C# 13前瞻:Extensions
从C#3开始,拓展方法
这一特性就得到了广泛的应用。
此功能允许你能够使用实例方法的语法调用某个静态方法,以下是一个获取/创建文件的静态方法:
public static async Task<StorageFile> GetOrCreateFileAsync(this StorageFolder folder,string name)
{
var item = await folder.TryGetItemAsync(name) as StorageFile;
item ??= await folder.CreateFileAsync(name);
return item;
}
可以采取如下方式调用此方法,但可读性较差:
await Extensions.GetOrCreateFileAsync(folder, "FileName");
通过为方法的第一个参数添加this
标记,我们还可以这样调用
await folder.GetOrCreateFileAsync("FileName");
拓展方法在C#的发展中有着举足轻重的作用,System.Linq
就使用了大量拓展方法极大简化了数据查询:
//筛选最高温大于30°C的每日天气数据并按照天气类型分组
var result = forecasts.Where(p => p.MaxTemperature > 30).GroupBy(p => p.WeatherType);
而现在,可拓展的内容不再局限于方法
我们可以拓展属性、索引器(有参属性)、静态成员甚至运算符等内容
注意:
extensions
功能尚未正式进入C#13的预览版,以下示例根据语言提案/Build 2024演示中的相关内容编写,正式版语法可能有所不同目前的Roslyn实现在feature/roles分支,可以自行编译尝试
示例1:隐式拓展
假定有以下类型
public class DailyWeather
{
public int MaxTemperature { get; set; }
public int MinTemperature { get; set; }
public string WeatherType { get; set; }
public List<HourlyWeather> HourlyForecasts { get; set; }
public class HourlyWeather
{
public int Temperature { get; set; }
public string WeatherType { get; set; }
}
}
定义以下隐式拓展(implicit extension
)
拓展的语法与类十分相似,它可以访问该类中的任意非private
或protected
成员,但不能有实例字段
public implicit extension DailyWeatherExtension for DailyWeather
{
//拓展属性:平均温度
public int AverageTemperature => (int)Math.Round(HourlyForecasts.Average(p => p.Temperature));
//拓展索引器:获取/修改某小时预报
public HourlyWeather this[int index]
{
get => HourlyForecasts[index];
set => HourlyForecasts[index] = value;
}
//拓展运算符:通过比较最高温大小支持">"/"<"运算符
public static bool operator >(DailyWeather a,DailyWeather b)
{
return a.MaxTemperature > b.MaxTemperature;
}
public static bool operator <(DailyWeather a, DailyWeather b)
{
return a.MaxTemperature < b.MaxTemperature;
}
//拓展静态方法:获取今日天气
public static async Task<DailyWeather> GetWeatherToday()
{
//从外部获取今日天气...
}
}
在代码中,我们就可以这样使用:
public async void PrintInfo(DailyWeather weather)
{
Console.WriteLine(weather.AverageTemperature);
var weatherToday = await DailyWeather.GetWeatherToday();
if(weather > weatherToday)
{
Console.WriteLine(weather[0].WeatherType);
}
}
这些“拓展”似乎就是类中真实存在的成员!原先需要继承才能部分实现的功能,使用一个extension
即可完美解决
示例2:显式拓展
通过声明一个显式拓展(explict extension
),我们可以使用类型转换将类型转换为拓展的类型并获得相应的成员
有如下天气数据JSON
{
"type": "clear",
"tempMax": 32,
"tempMin": 20,
"hourly": [
{
"time": "2024-06-09T00:00",
"type": "cloudy",
"temp": 20
},
{
"time": "2024-06-09T06:00",
"type": "clear",
"temp": 24
},
{
"time": "2024-06-09T12:00",
"type": "clear",
"temp": 32
},
{
"time": "2024-06-09T18:00",
"type": "cloudy",
"temp": 26
},
{
"time": "2024-06-09T23:00",
"type": "rain",
"temp": 21
}
]
}
定义如下显式拓展:
explicit extension DailyWeather for JsonElement
{
public string WeatherType => this.GetProperty("type").GetString()!;
public string MaxTemperature => this.GetProperty("tempMax").GetString()!;
public string MinTemperature => this.GetProperty("tempMin").GetString()!;
public IEnumerable<HourlyWeather> HourlyForecasts => this.GetProperty("hourly")!.EnumerateArray();//此处有隐式类型转换
}
explicit extension HourlyWeather for JsonElement
{
public DateTime Time => this.GetProperty("time").GetDateTime()!;
public int Temperature => this.GetProperty("temp").GetInt32()!;
public string WeatherType => this.GetProperty("type").GetString()!;
}
现在,我们可以用类型安全的方式访问JSON中的内容
var data = jsonData.ParseAsJson();
var weather = (DailyWeather)data;
Console.WriteLine($"今日天气:{weather.WeatherType}");
foreach(HourlyWeather hourly in weather.HourlyForecasts)//此处有隐式类型转换
{
Console.WriteLine($"{hourly.Time.Hour}时的天气:{hourly.WeatherType}");
}