SingleOrDefault

This C# method returns an element if a single one is present. With it you receive the single matching element, or the default value if no element is found.

Single

SingleOrDefault has one difference to Single. With Single, an exception is thrown when zero matching elements are found. But with SingleOrDefault, the default value is returned.

SingleOrDefault example

This program allocates an array of 3 integers. Next, the SingleOrDefault method is called. A Predicate is specified to search for a single element equal to 5.

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        int[] array = { 1, 2, 3 };

        // Default is returned if no element found.
        int a = array.SingleOrDefault(element => element == 5);
        Console.WriteLine(a);

        // Value is returned if one is found.
        int b = array.SingleOrDefault(element => element == 1);
        Console.WriteLine(b);

        try
        {
            // Exception is thrown if more than one is found.
            int c = array.SingleOrDefault(element => element >= 1);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.GetType());
        }
    }
}
0
1
System.InvalidOperationException

Single example

This program creates an array of 5 ints. Only one of the elements is greater than 999. Next, Single is called with a Predicate lambda expression.

using System;
using System.Linq;

class Program
{
    static void Main()
    {
        // Find only element > 999.
        int[] array1 = { 1, 3, 1000, 4, 5 };
        int value1 = array1.Single(element => element > 999);

        // Ensure only one element.
        int[] array2 = { 4 };
        int value2 = array2.Single();

        Console.WriteLine(value1);
        Console.WriteLine(value2);

        // See exception when more than one element found.
        try
        {
            int value3 = array1.Single(element => element > 0);
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.GetType());
        }
    }
}
1000
4
System.InvalidOperationException

Single, no match

What happens if no single matching element is found? Single() tries to use a predicate that has 5 matching elements, so an exception is thrown.

Discussion

Is Single a useful method? Unfortunately, methods that can throw an exception in a normal circumstance such as Single may be less useful.

A summary

SingleOrDefault helps ensure zero or one elements exist matching a condition. As with Single, it can be used with no parameters to match all elements.