is incorrect.

Solution

1. Step 1: Understand foreach restrictions

   Modifying a collection (like removing items) while using foreach causes runtime errors.

2. Step 2: Check other options

   Semicolon is not needed after foreach header. Variable 'num' is declared. List initialization is correct.

3. Final Answer:

   Cannot modify collection while iterating with foreach. -> Option C

4. Quick Check:

   Modifying collection in foreach = error [OK]

Hint: Don't change collection inside foreach loop [OK]

Common Mistakes:

• Trying to remove items inside foreach
• Confusing foreach syntax errors
• Ignoring runtime exceptions

5. Given a dictionary Dictionary<string, int> scores, how can you use a foreach loop to print all keys with values greater than 50?

hard

A. foreach (var pair in scores) { Console.WriteLine(pair.Value > 50); }

B. foreach (string key in scores.Values) { if (key > 50) Console.WriteLine(key); }

C. foreach (int value in scores.Keys) { if (value > 50) Console.WriteLine(value); }

D. foreach (var pair in scores) { if (pair.Value > 50) Console.WriteLine(pair.Key); }

Solution

1. Step 1: Understand dictionary iteration

   Iterating a dictionary with foreach gives key-value pairs as 'var pair'.

2. Step 2: Filter and print keys with values > 50

   Check pair.Value > 50, then print pair.Key. foreach (var pair in scores) { if (pair.Value > 50) Console.WriteLine(pair.Key); } does this correctly.

3. Step 3: Analyze incorrect options

   foreach (string key in scores.Values) { if (key > 50) Console.WriteLine(key); } tries to iterate values as strings (wrong). foreach (int value in scores.Keys) { if (value > 50) Console.WriteLine(value); } iterates keys as ints (wrong). foreach (var pair in scores) { Console.WriteLine(pair.Value > 50); } prints booleans, not keys.

4. Final Answer:

   foreach (var pair in scores) { if (pair.Value > 50) Console.WriteLine(pair.Key); } -> Option D

5. Quick Check:

   Use var pair and check Value > 50 [OK]

Hint: Use var pair to access key and value in dictionary [OK]

Common Mistakes:

• Iterating keys as values or vice versa
• Printing booleans instead of keys
• Wrong variable types in foreach