具有多个泛型类型参数的类型推断

本文关键字：类型 泛型类型参数 | 更新日期: 2023-09-27 18:04:15

我不明白为什么c#在以下完整的情况下不推断类型:

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，编译器推断类型。当我添加第二个时，编译器也不推断(它显然不能推断第二个，但我不确定为什么它不能推断第一个)。我的问题是为什么容器的类型不推断?

具有多个泛型类型参数的类型推断

原因是第二个类型参数没有在任何函数参数中使用。因此，不可能纯粹从参数使用推断出它的类型。

如果你有一个这样的方法签名(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)

那么它将能够从参数推断出泛型类型。

如果你想避免指定第一个参数，你可以将方法分成两个函数，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>()
}

然后调用

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