Interpreter Pattern

Greetings, fellow developers! Join me at .Net Core Central for another insightful journey into the world of design patterns. In today’s blog post, we’ll unravel the intricacies of the Interpreter Design Pattern, a valuable addition to the renowned Gang of Four design patterns.

Navigating the Landscape of Interpreter Design Pattern

The Interpreter Design Pattern, nestled within the category of behavioral design patterns, stands out as a powerful tool. Its primary objective is to provide a means of interpreting sentences in a language by defining a representation of its grammar along with an interpreter. This interpreter, in turn, leverages the grammar representation to comprehend and execute sentences in the given language.

In simpler terms, imagine having a language, defining its grammar, and employing an interpreter to understand and execute sentences in that language. To illustrate this pattern, we’ll dive into a real-world example—building a class that acts as a command-line interface for communication with underlying software, focusing on string conversion.

Crafting the Interpreter Pattern Framework

The journey begins with the creation of a Context class, a simple entity holding both a value and an expression, both represented as strings. The value can be manipulated by the interpreter, making it a central element in our pattern.

namespace InterpreterPattern.Demo
{
    internal class Context
    {
        public Context(string expression, string value)
        {
            Expression = expression;
            Value = value;
        }

        public string Expression { get; }
        public string Value { get; set; }
    }
}

Next, we define the IExpression interface, featuring a single method, Evaluate, which takes a Context for evaluation. For our example, we focus on two expression types: LowercaseExpression and UppercaseExpression, each handling the conversion of strings to lowercase and uppercase, respectively.

namespace InterpreterPattern.Demo;

internal interface IExpression
{
    void Evaluate(Context context);
}

internal class LowerCaseExpression : IExpression
{
    public void Evaluate(Context context)
    {
        context.Value = context.Value.ToLower();
    }
}

internal class UpperCaseExpression : IExpression
{
    public void Evaluate(Context context)
    {
        context.Value = context.Value.ToUpper();
    }
}

The Interpreter Pattern in Action

The heart of our implementation lies in the Interpreter class. Armed with a single Interpret method, it takes a Context and embarks on the journey of interpreting the incoming message. It dissects the expression, identifies the appropriate expression types, evaluates them, and prints the modified value.

namespace InterpreterPattern.Demo;

internal class Interpreter
{
    public void Interpret(Context context)
    {
        var expressions = context.Expression.Split(' ');
        var expressionTypes = new List<IExpression>();

        foreach (var expression in expressions)
        {
            if(expression == "-l")
                expressionTypes.Add(new LowerCaseExpression());
            else if(expression == "-u")
                expressionTypes.Add(new UpperCaseExpression());
        }

        foreach (var item in expressionTypes)
        {
            item.Evaluate(context);
        }

        Console.WriteLine(context.Value);
    }
}

Interactive Demonstration

To bring our implementation to life, we step into the program class. Here, we prompt the user for a sentence and an expression. The interpreter then transforms the input based on the specified expression—whether converting to lowercase or uppercase.

using InterpreterPattern.Demo;

Console.WriteLine("Provide a word with expression");
var word = Console.ReadLine();

var value = word.Substring(0, word.IndexOf("-"));
var expressions = word.Substring(word.IndexOf("-"));

var interpreter = new Interpreter();
interpreter.Interpret(new Context(expressions, value));

Embracing Flexibility with Expressions

One of the strengths of the Interpreter Design Pattern is its adaptability. Adding new expressions becomes seamless. For instance, incorporating a WebsocketExpression involves creating a new class implementing IExpression. This extensibility underscores the pattern’s versatility.

Conclusion: Decoding the Power of Interpreter

While the use cases for the Interpreter Design Pattern may not be prevalent in everyday scenarios, its significance shines in specialized situations requiring language interpretation. This pattern provides an elegant solution, separating concerns and facilitating extensibility.

And that concludes our exploration of the Interpreter Design Pattern in .NET development. If this journey resonated with you, don’t forget to go through the rest of the blog posts.

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