Solid Principles / Builder Pattern

Question

I am creating a very small application to demonstrate solid principles and also a brief implementation of a builder pattern, does anyone have any feedback as to how this could be improved or how it breaks the solid principles? The app is a small app that just converts units, e.g. yards to meters, inches to cms.

Interface

public interface IConverter
{
    double ConversionRate { get; set; }
    string[] ConvertedUnits { get; set; }
    string DisplayText { get; set; }

    string[] Convert(string input);
}

Class implementing interface

 public class Converter : IConverter
    {
        public double ConversionRate { get; set; }
        public string[] ConvertedUnits { get; set; }
        public string DisplayText { get; set; }

        public Converter(double conversionRate, string[] convertedUnits, string displayText)
        {
            ConversionRate = conversionRate;
            ConvertedUnits = convertedUnits;
            DisplayText = displayText;
        }

        public string[] Convert(string input)
        {
            if (!input.Contains('\n'))
            {
                double d = double.Parse(input, CultureInfo.InvariantCulture) * ConversionRate;

                string[] array = new string[1];
                array[0] = d.ToString();

                return array;
            }
            else
            {
                double[] doubles = new double[input.Split('\n').Count()];

                for (int i = 0; i < input.Split('\n').Count(); i++)
                {
                    double value = double.Parse(input.Split('\n')[i]);
                    doubles[i] = value;

                    string.Format("{0}", value * ConversionRate);
                }

                string[] strings = new string[doubles.Length];

                for (int i = 0; i < input.Split('\n').Length; i++)
                {
                    strings[i] = string.Format("{0}", doubles[i] * ConversionRate);
                }

                return strings;
            }
        }
    }

Builder (abstract class)

public abstract class ConverterBuilder
    {
        protected double _conversionRate;
        protected string[] _convertedUnits;
        protected string _displayText;

        public ConverterBuilder AddConversionRate(double conversionRate)
        {
            _conversionRate = conversionRate;
            return this;
        }

        public ConverterBuilder AddConversionRate(string[] convertedUnits)
        {
            _convertedUnits = convertedUnits;
            return this;
        }

        public ConverterBuilder AddDisplayText(string displayText)
        {
            _displayText = displayText;
            return this;
        }

        public Converter Build()
        {
            return new Converter(_conversionRate, _convertedUnits, _displayText);
        }

    }

Builder implementing abstract builder class

public class UnitConverterBuilder : ConverterBuilder
{
    public UnitConverterBuilder()
    {

    }
}

Controller

public IActionResult Convert(string text, double conversionRate)
{
    Converter converter = new UnitConverterBuilder()
    .AddConversionRate(conversionRate)
    .Build();

    string output = "";
    for (int i = 0; i < converter.Convert(text).Count(); i++)
    {
        output += converter.Convert(text)[i] + Environment.NewLine;
    }

    return Content(output);

}

View

Enter values to convert

<form asp-controller="Home" asp-action="Convert" method="post">
    <textarea id="text" name="text" rows="10" cols="40"></textarea>
    <select name="conversionRate">
        <option value="">Please Select</option>
        <option value="0.9144">Yards to Meters</option>
        <option value="2.54">Inches To Centimeters</option>
    </select>
    <button>Convert</button>
</form>

The app is built using .net core, any feedback greatly appreciated :)

Answer 1

These are not necessarily related to SOLID principle but here are a few things I would mention if I was reviewing this code at work

• Excessive use of char '/n'. If you wanted to change from '/n' to '/r/n' you would have to change it 5 times in Converter.Convert. A better way to handle this would be to store this in a variable or perhaps allow it to be set via the constructor.
• You can use var instead of explicitly stating the variable type e.g var d = double.Parse(input, CultureInfo.InvariantCulture) * ConversionRate;
• Variable names, clean code should be easy to read so rather than using names like d and doubles use names that are easy for someone reading the code to understand
• You don't need to specify the properties of Converter on the interface IConverter, this is because the interface only needs to expose the behaviors of the object and the properties are more of an implementation detail. Removing the properties will allow to you to have multiple objects that implement IConvert without being forced to have those specific properties (interface segregation)

I hope this helped :)