(注:本文参照 NickChapsas的Attributes get a feature long-overdue in C# 11)
今天看一个泛型特性的例子,这个功能在C#11才受支持。
在asp.net core mvc中,可以给action添加filter,达到拦截作用,实现如下:
public class MyFilter : IAsyncActionFilter
{public async Task OnActionExecutionAsync(ActionExecutingContext context, ActionExecutionDelegate next){Console.WriteLine("Action前执行");await next();Console.WriteLine("Action后执行");}
}
使用方式,在Action上添加ServiceFilter特性即可,如下:
[ServiceFilter(typeof(MyFilter))]
public IEnumerable<WeatherForecast> Get()
{return Enumerable.Range(1, 5).Select(index => new WeatherForecast{Date = DateTime.Now.AddDays(index),TemperatureC = Random.Shared.Next(-20, 55),Summary = Summaries[Random.Shared.Next(Summaries.Length)]}).ToArray();
}
在运行前记得把MyFilter注放到Service容器中:
builder.Services.AddScoped<MyFilter>();
为了支持C#11,在项目文件.csproj中,PropertyGroup中添加一行<LangVersion>preview</LangVersion>
<Project Sdk="Microsoft.NET.Sdk.Web"><PropertyGroup><TargetFramework>net7.0</TargetFramework><Nullable>enable</Nullable><ImplicitUsings>enable</ImplicitUsings><LangVersion>preview</LangVersion></PropertyGroup><ItemGroup><PackageReference Include="Microsoft.AspNetCore.OpenApi" Version="7.0.0-preview.4.22251.1" /><PackageReference Include="Swashbuckle.AspNetCore" Version="6.3.1" /></ItemGroup>
</Project>
现在就可以定义一个继承IFilterFactory的特性类了,并且是泛型的。
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = true, Inherited = true)]
public class GSWFilterAttribute<TFilter> : Attribute, IFilterFactory, IOrderedFilter where TFilter : IAsyncActionFilter
{public bool IsReusable { get; set; }public int Order { get; set; }public IFilterMetadata CreateInstance(IServiceProvider serviceProvider){if (serviceProvider != null){var filter = (IFilterMetadata)serviceProvider.GetRequiredService(typeof(TFilter));if (filter is IFilterFactory filterFactory){filter = filterFactory.CreateInstance(serviceProvider);}return filter;}else{throw new ArgumentNullException(nameof(serviceProvider));}}
}
使用时,直接把泛型类型放上就可以了,如下:
[GSWFilter<MyFilter>]
public IEnumerable<WeatherForecast> Get()
{return Enumerable.Range(1, 5).Select(index => new WeatherForecast{ Date = DateTime.Now.AddDays(index),TemperatureC = Random.Shared.Next(-20, 55),Summary = Summaries[Random.Shared.Next(Summaries.Length)]}).ToArray();
}
虽然两种方法实现的功能是一样的,但后一种看起来更优雅一些。同时说明一下,如果多个MyFilter功能的过滤器,可以增加Order属性,如下:
[GSWFilter<MyFilter1>(Order = 2)]
[GSWFilter<MyFilter2>(Order = 1)]
public IEnumerable<WeatherForecast> Get()