Constructor chaining with intermediate variables

Question

I have the following situtation with overloaded constructors which I'm struggling to find a nice solution to. I can't see how to use an intermediate assignment with constructor chaining.

The following isn't valid but shows what I want to do

public MyThing(IServiceLocator services, int? userId) { // blah.... } public MyThing(IServiceLocator services, string userName) { User user = services.UserService.GetUserByName(userName); int userId = user == null ? null : (int?)user.Id; // call the other constructor this(services, userId); }

The only way I know to write the above in valid code is

public MyThing(IServiceLocator services, string userName) : this(services, services.UserService.GetUserByName(userName) == null ? null : (int?)services.UserService.GetUserByName(userName).Id)

which is not only ugly code, but also requires the database call twice (unless the compiler is clever enough to work that out, which I doubt).

Is there a better way to write the above?

bniwredyc · Accepted Answer · 2010-12-07 20:56:48Z

What about this:

public MyThing(IServiceLocator services, string userName) { User user = services.UserService.GetUserByName(userName); int? userId = user == null ? null : (int?)user.Id; Initialize(services, userId); } public MyThing(IServiceLocator services, int? userId) { Initialize(services, userId); } private void Initialize(IServiceLocator services, int? userId) { // initialization logic }

EDIT

If I were you I'd replace constructor with factory method like this:

private MyThing(IServiceLocator services, int? userId) { // blah.... } public static Create(IServiceLocator services, int? userId) { return new MyThing(services, userId); } public static Create(IServiceLocator services, string userName) { User user = services.UserService.GetUserByName(userName); int userId = user == null ? null : (int?)user.Id; return new MyThing(services, userId); }

Usage:

var myThing = MyThing.Create(services, 123); var myOtherThing = MyThing.Create(services, "userName");

Replace Constructor with Factory Method (refactoring.com)

That works pretty well for my example. If you start adding more constructors though you end up with a lot of duplicated code (i.e. if you in reality you want chains of more than 2 constructors).
+1 If the constructor gets complicated, use separate initialization logic.
@fearofawhackplanet, can you give an example of situalition with a lot of duplicated code?

Boris Pavlović · Accepted Answer · 2010-12-07 11:47:34Z

There is, yes. I know that the example is in Java but it's such a nice solution to your problem that some effort of porting to C# does make sense.

LukeH · Accepted Answer · 2010-12-07 12:01:06Z

You can use a static helper method:

public MyThing(IServiceLocator services, int? userId) { // blah.... } public MyThing(IServiceLocator services, string userName) : this(services, GetUserId(services, userName)) { } private static int? GetUserId(IServiceLocator services, string userName) { User user = services.UserService.GetUserByName(userName); return (user == null) ? (int?)null : user.Id; }

Collectives™ on Stack Overflow

Constructor chaining with intermediate variables

3 Answers 3

3 Comments

Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

3 Comments

Comments

Comments

Linked

Related