使用接口和通用约束进行类型推断 [英] Type inference with interfaces and generic constraints

问题描述

我正在构建一个通用查询调度器。这个想法如下：

• 通用接口 IQuery< T> 查询对象
  
• 具体的查询类有一些getter和setter（实现 IQuery< T> ）
  
• 使用接收TQuery的单个方法（在上具有约束条件）的TQuery：IQuery< TReturnType>的通用接口 IQueryHandler< TReturnType，TQuery> ; ）并返回一个 TReturnType


• 实现 IQueryHandler 接收具体的 ConcreteQuery 并返回一个TQuery


• A QueryDispatcher 有一个接收查询并返回结果的单一方法。具体类将从DI容器中查找正确的处理程序。

问题是我没有得到类型推断在 QueryDispatcher 上调用 Fetch 方法。有什么我可以做的类型推断，或者这只是一个限制的C＃？

理论上它应该知道类型参数是什么，因为它接收一个参数 TQuery ，它有一个约束是一个 IQuery< TReturnType 。

这是代码：

  class ConcreteClass //这是我想要查询对象的类
 {
} 
  

查询接口+具体查询类

 接口IQuery< T> 
 {
} 
 
公共类ConcreteQuery：IQuery< ConcreteClass> 
 {
} 
  

QueryHandler接口+具体查询处理程序

  interface IQueryHandler< TReturnType，TQuery>其中TQuery：IQuery< TReturnType> 
 {
 TReturnType Fetch（TQuery查询）; 
} 
 
 class ConcreteQueryHandler：IQueryHandler< ConcreteClass，ConcreteQuery> 
 {
 ConcreteClass Fetch（ConcreteQuery query）
 {
} 
} 
  

QueryDispatcher（使用DI容器来解析正确的处理程序）

pre $ class QueryDispatcher
{
TReturnType Fetch< TReturnType，TQuery>（TQuery查询）
其中TQuery：IQuery< TReturnType>
{
return myDIcontainer.Get< IQueryHandler< T，TQuery>>（）。Fetch（query）;






$ b现在当我使用这个QueryDispatcher时，错误：

  var queryDispatcher = new QueryDispatcher（）; 
 var c = queryDispatcher.Fetch（new ConcreteQuery（））; 
  

当我提供类型参数时，一切正常：

  var c = queryDispatcher.Fetch< ConcreteClass，ConcreteQuery>（new ConcreteQuery（））; 
  

解决方案

您需要的是两级类型推理，编译器不会这样做。

您可能想重新考虑您的查询调度程序。把它看作是一个需要 IQuery< TReturnType> 并且返回一个 TReturnType 。

尝试将您的查询调度程序更改为：

  class QueryDispatcher 
 {
 public TReturnType Fetch< TReturnType>（IQuery< TReturnType> query）
 {
 return myDIcontainer 
 .Get< IQueryHandler< TReturnType，IQuery< TReturnType>>>（）
。取（查询）; 
} 
} 
  

I'm building a generic query dispatcher. The idea is as follows:

• A generic interface IQuery<T> that represents a query object
• A concrete query class has some getters and setters (implementing IQuery<T>)
• A generic interface IQueryHandler<TReturnType, TQuery> with a single method that receives a TQuery (with a constraint on TQuery: IQuery<TReturnType>) and returns a TReturnType
• Concrete handlers implementing IQueryHandler that receive a concrete ConcreteQuery and return a TQuery
• A QueryDispatcher that has a single method that receives a Query and returns a result. The concrete class will look up the correct handler from a DI container.

The problem is that I don't get type inference when I call the Fetch method on the QueryDispatcher. Is there anything I can do to get type inference or is this just a limitation of c#?

In theory it should know what the type arguments are since it receives a parameter of TQuery which has a constraint on being an IQuery<TReturnType.

This is the code:

class ConcreteClass    // This is the class of which I want to query objects
{
}

Query interface + concrete query class

interface IQuery<T>
{
}

public class ConcreteQuery : IQuery<ConcreteClass>
{
}

QueryHandler interface + concrete Query handler

interface IQueryHandler<TReturnType, TQuery> where TQuery : IQuery<TReturnType>
{
    TReturnType Fetch(TQuery query);
}

class ConcreteQueryHandler : IQueryHandler<ConcreteClass, ConcreteQuery>
{
    ConcreteClass Fetch(ConcreteQuery query)
    {
    }
}

QueryDispatcher (uses a DI container to resolve the correct handler)

class QueryDispatcher
{
    TReturnType Fetch<TReturnType, TQuery>(TQuery query) 
        where TQuery : IQuery<TReturnType>
    {
        return myDIcontainer.Get<IQueryHandler<T, TQuery>>().Fetch(query);
    }
}

Now when I use the QueryDispatcher like this I get an error:

var queryDispatcher = new QueryDispatcher();
var c = queryDispatcher.Fetch(new ConcreteQuery());

When I provide the type arguments everything works correctly:

var c = queryDispatcher.Fetch<ConcreteClass, ConcreteQuery>(new ConcreteQuery());

解决方案

What you want is a two level type inference and the compiler won't do it.

You might want to rethink your query dispatcher. Think of it as something that takes an IQuery<TReturnType> and returns a TReturnType.

Try changing your query dispatcher to:

class QueryDispatcher
{
    public TReturnType Fetch<TReturnType>(IQuery<TReturnType> query) 
    {
        return myDIcontainer
            .Get<IQueryHandler<TReturnType, IQuery<TReturnType>>>()
            .Fetch(query);
    }
}

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