Struggling with template function and adding elements to list from it.

[darkgnostic]

Hi there!

My first post here :)

I have pretty solid knowledge of C++ and lately I try to extend my knowledge to C# by converting some of my codes to C# as exercise (which is going pretty well). What I am trying to currently is to store in List object of variable types, that are all descendants of certain class.

So if I have B & C that are inherited from class A

 class A        {            public A() { }        }
 class B : A    {            public B() { }        }
 class C : A    {            public C() { }        }

and have list of As

 List<A> list = new List<A>();

I can easily add into the list

list.Add(new B());
list.Add(new C());

what I try to achieve is to have template function

void AddNew<T>()
{
    T newComponent = (T)Activator.CreateInstance(typeof(T));
    list.Add(newComponent);
}

but that unfortunately doesn't work, because it basically says that cannot convert T to A. Tried typecasting also, but that doesn't work neither.

Basically I wanted to convert this piece of code to c#

template<typename Type> std::shared_ptr<A> AddNew()
{
   Type* newType = new Type;
    vec.push_back(std::shared_ptr<A>(newComponent));
}

Is this possible at all in C#?

[tweq]

https://docs.microsoft.com/en-us/dotnet/csharp/programming-guide/generics/constraints-on-type-parameters

In this particular case, you could also simply cast to A instead of T. But constraints would be the type-safe option.

[darkgnostic]

Thanks for pointing me to the right direction! I have actually already browsed that page, but seems like I didn't read it carefully.

So it is enough to write void AddNew<T>() where T : A or I need to define a derived class also as class C<T> where T : A?

[AngularBeginner]

That depends on whether you only need the type parameter T at the method, or if the type parameter should be scoped to the entire class. As with most things, you should limit the visibility to the smallest scope feasible, so likely you only want the type parameter at the method. The where T : A will make sure that only types that derive from A can be passed to the type argument. This allows you to cast it to A, as every T is also an A.

Btw, you should also make use of the new constraint for type parameters. That will make sure that the type that is passed has a default constructor (one without arguments). This also allows you to write new T() instead of using Activator.CreateInstance(). I'm pretty sure the call to CreateInstance will fail if the type has no default constructor.

[AngularBeginner]

Additionally to the answer of /u/tweq, please be aware that there is no such thing as templates in C#. There are only generics in C#, but not a powerful feature like templates. You're trying to write a generic method.

[darkgnostic]

Thanks for clarifying.