Is there a pattern that allows a constructor to be called only from a specific factory and from nowhere else?

Question

We have a class, say LegacyUserSettingsService. LegacyUserSettingsService implements an interface, IUserSettingsService.

You can get an instance of the IUserSettingsService by calling our ApplicationServicesFactory. The factory uses Spring.NET to construct the concrete LegacyUserSettingsService.

The trouble is that new developers sometimes do their own thing and construct new instances of the LegacyUserSettingsService directly (instead of going via the factory).

Is there a way to protect the constructor of the concrete class so it can only be called from the factory? A well-known pattern perhaps?

Note that the concrete class resides in a different assembly (separate from the Factory's assembly, so the internal keyword is not a solution). The factory assembly references the other assembly that contains the concrete class.

Any ideas?

Answer 1

Mark LegacyUserSettingsService constructors with ObsoleteAttribute with IsError set to true. This is purely a "compile-time attribute", which will result in compilation error everywhere a constructor is used, but an instance can be created via reflection by Spring.NET just fine.

Answer 2

On the contrary, internal would be a great solution, the key though is to make the factory assembly a "Friend Assembly" of the other. See Friend Assemblies for more info.

Make LegacyUserSettingsService an internal class then in the assemblyinfo.cs of the project that contains that class add the line [assembly:InternalsVisibleTo("Spring.Core")]. This should give Spring.NET access to the type but no other assembly.

Answer 3

The only way I know is to make the constructor private and have static factory methods on the class itself which can be called from the outside.

Of course that would not the meet the requirements of your questions. So what about taking a less technical approach involving, communication and code-review?

Answer 4

We found a workable solution. Since our factory is using Spring.NET we can simply make LegacyUserSettingsService's constructor private. Spring will still be able to call the private constructor.

And nobody else will be able to (unless they try really really hard and go the reflection route).

Answer 5

You could always have a private constructor that you invoke using System.Reflection.ConstructorInfo or a System.Linq.Expressions.Expression>. This would stop the casual developer from creating instances directly. Your factory object would then be able to create the instance, but those pesky developers would be thwarted!