具有泛型 class 类型约束的泛型方法

Question

我有一个通用接口，它有一个带有另一个通用类型的方法。该方法的约束应该只允许从 class 的泛型派生的类型。我尝试了以下代码和其他一些可能性，但 none 目前有效：

generic <class ObjectType>
public interface class IFoo
{
public:
    generic <class ObjectTypeInherited> where ObjectTypeInherited : ObjectType
    ObjectTypeInherited Test();
};

public ref class Derived : IFoo<System::Object^>
{
public:
generic <class ObjectTypeInherited> where ObjectTypeInherited :     System::Object^
    virtual ObjectTypeInherited Test()
    {
        return ObjectTypeInherited();
    }
};

int main(array<System::String ^> ^args)
{
    auto derived = gcnew Derived();
    auto nullString = derived->Test<System::String^>();
    if (nullString == nullptr)
            System::Console::WriteLine("Got a null string!");
    return 0;
}

我的代码收到以下错误：

error C3766: 'Derived' must provide an implementation for the interface

方法'ObjectTypeInherited IFoo::Test(void)'

如果我将其更改为：

generic <class ObjectTypeInherited> where ObjectTypeInherited :     System::Object^

我收到一个不同的错误：

error C3284: the constraints for generic parameter 'ObjectTypeInherited' of function 'ObjectTypeInherited Derived::Test(void)' must match the constraints for generic parameter 'ObjectTypeInherited' of function 'ObjectTypeInherited IFoo<System::Object ^>::Test(void)'

有人知道如何正确实施吗？

编辑：System::Object^ 和 System::String^ classes 仅用作这里的例子。通常我会使用 2 个自己的 ref class 像 MyObject 和 MyDerivedObject : MyObject

Answer 1

首先 generic <class ObjectTypeInherited> where ObjectTypeInherited : System::Object^。应该 generic <class ObjectTypeInherited> where ObjectTypeInherited : System::Object.

没有^在这里签名。

然后乐趣开始了...

不知何故，C++\CLI 编译器不希望理解 class 不必是通用的才能实现 IFoo<ObjectType>::Test。所以我为这个奇怪的问题创建了一个解决方法。

您可以使用 System::Object 而不是 MyObjectType。我使用 MyObjectType 只是为了多功能性。

public ref class MyObjectType
{
};

generic <class ObjectType>
public interface class IFoo
{
public:
    generic <class ObjectTypeInherited> where ObjectTypeInherited : ObjectType
    ObjectTypeInherited Test();
};

//All the magic is here. Somehow the C++\CLI compiler does not wish to
//understand that the DerivedBase class does not have to be generic in order
//to implement the IFoo<ObjectType>::Test. As you can see the Whatever type
//name is never used. Although without it the compiler gives the C3284
//error.
generic <class Whatever>
public ref class DerivedBase : public IFoo<MyObjectType^>
{
public:
    //Why not make use of explicit syntax just because we can.
    generic <class ObjectTypeInherited> where ObjectTypeInherited : MyObjectType
    virtual ObjectTypeInherited Test() = IFoo<MyObjectType^>::Test
    {
        return ObjectTypeInherited();
    }
};

//In order to overcome the fake genericness of DerivedBase I provide
//another successor.
public ref class Derived : public DerivedBase<Object^>
{
};

我对 Roslyn 编译器或 C++\CLI 没有任何深入的了解，但在我看来这似乎是一个错误。

如果我对 C# 托管代码执行相同的操作，它应该看起来像这样简单。这就是我要阻止自己的地方。即使没有任何奇怪的编译器行为，托管声明在 C# 端也比 C++\CLI 更容易维护。

public class MyObjectType
{
}

public interface IFoo<ObjectType>
{
    ObjectTypeInherited Test<ObjectTypeInherited>() where ObjectTypeInherited : ObjectType;
}

public class Derived : IFoo<MyObjectType>
{
    //Explicit
    ObjectTypeInherited IFoo<MyObjectType>.Test<ObjectTypeInherited>()
    {
        return null;
    }

    //Implicit
    public ObjectTypeInherited Test<ObjectTypeInherited>() where ObjectTypeInherited : MyObjectType
    {
        return null;
    }
}

具有泛型 class 类型约束的泛型方法

Generic method with generic class type constraint

.net

generics

c++-cli