DefaultIfEmpty

Created: November-22, 2018

如果 Sequence 不包含任何元素，則 DefaultIfEmpty 用於返回預設元素。此元素可以是型別的預設值或該型別的使用者定義例項。例：

var chars = new List<string>() { "a", "b", "c", "d" };

chars.DefaultIfEmpty("N/A").FirstOrDefault(); // returns "a";

chars.Where(str => str.Length > 1)
     .DefaultIfEmpty("N/A").FirstOrDefault(); // return "N/A"

chars.Where(str => str.Length > 1)
        .DefaultIfEmpty().First(); // returns null;

左連線中的用法：

使用 DefaultIfEmpty，如果找不到匹配項，傳統的 Linq Join 可以返回預設物件。因此充當 SQL 的左連線。例：

var leftSequence = new List<int>() { 99, 100, 5, 20, 102, 105 };
var rightSequence = new List<char>() { 'a', 'b', 'c', 'i', 'd' };

var numbersAsChars = from l in leftSequence
                     join r in rightSequence
                     on l equals (int)r into leftJoin
                     from result in leftJoin.DefaultIfEmpty('?')
                     select new
                     {
                         Number = l,
                         Character = result
                     };

foreach(var item in numbersAsChars)
{
    Console.WriteLine("Num = {0} ** Char = {1}", item.Number, item.Character);
}

ouput: 

Num = 99         Char = c
Num = 100        Char = d
Num = 5          Char = ?
Num = 20         Char = ?
Num = 102        Char = ?
Num = 105        Char = i

在使用 DefaultIfEmpty（沒有指定預設值）並且將導致右序列上沒有匹配項的情況下，必須確保在訪問其屬性之前物件不是 null。否則會產生一個 NullReferenceException。例：

var leftSequence = new List<int> { 1, 2, 5 };
var rightSequence = new List<dynamic>()
    {
        new { Value = 1 },
        new { Value = 2 },
        new { Value = 3 },
        new { Value = 4 },
    };

var numbersAsChars = (from l in leftSequence
                        join r in rightSequence
                        on l equals r.Value into leftJoin
                        from result in leftJoin.DefaultIfEmpty()
                        select new
                        {
                            Left = l,
                            // 5 will not have a matching object in the right so result 
                            // will be equal to null. 
                            // To avoid an error use:
                            //    -  C# 6.0 or above - ?. 
                            //    -  Under           - result == null ? 0 : result.Value
                            Right = result?.Value
                        }).ToList();

左連線中的用法 ：

左連線中的用法：