我不明白为什么 C# 在以下完整情况下不推断类型:
public interface IThing {}
public class Thing1 : IThing {}
public class Thing2 : IThing {}
public interface IContainer {}
public class Container1 : IContainer
{
public IThing A { get { return new Thing1(); } }
public IThing B { get { return new Thing2(); } }
}
public class Container2 : IContainer
{
public IThing C { get { return new Thing1(); } }
public IThing D { get { return new Thing2(); } }
}
public class SomeClass
{
public void PerformTask() {}
}
public static class ExtensionMethods
{
// This function behaves as I would expect, inferring TContainer
public static TContainer DoStuffWithThings<TContainer>(this TContainer container, Func<TContainer, IThing> getSomething, Func<TContainer, IThing> getSomethingElse)
where TContainer : IContainer
{
var something = getSomething.Invoke(container);
var somethingElse = getSomethingElse.Invoke(container);
// something and something else are the things we specify in our lambda expressions with respect to the container
return container;
}
// The method in question
public static TCustomReturnType DoStuffWithThings<TContainer, TCustomReturnType>(this TContainer container, Func<TContainer, IThing> getSomething, Func<TContainer, IThing> getSomethingElse)
where TContainer : IContainer
where TCustomReturnType : new()
{
var something = getSomething.Invoke(container);
var somethingElse = getSomethingElse.Invoke(container);
// Do stuff with the things just as above
// This time we return our custom type
return new TCustomReturnType();
}
}
public class Driver
{
public static void Main(string[] args)
{
var container1 = new Container1();
var container2 = new Container2();
// I can do stuff with the things for each container, returning the container each time.
container1.DoStuffWithThings(c => c.A, c => c.B)
.DoStuffWithThings(c => c.B, c => c.A);
container2.DoStuffWithThings(c => c.C, c => c.D)
.DoStuffWithThings(c => c.D, c => c.C);
// Now we try to do the same but with the custom return type
container1.DoStuffWithThings<Container1, SomeClass>(c => c.A, c => c.B)
.PerformTask();
// As you can see, the compiler requires us to specify Container1 as the container type.
// Why is it not inferred? It is called from an instance of Container1.
// The behavior I expect is for container1.DoStuffWithThings<SomeClass>(...) to infer
// the container type and return a new instance of SomeClass.
}
}
基本思想是,当只有一个泛型类型参数时,编译器会推断类型。当我添加第二个时,编译器也不会推断(它显然无法推断出第二个,但我不确定为什么它不能推断出第一个)。我的问题是为什么没有推断出容器的类型?