Introducing Dependency Injection into Interpolation Code

Greetings!

In one of my previous blogs I have touched upon the idea of programming to interfaces. I did not expect that the blog would be getting so many hits as it does. As far as writing maintainable loosely coupled code, I have a long way to go to become good at. Thus I’ve decided to investigate the most useful design patters to learn and introduce into the hobby and professional coding practices. And I am not going to start small…

One of the seriously misunderstood software design patterns is dependency injection. Many people like writing about it on their sites and blogs, but not many people do a decent job at describing it correctly. I am not claiming that I will now fix this for my blog readers. Instead, I will tel you that I am reading a fantastic book about it: Dependency Injection in .NET by Mark Seemann. So, if you are looking for a good material on this topic, please check out this recommendation.

My last post was on linear and cubic spline interpolation. The implementation can be improved, since I’ve hard-coded at least one inflexible dependency: outputting the results to the console screen. Anyone who wishes to use my example code without using the console screen (e.g. writing to a file instead) would need to rewrite at least twelve lines of code!  We can definitely improve upon this by decoupling the console writer and substituting a generic class that is ‘injected‘ with the user-specified writer through a constructor (aka constructor injection).

To begin with, I made a small change to the Tridiagonal class and removed console-based output. The Solve method now throws ArgumentException whenever passed matrix is not square or something else went wrong. I then made a similar change to the extensions. After all, it is not a good idea to tie extensions to a specific output class like Console. The LinearInterpolation and CubicSplineInterpolation classes now have private constructors. The basic checks on user input is now done by the initializing methods, which could have been defined as properties. Also, the interpolating classes do not display any messages when something goes wrong. Instead, they throw appropriate exceptions, which the calling method will deal with. Finally, I have added a new UserOutput namespace which defines an interface and a very generic class that will work with any concrete implementation of Write method declared by the interface:

namespace UserOutput
{
    #region GENERIC_OUTPUT
    internal interface IWriter
    {
        void Write(string message);
    }

    internal class Output
    {
        private readonly IWriter writer;

        internal Output(IWriter writer)
        {
            if (writer == null)
            {
                throw new ArgumentNullException("Null writer interface.");
            }

            this.writer = writer;
        }

        internal void Display(string message)
        {
            writer.Write(message);
        }
    }
    #endregion
}

The concrete implementation is another class defined as:

internal class ConsoleWriter : UserOutput.IWriter
    {
        public void Write(string message)
        {
            Console.WriteLine(message);
        }
    }

The Main method creates an instance of the IWriter interface and sets it to the ConsoleWriter. The linear and cubic spline interpolations are called through the IInterpolate interface which they both define. We no longer need two separate instances of each class. The full code can be downloaded here as a pdf file.

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One Response to Introducing Dependency Injection into Interpolation Code

  1. Elena says:

    Would be great to know if anyone found this blog interesting or even useful.

    Like

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