[英]Type inference with interfaces and generic constraints
I'm building a generic query dispatcher. 我正在构建一个通用的查询调度程序。 The idea is as follows: 这个想法如下:
IQuery<T>
that represents a query object 通用接口IQuery<T>
,表示查询对象 IQuery<T>
) 具体的查询类有一些getter和setter(实现IQuery<T>
) IQueryHandler<TReturnType, TQuery>
with a single method that receives a TQuery (with a constraint on TQuery: IQuery<TReturnType>
) and returns a TReturnType
通用接口IQueryHandler<TReturnType, TQuery>
带有一个接收TQuery的方法(对TQuery: IQuery<TReturnType>
有约束TQuery: IQuery<TReturnType>
)并返回TReturnType
IQueryHandler
that receive a concrete ConcreteQuery
and return a TQuery 实现IQueryHandler
具体处理程序,它接收具体的ConcreteQuery
并返回一个TQuery QueryDispatcher
that has a single method that receives a Query and returns a result. 一个QueryDispatcher
,它有一个接收Query并返回结果的方法。 The concrete class will look up the correct handler from a DI container. 具体类将从DI容器中查找正确的处理程序。 The problem is that I don't get type inference when I call the Fetch
method on the QueryDispatcher
. 问题是当我在QueryDispatcher
上调用Fetch
方法时,我没有得到类型推断。 Is there anything I can do to get type inference or is this just a limitation of c#? 有什么我可以做的类型推断或这只是c#的限制?
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
. 理论上它应该知道类型参数是什么,因为它接收到TQuery
的参数,该参数具有作为IQuery<TReturnType
的约束。
This is the code: 这是代码:
class ConcreteClass // This is the class of which I want to query objects
{
}
Query interface + concrete query class 查询界面+具体查询类
interface IQuery<T>
{
}
public class ConcreteQuery : IQuery<ConcreteClass>
{
}
QueryHandler interface + concrete Query handler QueryHandler接口+具体的查询处理程序
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) QueryDispatcher(使用DI容器来解析正确的处理程序)
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: 现在,当我像这样使用QueryDispatcher时,我收到一个错误:
var queryDispatcher = new QueryDispatcher();
var c = queryDispatcher.Fetch(new ConcreteQuery());
When I provide the type arguments everything works correctly: 当我提供类型参数时,一切正常:
var c = queryDispatcher.Fetch<ConcreteClass, ConcreteQuery>(new ConcreteQuery());
What you want is a two level type inference and the compiler won't do it. 你想要的是一个两级类型推断,编译器不会这样做。
You might want to rethink your query dispatcher. 您可能想要重新考虑您的查询调度程序。 Think of it as something that takes an IQuery<TReturnType>
and returns a TReturnType
. 将其视为采用IQuery<TReturnType>
并返回TReturnType
。
Try changing your query dispatcher to: 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>>>()
.Fetch(query);
}
}
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