Question

I may be going about this backwards… I have a class which is like a document and another class which is like a template. They both inherit from the same base class and I have a method to create a new document from a template (or from another document, the method it is in the base class). So, if I want to create a new document from a template, I just instantiate the template and call GetNewDoc() on it;

Document doc = mytemplate.GetNewDoc(); 

In the Document class I have a blank constructor creating a new, blank document as well as another constructor that takes a document ID so I can load the document from the database. However, I would also like a constructor that takes a Template ID. This way I can do

Document doc = New Document(TemplateID) 

Because the template class already has the ability to return a document, I’d like the constructor to do something like

Template temp = new Template(TemplateID); this = temp.GetNewDoc(); 

Of course, I can’t do this as ‘this’ is read-only – and it feels odd anyway. I have a feeling I am being very stupid here so feel free to shout 🙂

The thing is that the object in question is pretty complex with several collections of child objects and database persistence over multiple tables so i don’t want to duplicate too much code. Though, I guess I could just get the new document from the template and then copy the fields/properties across as the collections should follow easily enough – it just seems like duplication.

A more elaborate code example:

using System; using System.Collections.Generic; using System.Text;  namespace Test { class Program {     static void Main(string[] args)     {         // This just creates the object and assigns a value         Instance inst = new Instance();         inst.name = 'Manually created';         Console.WriteLine('Direct: {0}', inst.name);          //This creates a new instance directly from a template         MyTemplate def = new MyTemplate();         Instance inst2 = def.GetInstance(100);         Console.WriteLine('Direct from template: {0}', inst2.name);          Instance inst3 = new Instance(101);         Console.WriteLine('Constructor called the template: {0}', inst3.name);         Console.ReadKey();      } }  public class Instance {     public string name;      public Instance(int TemplateID)     {         MyTemplate def = new MyTemplate();         //If I uncomment this line the build will fail         //this = def.GetInstance(TemplateID);     }      public Instance()     {     } }  class MyTemplate {     public Instance GetInstance(int TemplateID)     {         Instance inst = new Instance();         //Find the template in the DB and get some values         inst.name = String.Format('From template: {0}', TemplateID.ToString());         return inst;     } } } 

Answer 1

If you want to be able to do anything other than create a new object just from the code in the constructor, don’t use a constructor in the first place.

Do you really need an Instance constructor taking an int? Why not turn it into a static factory method:

public static Instance CreateInstance(int id) {     MyTemplate def = new MyTemplate();     return def.GetInstance(id); } 

Static methods have various advantages over constructors – although some disadvantages too. (There’s a separate SO question on that – worth a look.)