棄元

棄元就是使用下劃線_作為一個占位符，但不占存儲空間。

元組（ValueTuple、Tuple）使用棄元例子。

using System;

namespace ConsoleApp4
{
    class Program
    {
        public static void Main()
        {
            // ValueTuple和Tuple都可以使用棄元
            var (_, _, _, pop1, _, pop2) = QueryCityDataForYears("New York City", 1960, 2010);
           
            Console.WriteLine($"Population change, 1960 to 2010: {pop2 - pop1:N0}");
        }

        private static (string,double,int,int,int,int) QueryCityDataForYears(string name,int year1,int year2)
        {
            int population1 = 0, population2 = 0;
            double area = 0;

            if (name == "New York City")
            {
                area = 468.48;
                if (year1 == 1960)
                {
                    population1 = 7781984;
                }
                if (year2 == 2010)
                {
                    population2 = 8175133;
                }
                return (name, area, year1, population1, year2, population2);
            }
            return ("", 0, 0, 0, 0, 0);
            }     
    }   
}

對象析構，析構函數的參數決定了元組的參數 。

using System;

namespace ConsoleApp4
{
    class Program
    {
        public static void Main()
        {
            var p = new Person("John", "Quincy", "Adams", "Boston", "MA");

            // Deconstruct(out string fname, out string lname, out string city, out string state) 
            var (fName,_,city,_) = p;
          
            Console.WriteLine($"Hello {fName} of {city}!");            
        }

        private static (string,double,int,int,int,int) QueryCityDataForYears(string name,int year1,int year2)
        {
            int population1 = 0, population2 = 0;
            double area = 0;

            if (name == "New York City")
            {
                area = 468.48;
                if (year1 == 1960)
                {
                    population1 = 7781984;
                }
                if (year2 == 2010)
                {
                    population2 = 8175133;
                }
                return (name, area, year1, population1, year2, population2);
            }
            return ("", 0, 0, 0, 0, 0);
            }     
    }

    public class Person
    {
        public string FirstName { get; set; }
        public string MiddleName { get; set; }
        public string LastName { get; set; }
        public string City { get; set; }
        public string State { get; set; }

        public Person(string fname, string mname, string lname,
                      string cityName, string stateName)
        {
            FirstName = fname;
            MiddleName = mname;
            LastName = lname;
            City = cityName;
            State = stateName;
        }

        // 返回FirstName、LastName的元組
        public void Deconstruct(out string fname,out string lname)
        {
            fname = FirstName;
            lname = LastName;
        }        

        public void Deconstruct(out string fname, out string mname, out string lname)
        {
            fname = FirstName;
            mname = MiddleName;
            lname = LastName;
        }

        public void Deconstruct(out string fname, out string lname,
                                out string city, out string state)
        {
            fname = FirstName;
            lname = LastName;
            city = City;
            state = State;
        }
    }
}

switch，case分支可以使用占位符，相當default的作用。

using System;
using System.Globalization;

namespace ConsoleApp4
{
    class Program
    {
        public static void Main()
        {
            object[] objects = { CultureInfo.CurrentCulture,
                           CultureInfo.CurrentCulture.DateTimeFormat,
                           CultureInfo.CurrentCulture.NumberFormat,
                           new ArgumentException(), null };
            foreach (var obj in objects)
                ProvidesFormatInfo(obj);

            Console.ReadLine();
        }

        private static void ProvidesFormatInfo(object obj)
        {
            switch (obj)
            {
                // 實現了IFormatProvider,if obj is IFormatProvider fmt 
                case IFormatProvider fmt:
                    Console.WriteLine($"{fmt} object");
                    break;
                case null:
                    Console.WriteLine("null");
                    break;
                case object _:
                    Console.WriteLine("without format information");
                    break;
            }
        }      
    }
}

具有out參數的調用方法，有時只想知道是否能轉換而不需要其轉換之後的值。

 bool isDate = DateTime.TryParse("8989", out _);

獨立的棄元 (好像沒有用....)

 _ = "";

當上下文存在下劃線標識符，則無法再用下劃線作為占位符，即不能使用棄元。

private static void ShowValue(int _)
{
    byte[] arr = { 0, 0, 1, 2 };
    // 此時無法作為占位符，因為和參數的有效標識符衝突
    _ = BitConverter.ToInt32(arr, 0);
    Console.WriteLine(_);
}
// The example displays the following output:
//       33619968

private static void ShowValue(int _)
{
    string value = _.ToString();
    int newValue = 0;
    // 無法作為占位符，類型和參數標識符_也不一樣，編譯報錯
    _ = Int32.TryParse(value, out newValue);
    return _ == newValue;
}