如何在没有第二个声明类型的情况下将辅助泛型参数传递给泛型类构造函数

Question

It is the first time i have gotten this requirement and i can't figure out how to make it work. I have a class that has a generic type. Then the constructor need to get an object of that type. So far so good. Now that same constructor need to receive a secondary generic type as a parameter of that constructor without having to define a second generic type in the class. So here some sample code simplified for understanding purpose

// a base class
public class Item {}

// one of many derived class
public class Switch : Item {}

// another class but unrelated to item
public class Sheet {}

// a derived of sheet
public class CoinSheet : Sheet {}

Now i need a generic class that has an Item as a type so a basic class would be the following and it works perfectly

public class Manager<T> where T : Item
{
    public T RelatedItem { get; set; } = null;
    
    public Manager(T relatedItem)
    {
        RelatedItem = relatedItem;
    }
}

The problem is that i now need to pass an object of type Sheet to the constructor without specifing it in the generic of the class. The following does NOT work but it's that i mean by that (based on the previous class definition

public class Manager<T> where T : Item
{
    public T RelatedItem { get; set; } = null;
    
    public Manager(T relatedItem, T2 relatedSheet) where T2 : Sheet
    {
        RelatedItem = relatedItem;
        InstanceGenerator.Create<T2>(relatedSheet);
    }
}

I proposed to call a method within the Manager class instead that can have the secondary generic type without altering the Manager class definition (which cannot change) but i was given a definitive no as an answer.

I know i can just change the parameter type of that second parameter to object and forget about generic and just hardcode the check for if the object is of type Sheet and find by reflection the InstanceGenerator.Create and invoke it but if it's avoidable i rather do so.

Answer 1

If you don't add it to the class, you can't add it to the constructor.

public class Manager<T> where T : Item
{
    public T RelatedItem { get; set; } = null;
    // THIS IS INVALID
    public Manager<T2>(T relatedItem, T2 relatedSheet) where T2 : Sheet
    {
        RelatedItem = relatedItem;
        InstanceGenerator.Create<T2>(relatedSheet);
    }
}

Which makes sense. The constructor will only be called once, so there is no point changing the T2 in the constructor if it is to affect the method.

Your two real options are

1. Direct use of Sheet.

public class Manager<T> where T : Item
{
    public T RelatedItem { get; set; } = null;
    
    public Manager(T relatedItem, Sheet relatedSheet)
    {
        RelatedItem = relatedItem;
        InstanceGenerator.Create<T2>(relatedSheet);
    }
}

2. The T2 to be added on class level.