c# 8 开关表达式:找不到开关表达式的最佳类型

c# 8 switch expression: No best type was found for the switch expression

我在我的启动 class (.net core 3.1) 中添加了一个代码到 return 基于参数的类型,但我得到了编译时错误。

我在 sharplab 中创建了一个 运行 示例。如果 switch 表达式包含字符串或其他对象,它运行正常。

工作示例 1:

var x = key switch
            {
                "myhandler1" => "something",
                "myhandler2" => "something else",
                _ => "default case"
            };

工作示例 2:

object obj =  s switch {
            "a" => new object(),
            "b" => new DateTime(),
            _ => throw new NotImplementedException()
        };

错误示例:

interface IHandler { }
public class BaseHandler { }
public class MyHandler1: BaseHandler, IHandler { }
public class MyHandler2: BaseHandler, IHandler { }

class Program
{
    static void Main(string[] args)
    {

        var key = "myhandler1";

        var handler = key switch
        {
            "myhandler1" => new MyHandler1(),
            "myhandler2" => new MyHandler2(),
            _ => throw new NotImplementedException()
        };

        var x = key switch
        {
            "myhandler1" => "something",
            "myhandler2" => "something else",
            _ => "default case"
        };

        Console.WriteLine("Hello World!");
    }
}

原问题(需要修复):

serviceCollection.AddTransient<Func<string, IHandler>>(sp => key =>
            {
                return key switch
                {
                    Constants.Brand => sp.GetService<Handler1>(),
                    Constants.Series => sp.GetService<Handler2>(),
                    _ => throw new NotImplementedException()

                };
}

找到这个 link:https://github.com/dotnet/csharplang/issues/2728

感谢 PavelMarc,修复如下:

serviceCollection.AddTransient<Func<string, IHandler>>(sp => key =>
            {
                return key switch
                {
                    Constants.Brand => (sp.GetService<Handler1>() as IHandler),
                    Constants.Series => (sp.GetService<Handler2>() as IHandler),
                    _ => throw new NotImplementedException()

                };
}

您应该显式声明一种处理程序,而不是 var

IHandler handler = key switch //or BaseHandler handler = key switch
{
    "myhandler1" => new MyHandler1(),
    "myhandler2" => new MyHandler2(),
    _ => throw new NotImplementedException()
};

在你的 sharplab 示例中,两个处理程序都实现了 IHandler 接口并继承了 BaseHandler class,编译器根本不知道要使用哪种类型,你应该告诉它他明确地

interface IHandler { }
public class BaseHandler { }
public class MyHandler1 : BaseHandler, IHandler { }
public class MyHandler2 : BaseHandler, IHandler { }

依赖注入示例也是如此,你应该显式声明一个类型(假设Handler1Handler2实现IHandler

return key switch
{
    Constants.Brand => sp.GetService<Handler1>(),
    Constants.Series => (IHandler) sp.GetService<Handler2>(),
    _ => throw new NotImplementedException()
};

你只能为一个常量做这件事,编译器足够聪明,可以为你完成剩下的工作

var 很挑剔——它希望事情明确,而且 handler 应该在这里的类型并不明显,因为 MyHandler1MyHandler2 是不同的类型;基本上,从 MyHandler1MyHandler2 中选择一些通用的基类型或实现的接口,并使用它而不是 var。在最坏的情况下,object 就足够了:

object handler = key switch
{
    "myhandler1" => new MyHandler1(),
    "myhandler2" => new MyHandler2(),
    _ => throw new NotImplementedException()
};

编译器自己不会尝试这样做。

您正在处理协方差问题。您已指定 Func 的 return 类型应为 IHandler,但此类型参数是不变的。因此,您实际上必须 return IHandler,而不仅仅是实现它的 class。

serviceCollection.AddTransient<Func<string, IHandler>>(sp => key =>
{
    return key switch
    {
        Constants.Brand => (IHandler)sp.GetService<Handler1>(),
        Constants.Series => (IHandler)sp.GetService<Handler2>(),
        _ => throw new NotImplementedException()
    };
}