First, Single, and their OrDefault variants in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of First() vs FirstOrDefault()

What is the output of the following C# code?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[] numbers = { 2, 4, 6, 8 };
        int first = numbers.First();
        int firstOrDefault = numbers.FirstOrDefault();
        Console.WriteLine($"{first} {firstOrDefault}");
    }
}

A 2 2

B 0 0

C 2 0

D Compilation error

💡 Hint

First() returns the first element; FirstOrDefault() returns first or default if none.

The array has elements, so First() returns 2, and FirstOrDefault() also returns 2 because the first element exists.

❓ Predict Output

intermediate

Behavior of Single() with multiple matches

What happens when you run this C# code?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[] numbers = { 1, 2, 2, 3 };
        int single = numbers.Single(x => x == 2);
        Console.WriteLine(single);
    }
}

A Compilation error

B InvalidOperationException

C 0

D 2

💡 Hint

Single() expects exactly one matching element.

Since there are two elements equal to 2, Single() throws InvalidOperationException due to multiple matches.

❓ Predict Output

advanced

Output of SingleOrDefault() with no matches

What is the output of this C# program?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[] numbers = { 1, 3, 5 };
        int result = numbers.SingleOrDefault(x => x == 2);
        Console.WriteLine(result);
    }
}

A 2

B InvalidOperationException

C Compilation error

D 0

💡 Hint

SingleOrDefault returns default if no element matches.

No element equals 2, so SingleOrDefault returns default(int) which is 0.

❓ Predict Output

advanced

Difference between FirstOrDefault() and SingleOrDefault() with multiple matches

What is the output of this C# code?

C Sharp (C#)

using System;
using System.Linq;

class Program {
    static void Main() {
        int[] numbers = { 5, 5, 5 };
        int firstOrDefault = numbers.FirstOrDefault(x => x == 5);
        Console.WriteLine(firstOrDefault);
        int singleOrDefault = numbers.SingleOrDefault(x => x == 5);
    }
}

A 5 InvalidOperationException

B 5 0

C Compilation error

D 5 5

💡 Hint

SingleOrDefault throws if more than one match exists.

FirstOrDefault returns the first matching element (5). SingleOrDefault throws InvalidOperationException because multiple matches exist.

🧠 Conceptual

expert

Choosing between First() and Single() in LINQ queries

Which statement is TRUE about using First() and Single() in C# LINQ queries?

A Single() returns the first matching element without checking for others.

B First() throws an exception if more than one element matches the condition.

C Single() throws an exception if zero or more than one element matches the condition.

D First() returns default value if no elements match the condition.

💡 Hint

Think about how Single() enforces uniqueness.

Single() expects exactly one matching element; it throws if none or multiple matches are found. First() throws only if no elements match.

Practice

1.

Which method will throw an exception if the collection does not have exactly one matching element?

First(), FirstOrDefault(), Single(), SingleOrDefault()

easy

A. SingleOrDefault()

B. First()

C. FirstOrDefault()

D. Single()

Solution

1. Step 1: Understand Single() behavior

   Single() expects exactly one matching element and throws if there are zero or more than one.

2. Step 2: Compare with other methods

   First() and FirstOrDefault() do not throw if multiple elements exist; SingleOrDefault() throws only if multiple elements exist but returns default if none.

3. Final Answer:

   Single() -> Option D

4. Quick Check:

   Throws on multiple matches = Single() [OK]

Hint: Single() throws if more than one match exists [OK]

Common Mistakes:

• Confusing Single() with First() which returns first match
• Thinking FirstOrDefault() throws on multiple matches
• Assuming SingleOrDefault() never throws

2.

Which of the following is the correct syntax to get the first element or default from a list numbers?

var result = numbers._____();

easy

A. Single

B. FirstOrDefault

C. First

D. SingleOrDefault

Solution

1. Step 1: Identify method for first or default

   FirstOrDefault() returns the first element or default if none found.

2. Step 2: Check syntax correctness

   numbers.FirstOrDefault() is valid syntax to get first or default.

3. Final Answer:

   FirstOrDefault -> Option B

4. Quick Check:

   FirstOrDefault() syntax correct for first or default [OK]

Hint: Use FirstOrDefault() to safely get first or default [OK]

Common Mistakes:

• Using Single() which throws if multiple elements
• Using First() which throws if empty
• Using SingleOrDefault() which expects single element

3.

What will be the output of this code?

var list = new List<int> { 5, 10, 15 };
var result = list.SingleOrDefault(x => x == 10);
Console.WriteLine(result);

medium

A. 10

B. 0

C. Throws exception

D. null

Solution

1. Step 1: Understand SingleOrDefault with predicate

   SingleOrDefault returns the only element matching predicate or default if none, throws if multiple.

2. Step 2: Check list and predicate

   List has one element equal to 10, so SingleOrDefault returns 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   SingleOrDefault returns single matching element = 10 [OK]

Hint: SingleOrDefault returns single match or default, throws if many [OK]

Common Mistakes:

• Expecting 0 as default when element exists
• Thinking it throws because of multiple elements in list
• Confusing with FirstOrDefault behavior

4.

Identify the error in this code snippet:

var items = new List<string> { "apple", "banana", "apple" };
var singleItem = items.Single(x => x == "apple");
Console.WriteLine(singleItem);

medium

A. Returns "apple" without error

B. Returns null because of duplicates

C. Throws InvalidOperationException due to multiple matches

D. Syntax error in lambda expression

Solution

1. Step 1: Analyze Single() with multiple matches

   Single() throws InvalidOperationException if more than one element matches the predicate.

2. Step 2: Check list contents

   List has two "apple" strings, so Single() throws exception.

3. Final Answer:

   Throws InvalidOperationException due to multiple matches -> Option C

4. Quick Check:

   Single() throws on multiple matches [OK]

Hint: Single() throws if more than one match found [OK]

Common Mistakes:

• Expecting Single() to return first match
• Thinking it returns null on duplicates
• Assuming syntax error in lambda

5.

You have a list of users and want to get the only user with the username "admin" or null if none exists. Which method should you use to avoid exceptions if there are no or multiple admins?

hard

A. FirstOrDefault()

B. SingleOrDefault() with try-catch

C. Single()

D. First()

Solution

1. Step 1: Understand requirements

   We want the only user named "admin" or null if none, but avoid exceptions if multiple exist.

2. Step 2: Evaluate methods

   SingleOrDefault() throws if multiple matches, so risky. FirstOrDefault() returns first or null safely even if multiple exist.

3. Final Answer:

   FirstOrDefault() -> Option A

4. Quick Check:

   FirstOrDefault() safely returns first or null without exceptions [OK]

Hint: Use FirstOrDefault() to avoid exceptions on multiple matches [OK]

Common Mistakes:

• Using SingleOrDefault() which throws on multiple matches
• Using Single() which throws on zero or multiple matches
• Using First() which throws if none found