Returning and using class types (reflection?)

Charles Wiltshire

Hi

I'd like to return a class type from a method call, then create an instance of that returned class. I'm guessing it's got something to do with reflection / the 'Type' object? Any help would be greatly appreciated!

In delphi you'd be able to do it as follows:

unit Unit2;


interface


type

  TEnum = (enString, enInteger);


  TClassType = class of TClass;


  TClass = class  // base class type

    class function ReturnClassType(Enum : TEnum) : TClassType;

  end;


  TClass1 = class(TClass)

  public

    v : string;

  end;


  TClass2 = class(TClass)

  public

    v : integer;

  end;


  TTesterClass = class

  public

    procedure Test;

  end;


implementation


uses

  Dialogs;


{ TClass }


class function TClass.ReturnClassType(Enum: TEnum): TClassType;

begin

  case Enum of

    enString : result:=TClass1;

    enInteger : result:=TClass2;

  end;

end;


{ TTesterClass }


procedure TTesterClass.Test;

var

  ClassType : TClassType;

  ValuedClass : TClass;

begin

  ClassType := TClass.ReturnClassType(enInteger);

  ValuedClass := ClassType.Create;


  ShowMessage(ValuedClass.ClassName);  // <- returns "TClass2"

end;


end.

3 Replies

Know the answer? Post it — somebody with the same question will find it here.

Charles Wiltshire18y ago
Thanks for the quick reply Alan, thats excellent - exactly what I was after works a treat.

Cheers again
Charlie
Alan18y ago
I was just playing around some more with this and one thing I forgot to do which will enable you to access members inherited from TClass, without the need for a cast to TClass1 or TClass2, is to replace this line:

object valuedClass = Activator.CreateInstance(classType);

with this one:

TClass valuedClass = (TClass)Activator.CreateInstance(classType);
Alan18y ago
Hi Charles,

The closest I could get to that in C# is:

using System;
using System.Windows.Forms;
using System.Reflection;

enum TEnum
{
   enString,
   enInteger
}

class TTesterClass
{
    static void Main()
    {
      Test();
    }

    static void Test()
    {
      Type classType = TClass.ReturnClassType(TEnum.enInteger);
      object valuedClass = Activator.CreateInstance(classType);
      MessageBox.Show(valuedClass.ToString());
    }

}

class TClass
{
    public static Type ReturnClassType(TEnum tEnum)
    {
       switch(tEnum)
       {
          case TEnum.enString:
             return typeof(TClass1);
          case TEnum.enInteger:
             return typeof(TClass2);
          default:
             return null;
       }
    }
}

class TClass1 : TClass
{
   public string v;
}

class TClass2 : TClass
{
   public int v;
}

I don't know about Delphi but in C# you'd eventually have to cast valuedClass to its actual type in order to be able to do anything useful such as setting its field.