C#9.0推出的4个新特性介绍

class Customers
{
    public int CustomerId { get; }
    public string CustomerName { get; set; }

    public Customers(int customerId)
    {
        CustomerId = customerId;
    }

    static void Main(string[] args)
    {
        var customers = new Customers(1045)
        {
            CustomerName = "Tyson"
        };

        //customerid 不能设置，因为该属性是只读
        customers.CustomerId = 1099;
    }
}

class Customers
{
    public int CustomerId { get; init; }
    public string CustomerName { get; set; }

    static void Main(string[] args)
    {
        var customers = new Customers()
        {
            CustomerId = 1045,
            CustomerName = "Tyson"
        };

        //CS8852:只能在对象初始值设定项中或在实例构造函数或...分配
        customers.CustomerId = 1099;
    }
}

class SalesOrder
{
    public int OrderId { get; init; }
    public string ProductName { get; init; }
    public int Quantity { get; init; }

    static void Main(string[] args)
    {
        SalesOrder order = new SalesOrder { OrderId = 1, ProductName = "Mobile", Quantity = 2 };

        //修改ProductName
        SalesOrder neWorder = new SalesOrder { OrderId = order.OrderId, ProductName = "Laptop", Quantity = order.Quantity };
    }
}

public record SalesOrder
{
    public int OrderId { get; init; }
    public string ProductName { get; init; }
    public int Quantity { get; init; }

    static void Main(string[] args)
    {
        SalesOrder order = new SalesOrder { OrderId = 1, ProductName = "Mobile", Quantity = 2 };
 
        SalesOrder newOrder = order with { ProductName = "Laptop" };
    }
}

using System;

namespace CSharp9
{
    class Program
    {
        static void Main(string[] args)
        {
            Console.WriteLine("Welcome!");
        }
    }
}

using System;

Console.WriteLine("Welcome !");

System.Console.WriteLine("Welcome !");

public class SalesOrder
{
    public int OrderId { get; set; }
    public string ProductName { get; set; }
    public int Quantity { get; set; }
    public int TotalCost { get; set; }

    public double GetTotalCost() => TotalCost switch
    {
        500 or 600  => 10,
        < 1000 => 10 * 1.5,
        <= 10000 => 10 * 3,
        _ => 10 * 5
    };
}

class CSharpFeatures
{
    static void Main(string[] args)
    {
        SalesOrder newOrderforCustomer1 = new SalesOrder() { OrderId = 1, ProductName = "Camera", Quantity = 1, TotalCost = 5000 };
        newOrderforCustomer1.GetTotalCost();
        SalesOrder newOrderforCustomer2 = new SalesOrder() { OrderId = 2, ProductName = "Pen", Quantity = 1, TotalCost = 500 };
        newOrderforCustomer2.GetTotalCost();
    }
}