class Pet
{
    public string Name { get; set; }
    public int Age { get; set; }
}

public static void DefaultIfEmptyEx1()
{
    // Create a list of Pet objects.
    List<Pet> pets =
        new List<Pet>{ new Pet { Name="Barley", Age=8 },
                       new Pet { Name="Boots", Age=4 },
                       new Pet { Name="Whiskers", Age=1 } };

    // Call DefaultIfEmtpy() on the collection that Select()
    // returns, so that if the initial list is empty, there
    // will always be at least one item in the returned array.
    string[] names =
        pets.AsQueryable()
        .Select(pet => pet.Name)
        .DefaultIfEmpty()
        .ToArray();

    string first = names[0];
    Console.WriteLine(first);
}

// Create a list of Pet objects.
    List<Pet> pets =
        new List<Pet>{ new Pet { Name="Barley", Age=8 },
                       new Pet { Name="Boots", Age=4 },
                       new Pet { Name="Whiskers", Age=1 } };

    // This query selects only those pets that are 10 or older.
    // In case there are no pets that meet that criteria, call
    // DefaultIfEmpty(). This code passes an (optional) default
    // value to DefaultIfEmpty().
    string[] oldPets =
        pets.AsQueryable()
        .Where(pet => pet.Age >= 10)
        .Select(pet => pet.Name)
        .DefaultIfEmpty("[EMPTY]")
        .ToArray();

    Console.WriteLine("First query: " + oldPets[0]);

    // This query selects only those pets that are 10 or older.
    // This code does not call DefaultIfEmpty().
    string[] oldPets2 =
        pets.AsQueryable()
        .Where(pet => pet.Age >= 10)
        .Select(pet => pet.Name)
        .ToArray();

    // There may be no elements in the array, so directly
    // accessing element 0 may throw an exception.
    try
    {
        Console.WriteLine("Second query: " + oldPets2[0]);
    }
    catch (Exception e)
    {
        Console.WriteLine("Second query: An exception was thrown: " + e.Message);
    }