Open and closed generics types

            1. What does open type mean?

            2. What does closed type mean?

            3. What does arity mean?

            4. How can we use type constructors?

A type without specified generic type parameters is called an open type. The C# compiler can compile code without defined type parameters. However it is unable to create instance of open types.


When type parameters are specified for a type with generic type parameters it is called a closed type and the CLR allows creating instances of closed types, if only they were specified completely.


class Program



        internal enum CarOptions


            SunRoof = 0x01,

            Spoiler = 0x02,

            FogLights = 0x04,

            TintedWindows = 0x08,



        internal sealed class StringDictionary<TValue> : Dictionary<string, TValue>


            static StringDictionary()


                if (!typeof(TValue).IsEnum)

                    throw new ArgumentException("The type must be an enumerated");





        static void Main(string[] args)


            object obj = CreateInstace(typeof(Dictionary<,>));

            obj = CreateInstace(typeof(StringDictionary<>));

            obj = CreateInstace(typeof(StringDictionary<Object>));

            obj = CreateInstace(typeof(StringDictionary<CarOptions>));




        private static Object CreateInstace(Type t)


            Object result = null;



                result = Activator.CreateInstance(t);


                Trace.WriteLine(String.Format("Instance of {0} is created", t.ToString()));


            catch(Exception ex)




            return result;




            System.ArgumentException: Cannot create an instance of System.Collections.Generic.Dictionary`2[TKey,TValue] because Type.ContainsGenericParameters is true.

            System.ArgumentException: Cannot create an instance of OpenClosedTypes.Program+StringDictionary`1[TValue] because Type.ContainsGenericParameters is true.

            A first chance exception of type ‘System.ArgumentException’ occurred in OpenClosedTypes.exe

            A first chance exception of type ‘System.Reflection.TargetInvocationException’ occurred in mscorlib.dll

            System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation. —> System.TypeInitializationException: The type initializer for ‘StringDictionary`1’ threw an exception. —> System.ArgumentException: The type must be enumerable

Instance of OpenClosedTypes.Program+StringDictionary`1 [OpenClosedTypes.Program+CarOptions] is created

The number of type parameters required by the type is called arity.

The C# has a feature is called constraints, which allows to constraint type parameters, but it has limitations, so we can use the type constructor to check type parameters before type will be created.

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