First, Single, and their OrDefault variants in C Sharp (C#)

Introduction

These methods help you find one item in a list or collection easily. They make your code simple and clear when you want just one element.

When you want the first item from a list, like the first name in a list of students.

When you expect exactly one item that matches a condition, like finding a user by their unique ID.

When you want to avoid errors if no item is found, by using the OrDefault versions to get a safe default value.

When you want to check if a list has any matching items and get the first or single one without extra code.

When you want to write clean and readable code instead of looping through collections manually.

Syntax

C Sharp (C#)

collection.First();
collection.FirstOrDefault();
collection.Single();
collection.SingleOrDefault();

First() returns the first item and throws an error if none found.

Single() expects exactly one item and throws if zero or more than one found.

Examples

Gets the first name from the list. Throws error if list is empty.

C Sharp (C#)

var firstName = names.First();

Gets the first name or null (default) if list is empty.

C Sharp (C#)

var firstOrDefault = names.FirstOrDefault();

Gets the only user with Id 5. Throws if none or more than one found.

C Sharp (C#)

var singleUser = users.Single(u => u.Id == 5);

Gets the user with Id 5 or null if none found. Throws if more than one found.

C Sharp (C#)

var singleOrDefaultUser = users.SingleOrDefault(u => u.Id == 5);

Sample Program

This program shows how to use First, FirstOrDefault, Single, and SingleOrDefault on a list of numbers. It prints the results to the console.

C Sharp (C#)

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

class Program
{
    static void Main()
    {
        List<int> numbers = new() { 10, 20, 30, 40 };
        
        int first = numbers.First();
        int firstOrDefault = numbers.FirstOrDefault();
        
        int single = numbers.Single(n => n == 20);
        int singleOrDefault = numbers.SingleOrDefault(n => n == 100);

        Console.WriteLine($"First: {first}");
        Console.WriteLine($"FirstOrDefault: {firstOrDefault}");
        Console.WriteLine($"Single (20): {single}");
        Console.WriteLine($"SingleOrDefault (100): {singleOrDefault}");
    }
}

Important Notes

Use First() when you want the first item and expect the list not to be empty.

Use FirstOrDefault() to avoid errors if the list might be empty; it returns default value (like 0 or null).

Single() is strict: it throws if there is not exactly one matching item.

SingleOrDefault() returns default if no match, but throws if more than one match.

Summary

First() gets the first item or throws if none.

FirstOrDefault() gets the first item or default if none.

Single() gets the only matching item or throws if zero or many.

SingleOrDefault() gets the only matching item or default if none, throws if many.

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