Foreach loop over collections in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of a foreach loop in C#?

A foreach loop is used to iterate over each element in a collection, such as an array or list, without needing to manage the loop counter manually.

beginner

How does a foreach loop differ from a for loop?

A foreach loop automatically accesses each element in a collection one by one, while a for loop requires manual control of the index to access elements.

intermediate

Can you modify the elements of a collection inside a foreach loop in C#?

No, you cannot modify the elements directly inside a foreach loop because the loop variable is read-only. To modify elements, use a for loop or other methods.

beginner

Write a simple foreach loop to print all items in a string array named fruits.

Example:

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

intermediate

What types of collections can be used with a foreach loop in C#?

Any collection that implements IEnumerable or IEnumerable<T> can be used with foreach, including arrays, lists, dictionaries, and custom collections.

Which keyword is used to iterate over each element in a collection in C#?

A foreach

B while

C do

D switch

The foreach keyword is designed to loop through each element in a collection easily.

What happens if you try to modify the loop variable inside a foreach loop?

A It modifies the original collection element

B It skips the current iteration

C It causes a compile-time error

D It modifies a copy of the element only

The loop variable in a foreach loop is read-only. Attempting to assign a new value to it causes a compile-time error.

Which of these collections can be used with foreach?

A Array

B All of the above

C Dictionary

D List

All these collections implement IEnumerable and can be iterated with foreach.

What is the correct syntax to start a foreach loop over a collection named items?

A foreach (var item in items) { }

B foreach (items in var item) { }

C foreach item in items { }

D foreach (items : item) { }

The correct syntax is foreach (var item in items) { }, where item is the loop variable.

Can foreach be used to iterate over a string in C#?

A Only if converted to a char array first

B No, strings cannot be iterated

C Only in .NET 6 and later

D Yes, because a string is a collection of characters

Strings implement IEnumerable<char>, so you can use foreach to iterate over each character.

Explain how a foreach loop works in C# and when you would use it.

Think about how you would read items from a list one by one.

Describe the limitations of modifying collection elements inside a foreach loop and how to handle such cases.

Consider why the loop variable is a copy and what alternatives exist.

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