c#操作符 ??和First,FirstOrDefault,Single,SingleOrDefault

1、操作符??

?? 运算符称作 null 合并运算符。如果此运算符的左操作数不为 null，则此运算符将返回左操作数；否则返回右操作数。

class NullCoalesce
    {
        static int? GetNullableInt()
        {
            return null;
        }

        static string GetStringValue()
        {
            return null;
        }

        static void Main()
        {
            int? x = null;

            // Set y to the value of x if x is NOT null; otherwise,
            // if x = null, set y to -1.
            int y = x ?? -1;

            // Assign i to return value of the method if the method's result
            // is NOT null; otherwise, if the result is null, set i to the
            // default value of int.
            int i = GetNullableInt() ?? default(int);

            string s = GetStringValue();
            // Display the value of s if s is NOT null; otherwise, 
            // display the string "Unspecified".
            Console.WriteLine(s ?? "Unspecified");
        }
    }

2、First,FirstOrDefault,Single,SingleOrDefault

string[] fruits1 = { "orange" };

string fruit1 = fruits1.SingleOrDefault();

Console.WriteLine(fruit1);

/*
 This code produces the following output:

 orange
*/

 

关于SingleOrDefault详见msdn

https://msdn.microsoft.com/zh-cn/library/bb342451(v=vs.110).aspx