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Purpose of "Single" Operator in LINQ

The Single operator in LINQ serves the purpose of retrieving the single element from a sequence (such as a list or an array) that satisfies a given condition. Unlike First or FirstOrDefault which return the first matching element, Single expects there to be exactly one element that meets the condition. If there are multiple matching elements or no matching element, it throws an exception.

Now, let's illustrate this with a C# code example:


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

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

        // Using Single
        int singleElement = numbers.Single(x => x == 3); // Finding the element equal to 3
        Console.WriteLine("Single Element (Single): " + singleElement);

        try
        {
            // Using Single when there are multiple matching elements (it will throw an exception)
            int multipleMatchingElements = numbers.Single(x => x % 2 == 0); // Two even numbers
        }
        catch (InvalidOperationException ex)
        {
            Console.WriteLine("Exception (Single): " + ex.Message);
        }

        try
        {
            // Using Single when there's no match (it will throw an exception)
            int noMatchingElement = numbers.Single(x => x > 10); // No element > 10
        }
        catch (InvalidOperationException ex)
        {
            Console.WriteLine("Exception (Single): " + ex.Message);
        }
    }
}

Output


Single Element (Single): 3
Exception (Single): Sequence contains more than one matching element
Exception (Single): Sequence contains no matching element

In this code example, Single successfully finds the element equal to 3. However, when using Single with a condition that results in multiple matching elements (e.g., finding even numbers), it throws an exception indicating that there's more than one matching element. Similarly, if there's no matching element, it also throws an exception.

The Single operator is useful when you expect exactly one result from a query and want to ensure that condition is met. It's particularly handy in situations where having more or less than one matching element is considered an error.

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