casting from class<Base> to class<Sub>

[aronmatthew]

I'm trying to create a class of type T where T: Control. The only problem is that when creating a class object of type T where T inherits from Control the cast is seemingly impossible.

 internal class Class1<T> where T: Control

 Class1<RichTextBox> textControl = new Class1<RichTextBox>();
 Class1<Control> control = textControl;

although in theory it could be possible in this case to bypass the template method and pass a Type to the constructor to create an instance of any derivation of Control which should prevent the need of such a cast

try
 {
     if (controlType.IsSubclassOf(typeof(Control)))
         Control = Activator.CreateInstance(controlType) as Control;
     else
         throw new Exception("invalid type");
 }
 catch(Exception ex)
 {

     Control = null;
 }

 

[Skydiver]

Works for me:

Spoiler

using System;
using System.Windows.Forms;
using System.Collections.Generic;
using System.Configuration;

namespace WinFormsNetCore
{
    class Foo<T> where T : Control
    {
        public static Control Create()
        {
            Control control = null;
            try
            {
                var controlType = typeof(T);
                if (typeof(T).IsSubclassOf(typeof(Control)))
                    control = Activator.CreateInstance(controlType) as Control;
                else
                    throw new InvalidOperationException("We need type parameter to be a Control");
            }
            catch (Exception ex)    // $BUG: Pokemon exception handling
            {
                control = null;
            }
            return control;
        }
    }


    class MainForm : Form
    {
        [STAThread]
        static void Main()
        {
            var x = Foo<TextBox>.Create();
            MessageBox.Show($"{x.GetType()}");
        }
    }
}

 

[Skydiver]

As well as this one. You just need to pay attention to the kind of object you are creating.

public static Foo<T> CreateFoo()
{
    Foo<T> control = null;
    try
    {
        var controlType = typeof(T);
        if (typeof(T).IsSubclassOf(typeof(Control)))
            control = Activator.CreateInstance(typeof(Foo<T>)) as Foo<T>;
        else
            throw new InvalidOperationException("We need type parameter to be a Control");
    }
    catch (Exception ex)    // $BUG: Pokemon exception handling
    {
        control = null;
    }
    return control;
}

 

[Skydiver]

Now, on the other hand what you are trying to do is:

class Base
{
}

class Foo : Base
{
}

class Wrapper<T> where T : Base
{
}

Wrapper<Foo> fooObject = new Wrapper<Foo>();
Wrapper<Base> baseObject = fooObject;

This will never work because the type Wrapper<Foo> is not the same type as Wrapper<Base>.

 

[aronmatthew]

for my applications this in theory could work:

  internal class Class1: Control
  {
      public Control Control
      {
          get { return base; }
          set { base(value); }
      }
  }

class1 = new Control();
class1 = new RichTextBox();

 

[Skydiver]

Yes. That works Class1 inherits from Control. It follows as IS-A relationship: Class1 "is a" Control.

But Wrapper<Foo> does not inherit from Wrapper<Base> from my post #4, nor Class1<RichTextBox> does inherit from Class1<Control> from your post #1. The same way an List<int> is not an int.

 

[aronmatthew]

for my applications this in theory could work:

  internal class Class1: Control
  {
      public Control Control
      {
          get { return base; }
          set { base(value); }
      }
  }

class1 = new Control();
class1 = new RichTextBox();

forgive me my computer was acting weird after going to redondo beach the activator method is working good for some reason the whole cs file changes type when inheritance fron Control on my end