弃元就是使用下划线_作为一个占位符，但不占存储空间。
元组（ValueTuple、Tuple）使用弃元例子。
using System;namespace ConsoleApp4
{class Program
{public static void Main()
{// ValueTuple和Tuple都可以使用弃元var (_, _, _, pop1, _, pop2) = QueryCityDataForYears("New York City", 1960, 2010);
Console.WriteLine($"Population change, 1960 to 2010: {pop2 - pop1:N0}");
}private static (string,double,int,int,int,int) QueryCityDataForYears(string name,int year1,int year2)
{int population1 = 0, population2 = 0;double area = 0;if (name == "New York City")
{
area = 468.48;if (year1 == 1960)
{
population1 = 7781984;
}if (year2 == 2010)
{
population2 = 8175133;
}return (name, area, year1, population1, year2, population2);
}return ("", 0, 0, 0, 0, 0);
}
}
}对象析构，析构函数的参数决定了元组的参数 。
using System;namespace ConsoleApp4
{class Program
{public static void Main()
{var p = new Person("John", "Quincy", "Adams", "Boston", "MA");// Deconstruct(out string fname, out string lname, out string city, out string state) var (fName,_,city,_) = p;
Console.WriteLine($"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的元组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，相当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)
{// 实现了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参数的调用方法，有时只想知道是否能转换而不需要其转换之后的值。
bool isDate = DateTime.TryParse("8989", out _);独立的弃元 (好像没有用....)
_ = "";
当上下文存在下划线标识符，则无法再用下划线作为占位符，即不能使用弃元。
private static void ShowValue(int _)
{byte[] arr = { 0, 0, 1, 2 };// 此时无法作为占位符，因为和参数的有效标识符冲突_ = BitConverter.ToInt32(arr, 0);
Console.WriteLine(_);
}// The example displays the following output:// 33619968private static void ShowValue(int _)
{string value = _.ToString();int newValue = 0;// 无法作为占位符，类型和参数标识符_也不一样，编译报错_ = Int32.TryParse(value, out newValue);return _ == newValue;
}
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