使用多个泛型类型参数键入推断

Question

我不明白为什么C#在以下完整情况下不会推断出类型:

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,编译器会推断出类型.当我添加一秒时,编译器也没有推断(它显然无法推断第二个,但我不确定为什么它不能推断第一个).我的问题是为什么没有推断出容器的类型？

Answer 1

原因是第二个类型参数没有在任何函数参数中使用.所以它的类型无法从参数用法中推断出来.

如果你有这样的方法签名(显然不等同于你的代码,只是一个例子):

public static TResult DoStuffWithThings<TContainer, TResult>(
    this TContainer container,
    Func<TContainer, TResult> getSomething)

然后,它会能够从参数推断泛型类型.

如果要避免指定第一个参数,可以将方法拆分为两个函数,并在泛型类型中隐藏中间参数,如下所示:

public static IntermediateResult<T> DoStuffWithThings<T>(this T container)

public class IntermediateResult<T>
{
    public WithReturnType<TResult>()
}

然后它称为

var result = container.DoStuffWithThings().WithReturnType<Result>();