Newbie : Issues with constructor chaining

Question

Hi All

I'm not sure if there is a forum for Newbies, so I'm going to post this here and if there is a FAQ on this and a newbie section then please let me know.

I hope you can help a newbie to C# programming with something that I am struggling regarding constructor chaining.

If I have multiple fields of the same type that I want to be populated on instaniation is there a way to indicate which field want to be populated in the chained call.

For example I have a class that has 2 private properties/variable:

class ClassName
{
        // Field data.
        private int     id;
        private int     age;

        // Chained constructors
        ClassName() : this(0) {}
        ClassName(int myID) : this(myID, 0) { }
        ClassName(int myAge) : this(0, myAge) { }

        // Master constructor
        ClassName(int myID, int myAge)
        {
                id = myID;
                age = myAge;
        }
}

Main would look like this say:

public static void Main()
{
        // Is this argumentfor ID or Age as they are both of type int?
        ClassName myClass = new class(10);
}

I hope you can see what my problem is? Is there a way to allocate values to exact constructor arguments or is this an issue with constructor chaining that has to be lived with, and I should ignore chaining when dealing with constructors that contain arguments with this same type?

Many thanks
Gary

Vulpes · Answer

Well you can't have two constructors which have the same number of arguments of the same type. So you'll have to delete one of the constructors which takes one argument in order that the application can compile. This of course means that only one of the fields can be initialized with the value passed to this constructor. As a result of constructor chaining the other argument will be zero. If you want to initialize both fields with non-zero values then you should call the constructor which takes both arguments directly i.e. no chaining is necessary. You'll also need to make the constructors public (they're private by default) so they can be accessed by code outside the class. The following compiles and runs OK: using System; class ClassName { // Field data. public int id; public int age; // Chained constructors public ClassName() : this(0) {} public ClassName(int myID) : this(myID 0) { } // Master constructor public ClassName(int myID int myAge) { id = myID; age = myAge; } } class Test { public static void Main() { ClassName myClass = new ClassName(10); Console.WriteLine(myClass.id); // 10 Console.WriteLine(myClass.age); // 0 ClassName myClass2 = new ClassName(11 20); Console.WriteLine(myClass2.id); // 11 Console.WriteLine(myClass2.age); // 20 Console.ReadKey(); } } Incidentally the default value for an int field is 0 anyway so the other field will be initialized to 0 here without the need for constructor chaining.

Vulpes · Answer

Yes named arguments work with constructors as well as 'ordinary' methods. As you say the combination of named and optional arguments greatly reduce the need for constructor chaining. It's a pity that these weren't introduced in C# 1.0 (they've always been in VB.NET) but it was argued at the time that optional arguments were bad because the default value of the argument had to be 'baked into' the method signature. If the default value were subsequently changed then any code which called that method had to be recompiled. In the end the C# team had to relent on this because they were trying to improve Office and COM interop which had always been a pain from C# because of the copious use of optional arguments in COM methods. The problem now is that everybody's got used to the previous way of working and so you don't see optional and named arguments used much outside of interop code.

Vulpes · Answer

Classes with private constructors are still useful where you need to control instantiation such as when implementing the singleton pattern. You can then have a public static 'factory' method for creating the single instance (if it hasn't been created already) which calls the private constructor internally.

Gary · Answer

Thanks for the details. Must admit I am tryint to think of a time when having a private defaulted constructor wouold be useful and there are many in C++ but not so much in C#. Anyhow. Thanks for helping.

Vulpes · Answer

Yes if you declare a private 'do nothing' constructor and make the class 'sealed' then you achieve something close to a static class and in fact that's what we used to do before static classes were introduced in C# 2.0. However it's not quite the same as a static class which doesn't allow a constructor at all (except for a static one). Also a static class doesn't allow you to add any instance members which you can do with an 'ordinary' class which has a private constructor.

Gary · Answer

Hi there Sorry another constructor question I know that C# provides the concept of static classes but is it possible to achive similar by declaring the default construtor as private with no other public constructors? I seem to remeber somthing similar in C++.

Gary · Answer

Just tried named argument passing in a consructor and worked excellently. This concept does now change my view on constructor chaining and if it is now really needed as you can have a single constructor and simply name the values you want to be set at instansiation. Although as you say that is only available in .NET/C# 4.0(+). What do you think regarding design is constructor chaining really needed in anything 4.0 onwards?

Gary · Answer

Named arguments excellent..thanks for the update. Does this apply to constructors and methods?

Gary · Answer

PS. Sorry the constructors are public in my test env. Stupid to have missed that.

Vulpes · Answer

Named argument passing was in fact introduced in C# 4.0 (VS 2010). Please check out this link for details: http://msdn.microsoft.com/en-us/library/dd264739.aspx

Gary · Answer

Thanks Vulpes. I thought that would be the case just wanted to be sure. I wonder if in future iterations of the language or the .NET platform named argument passing will be implemented as is available with Oracles PL/SQL...it would make life so much easier. Thanks for the help though.