Sunday, September 21, 2014

A First Look at Using C# 6 - Primary Constructors

Update 2nd October 2014: Yesterdaty primary constructors was pulled from the planned features of C# 6, rendering this post somewhat irrelevant. Sorry, but that's the kind of thing that happens in open development processes.

The other night I moved Nancy.Linker to C# 6 - this is the first part of a summary of what I changed along with quick introductions to the C# 6 features I used. Nancy.Linker is a small library (1 interfaces, 2 production classes, 2 test class), so not everything comes into play there - but the 3 C# 6 features I'm most eager to get, namely primary constructors, auto-property initializers and getter-only auto-properties, all did. For the full overview of C# 6 features visit the language features status page on the Roslyn codeplex site.

Before diving in I will note that this is based on code written using the 'experimental' flag for the compiler that comes with Visual Studio 14 CTP 3. Since this is a pre-release of the compiler things may change, but judging from the language features status page linked above the features described here are fairly stable.

Primary constructors

The first feature I used was primary constructors. In Nancy.Linker only 1 of the production classes and 1 of the test classes have non-default constructors, and they only have 1 constructor each. Classes with 1 explicit constructor are prime candidates for primary constructors, so i gave both a primary constructor.

Primary constructors add new C# syntax for declaring a constructor. While multiple constructors are still allowed only one of them can be the primary one. From the outside a primary constructor is no different than any other constructor; its primary-ness is only visible inside the class. The declaration of a primary constructor is part of the class declaration where an argument list is added:

which is equivalent the old ResourceLinker constructor:

Once a primary constructor is declared the variables in the argument list are available for initializing fields and properties in the class. In the ResourceLinker class the old constructor assigned the two constructor arguments directly to private fields. With primary constructor syntax this becomes:

I like this for two reasons.
  1. By pulling the constructor argument list all the way up to the line declaring the class primary constructors place more emphasis on this list. I find that the constructor argument list is quite important, because it shows what is needed to create an instance of the class. Particularly so if you use DI and prefer constructor injection.
  2. It saves a few lines of trivial initializtion code.

Primary Constructor Bodies

My previous experience with primary constructors is from Scala in which everything between the open curly brace at the start of the class and the closing one at the end is the body of the primary constructor. You can put arbitrary code into the class definition and it will run when instances are created. In my experience this is done fairly often in Scala code, and it is a feature I've been happy with in Scala. In C# 6 adding is a body to a primary constructor is - in my opinion - somewhat less elegant because it requires you to add a scope somewhere in the class where all the primary constructor body code is placed.

The tests for Nancy.Linker uses a small NancyModule called test module, which sets up a few routes, that the tests then use. Before that class looked like this:

Take line 8 as an example. This is not simlpy an assignment to a field or a property. Therefore it has to be part of the primary constructor body, so moving this class over to having a primary constructor means turning it into:

See that pair of curly braces around the constructor body? That is that extra scope delimiting the body of the primary constructor.

I still like the changes to this class for the same reasons as stated above, but I don't like the necessity of putting all the primary constructor body code into a scope because:
  1. It is less flexible than the Scala counterpart, because all the code has to be together.
  2. Aestetics: The code is at an extra level of indentation and has to be surrounded by a pair of somewhat dangling braces.

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