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.