First vs Single

In LINQ, First and Single are methods used to retrieve elements from a sequence, but they behave differently in terms of handling multiple elements and the conditions under which they throw exceptions. Here’s a comparison of First and Single:

First

Use First when you are interested in the first occurrence of an element (or the first element if no condition) and don’t mind if there are multiple matches.

Example

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

class Program
{
    static void Main()
    {
        var numbers = new List<int> { 1, 2, 3, 4, 5 };

        // Get the first number (no predicate)
        var firstNumberone = numbers.First();
        Console.WriteLine("FIrst = "+firstNumberone.ToString());

        // Get the first number greater than 2
        var firstNumberGreaterThanTwo = numbers.First(n => n > 2);
        Console.WriteLine("Second = "+firstNumberGreaterThanTwo.ToString());

        // Throws InvalidOperationException if no element is found.
        var firstNumberfour = numbers.First(n => n ==6); 
        Console.WriteLine("Three = "+firstNumberfour.ToString());
    }
}

Output

Single

Use Single when you expect exactly one matching element and want to ensure that no more than one match exists.

Example

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

class Program
{
    static void Main()
    {
        var singlenumbers = new List<int> { 2 };

        //Gets the single element, throws if there’s not exactly one.
        var firstNumbersingle = singlenumbers.Single();
        Console.WriteLine("FIrst = " + firstNumbersingle.ToString());

        var numbers = new List<int> { 1, 2, 3, 4, 5 };

        //  //Gets the single element, throws if there’s not exactly one.
        var firstNumberfour = numbers.Single(n => n == 5);
        Console.WriteLine("Third = " + firstNumberfour.ToString());

        //Gets the single element, throws if there’s not exactly one.
        var firstNumberone = numbers.Single();
        Console.WriteLine("FIrst = " + firstNumberone.ToString());  // Error

        //Gets the single element, throws if there’s not exactly one.
        var firstNumberGreaterThanTwo = numbers.Single(n => n > 2); 
        Console.WriteLine("Second = " + firstNumberGreaterThanTwo.ToString()); //Error
    }
}

Output

Difference Between First and Single in LINQ