The factory pattern is used if a class is instantiated throug code depended of certain values.

Example of factory pattern

First query the assembly to get the class with the iFilter inteface and stores it in a hastable: filters.

        public FilterFactory()
        {
            Assembly asm = Assembly.GetExecutingAssembly();
            Type[] allTypes = asm.GetTypes();
            foreach (Type type in allTypes)
            {
                if (type.IsClass && !type.IsInterface)
                {
                    Type filter = type.GetInterface("IFilter");

                    if (filter != null)
                    {

                        FilterTypes filterType = FilterTypes)type.GetProperty("Key").GetValue(null);
                        filters[filterType] = type;
                    }
                }
            }
        }

Next is the method wich returns the class, first it quyries the hasttable for the type and next it instantiates the class from the string:

        public IFilter GetFilter(FilterTypes filtertype)
        {
            IFilter filter = null;
            if(filters.Contains(filtertype))
            {
                filter = (IFilter) Activator.CreateInstance( (Type) filters[filtertype]);
            }
            return filter;
        }