Question

I am making some abstract base classes. The base object is defined as:

    public abstract class Bar
    {
        public bool Enabled;
    }

The base collection is defined as:

 public abstract class Foo<T> : List<T>
        where T : Bar
    {
        public Foo<T> GetAllEnabled()
        {
            Foo<T> ret = new Foo<T>();

            foreach (T item in this)
            {
                if (item.Enabled)
                {
                    ret.Add(item);
                }
            }

            return ret;
        }
    }

Naturally, I get an error trying do create a new Foo<T>(), because Foo is an abstract class. Without the abstract modifier, everything works fine, but I want the modifier there because I want to make sure the class gets derived.

I have tried

     Type U = this.GetType();
        Foo ret = new U();

but I get an error "The type or namespace name 'U' could not be found (blah blah blah)".

I think my attempted fix should work, and I must just have the syntax wrong. What am I missing here?

Answer 1

Type U = this.GetType();
Foo ret = new U();

Type U will contain information on the data type, it is not the equivalent of a generic type parameter that you can then instantiate. You can use U to create a type through Activator.CreateInstance, which has a few overloads and accepts Type objects, but then you’re left with needing to cast the resulting object to an appropriate type in order to be able to use it.

Type type = this.GetType();
Foo<T> ret = (Foo<T>)Activator.CreateInstance(type);