.DefaultIfEmpty()

Replace a collection with a default value

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

class Program
{
    static void Main()
    {
        // Empty list.
        List<int> list = new List<int>();
        var result = list.DefaultIfEmpty();

        // One element in collection with default(int) value.
        foreach (var value in result)
        {
            Console.WriteLine(value);
            // output: 0
        }

        result = list.DefaultIfEmpty(-1);

        // One element in collection with -1 value.
        foreach (var value in result)
        {
            Console.WriteLine(value);
            // output : -1
        }
    }
}

Reference

C# DefaultIfEmpty Method - Dot Net Perls

SO: Sequence contains no elements exception in linq without even using Single