C# Linq FirstorDefault Method

The FirstOrDefault() method in C# LINQ returns the first element of a sequence, or a default value if the sequence is empty or no element matches the specified condition. This method is useful when you need to safely retrieve the first element without throwing an exception.

Syntax

Following is the syntax for using FirstOrDefault() −

// Without condition
public static T FirstOrDefault<T>(this IEnumerable<T> source);

// With condition
public static T FirstOrDefault<T>(this IEnumerable<T> source, Func<T, bool> predicate);

Parameters

• source − The sequence to return the first element from.

• predicate (optional) − A function to test each element for a condition.

Return Value

Returns the first element that satisfies the condition or the default value for the type if no such element is found. For reference types, the default is null. For value types like int, it's 0, and for bool, it's false.

Using FirstOrDefault() with Empty Collection

When working with an empty collection, FirstOrDefault() returns the default value for the type instead of throwing an exception −

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

class Demo {
    static void Main() {
        List<double> val = new List<double> { };
        double d = val.FirstOrDefault();
        Console.WriteLine("Default Value = " + d);
        
        if (d == 0.0D) {
            d = 0.1D;
        }
        Console.WriteLine("Default Value changed = " + d);
    }
}

The output of the above code is −

Default Value = 0
Default Value changed = 0.1

Using FirstOrDefault() with Non-Empty Collection

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

class Program {
    static void Main() {
        List<int> numbers = new List<int> { 5, 10, 15, 20, 25 };
        List<string> names = new List<string> { "Alice", "Bob", "Charlie" };
        
        int firstNumber = numbers.FirstOrDefault();
        string firstName = names.FirstOrDefault();
        
        Console.WriteLine("First number: " + firstNumber);
        Console.WriteLine("First name: " + firstName);
    }
}

The output of the above code is −

First number: 5
First name: Alice

Using FirstOrDefault() with Condition

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

class Program {
    static void Main() {
        List<int> numbers = new List<int> { 1, 3, 5, 8, 12, 15 };
        
        int firstEven = numbers.FirstOrDefault(x => x % 2 == 0);
        int firstGreaterThan20 = numbers.FirstOrDefault(x => x > 20);
        
        Console.WriteLine("First even number: " + firstEven);
        Console.WriteLine("First number > 20: " + firstGreaterThan20);
    }
}

The output of the above code is −

First even number: 8
First number > 20: 0

Comparison with First() Method

Conclusion

The FirstOrDefault() method provides a safe way to retrieve the first element from a collection without throwing exceptions. It returns the default value for the type when no matching element is found, making it ideal for scenarios where the absence of elements is a valid case.