Instantiate a generic type knowing common base class

Question

Im trying to accomplish something which may seem a bit convoluted but would be quite helpful in my situation, looks something like this.

    public class CommonBaseClass {}
    public class Type1Object : CommonBaseClass {}
    public class Type2Object : CommonBaseClass {}
    public class Type3Object : CommonBaseClass {}

    public static Dictionary<string, Type> DataTypes = new Dictionary<string, Type>()
    {
        { "type1" , typeof(Type1Object) },
        { "type2" , typeof(Type2Object) },
        { "type3" , typeof(Type3Object) }
    };

    public static CommonBaseClass GetGenericObject(string type)
    {
        return new DataTypes[type]();     //How to instantiate generic class?
    }

Because I can guarantee that all the constructors have the same signature I know this will work, just not sure how to let the compiler know.

Thanks in advance

Answer 1

I don't see any generics in here really, but it looks like you want:

return (CommonBaseClass) Activator.CreateInstance(DataTypes[type]);

If you need to use parameterized constructors, use an alternative overload of Activator.CreateInstance .

Alternatively, consider changing your dictionary to be of delegates:

private static Dictionary<string, Func<CommonBaseClass>> DataTypes =
    new Dictionary<string, Func<CommonBaseClass>>
    {
        { "type1", () => new Type1Object() }
        { "type2", () => new Type2Object() },
        { "type3", () => new Type3Object() }
    };

public static CommonBaseClass GetGenericObject(string type)
{
    return DataTypes[type]();
}

Answer 2

Try this:

public class Foo
{
   public static CommonBaseClass GetGenericObject<T>() where T : CommonBaseClass
   {
      return (CommonBaseClass)Activator.CreateInstance<T>();
   }

   public void Test()
   {
      CommonBaseClass b = GetGenericObject<Type1Object>();
   }
}

Using generics can solve this problem a lot better than having a dictionary of type mapping.

Answer 3

A small test application:

namespace ConsoleApplication
{
    class Program
    {
        static void Main(string[] args)
        {
            var dataTypes = new Dictionary<string, Type>
            {
                {"type1", typeof (Type1Object)},
                {"type2", typeof (Type2Object)},
                {"type3", typeof (Type3Object)}
            };

            Func<string, CommonBaseClass> GetGenericObject = t =>
            {
                return (CommonBaseClass)Activator.CreateInstance(dataTypes[t]);
            };

            var myGenericObject = GetGenericObject("type1");
        }
    }

    public class CommonBaseClass { }
    public class Type1Object : CommonBaseClass { }
    public class Type2Object : CommonBaseClass { }
    public class Type3Object : CommonBaseClass { }
}