First, Single, and their OrDefault variants in C Sharp (C#) - Mini Project: Build & Apply

Using First, Single, and Their OrDefault Variants in C#

📖 Scenario: You are working on a simple program that manages a list of book titles in a library. You want to find specific books using different methods that help you get the first match, a single match, or a safe default if no match is found.

🎯 Goal: Build a C# program that uses First, Single, FirstOrDefault, and SingleOrDefault methods to find books in a list safely and correctly.

📋 What You'll Learn

Create a list of book titles with exact values.

Add a search keyword variable.

Use First and Single methods to find books.

Use FirstOrDefault and SingleOrDefault methods to handle cases with no matches.

Print the results clearly.

💡 Why This Matters

🌍 Real World

Finding specific items in lists or collections is common in apps like libraries, stores, or contact lists.

💼 Career

Understanding these LINQ methods helps you write safer and clearer code when searching data in professional C# development.

1

Create a list of book titles

Create a List<string> called books with these exact titles: "C# Basics", "LINQ Fundamentals", "C# Basics", "Advanced C#".

C Sharp (C#)

// Create the list of books below
// Your code here

Hint

Use new List<string> { ... } to create the list with the exact titles.

2

Add a search keyword variable

Add a string variable called searchKeyword and set it to "C# Basics".

C Sharp (C#)

using System;
using System.Collections.Generic;

class Program
{
    static void Main()
    {
        List books = new List { "C# Basics", "LINQ Fundamentals", "C# Basics", "Advanced C#" };
        // Add the searchKeyword variable below
        // Your code here
    }
}

Hint

Declare a string variable with the exact name and value.

3

Use First and Single to find books

Use First to find the first book in books that equals searchKeyword and store it in firstBook. Then use Single to find the single book in books that equals "LINQ Fundamentals" and store it in singleBook.

C Sharp (C#)

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

class Program
{
    static void Main()
    {
        List books = new List { "C# Basics", "LINQ Fundamentals", "C# Basics", "Advanced C#" };
        string searchKeyword = "C# Basics";
        // Use First to find firstBook
        // Use Single to find singleBook
        // Your code here
    }
}

Hint

Use books.First(book => book == searchKeyword) and books.Single(book => book == "LINQ Fundamentals").

4

Use OrDefault variants and print results

Use FirstOrDefault to find a book matching "Nonexistent Book" and store it in firstOrDefaultBook. Use SingleOrDefault to find a book matching "Advanced C#" and store it in singleOrDefaultBook. Then print all four variables: firstBook, singleBook, firstOrDefaultBook, and singleOrDefaultBook.

C Sharp (C#)

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

class Program
{
    static void Main()
    {
        List books = new List { "C# Basics", "LINQ Fundamentals", "C# Basics", "Advanced C#" };
        string searchKeyword = "C# Basics";
        string firstBook = books.First(book => book == searchKeyword);
        string singleBook = books.Single(book => book == "LINQ Fundamentals");
        // Use FirstOrDefault and SingleOrDefault
        // Print all results
        // Your code here
    }
}

Hint

Use FirstOrDefault and SingleOrDefault with the exact search strings. Use Console.WriteLine to print each result. Use ?? "null" to show "null" if the result is empty.

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