扩展方法

1namespace MainProgram;
2
3public class Printer
4{
5    public string Message { get; set; }
6
7    public Printer(string message)
8    {
9        Message = message;
10    }
11
12    // 我们可以修改此方法或添加另一个方法
13    public void PrintWithDashes()
14    {
15        Console.WriteLine($"###{Message}###");
16    }
17}

1using MainProgram;
2
3string message = "Hello World";
4Printer printer = new(message);
5
6printer.PrintWithDashes();

1namespace MainProgram;
2
3// 因为我们无法更改 Printer 类的源代码，
4// 所以创建一个从 Printer 继承的新类
5public class StarPrinter : Printer
6{
7    public StarPrinter(string message) : base(message)
8    {
9        
10    }
11
12    // 在这里我们可以添加额外的功能
13    public void PrintWithStars()
14    {
15        Console.WriteLine($"***{Message}***");
16    }
17}

1using MainProgram;
2
3string message = "Hello World";
4StarPrinter printer = new(message);
5
6// 我们仍然拥有旧的方法
7printer.PrintWithDashes();
8
9// 但同时也有了这个新方法
10printer.PrintWithStars();

1namespace Extensions;
2
3// 静态类
4public static class StringExtensionMethods
5{
6    // 静态方法
7    // 注意 "this" 关键字
8    public static string Reverse(this string stringObject)
9    {
10        char[] charArray = stringObject.ToCharArray();
11        Array.Reverse(charArray);
12        return new string(charArray);
13    }
14}

如果在类型上定义了与现有实例方法同名且签名相同的方法，那么实例方法始终具有优先权。扩展方法将被“隐藏”，永远不会被调用。

1using Extensions;
2
3string name = "John Doe";
4string company = "PageFlux";
5
6// 直接在字符串对象上调用该方法
7// 就好像该方法是定义在字符串类内部一样
8Console.WriteLine(name.Reverse());
9Console.WriteLine(company.Reverse());

1namespace MainProgram;
2
3// 密封类不能被继承
4public sealed class Person
5{
6    public string FirstName { get; set; }
7    public string LastName { get; set; }
8    public int Age { get; set; }
9    public string Occupation { get; set; }
10    public decimal AnnualSalary { get; set; }
11}

1using MainProgram;
2
3namespace Extensions;
4
5public static class PersonExtensionMethods
6{
7    public static decimal GetMonthlySalary(this Person person)
8    {
9        return person.AnnualSalary / 12;
10    }
11}

1using Extensions;
2using MainProgram;
3
4Person person = new()
5{
6    FirstName = "John",
7    LastName = "Doe",
8    Age = 34,
9    Occupation = "Software Developer",
10    AnnualSalary = 120000m
11};
12
13Console.WriteLine($"Hello, my name is {person.FirstName}.");
14Console.WriteLine($"I work as a {person.Occupation}.");
15Console.WriteLine($"I earn {person.GetMonthlySalary()} dollars per month.");