Interpreter –  It provides to evaluate the language grammar and description. This idea bases on having the class for each symbol in a specialized computer language.

From Design Pattern – Behavioral Patterns

Structure

Applicability

• It involves implementing an expression interface that interprets a particular context, such as SQL parsing, symbol processing engine, etc.
• It performs upon a hierarchy of expressions which every here is a terminal or no terminal.
• The interpreter pattern tree structure is somewhat similar to defined by the composite pattern with terminal expressions leaf object and non-terminal expression being composites.
• The tree contains the expressions to be evaluated and is usually generated by a parser, which is not part of the design pattern – interpreter.
• Map a domain to a language, the language to a grammar, and the grammar to a hierarchical, object-oriented design.

Note: It is often used on the specialized database query language such as SQL, the technical computer language often used to describe communication protocols. The most general-purpose computer language incorporates several specialized languages.

Pros and Cons

How to Implement

#1 Basic Sample

static void Main(string[] args)
{
	List<AbstractExpression> objExpression = new List<AbstractExpression>();
	Context context = new Context(DateTime.Now);
	Console.WriteLine("Please select the Expression  : MM DD YYYY or YYYY MM DD or DD MM YYYY ");

	context.expression = Console.ReadLine();
	string[] strArray = context.expression.Split(' ');

	foreach (var item in strArray)
	{
		if (item == "DD")
			objExpression.Add(new DayExpression());
		else if (item == "MM")
			objExpression.Add(new MonthExpression());
		else if (item == "YYYY")
			objExpression.Add(new YearExpression());
    	}

	objExpression.Add(new SeparatorExpression());

	foreach (var obj in objExpression)
	{
		obj.Evaluate(context);
    	}

	Console.WriteLine(context.expression);

	Console.ReadKey();    
}

public class Context
{
	public string expression { get; set; }
	public DateTime date { get; set; }
	public Context(DateTime date) => this.date = date;  
}

public interface AbstractExpression
{
	void Evaluate(Context context);
}

public class DayExpression : AbstractExpression
{
	private string expression;

	public void Evaluate(Context context)
	{
		expression = context.expression;
		context.expression = expression.Replace("DD", context.date.Day.ToString());
    	}
}

public class MonthExpression : AbstractExpression
{
	private string expression;

	public void Evaluate(Context context)
	{
		expression = context.expression;
		context.expression = expression.Replace("MM", context.date.Month.ToString());
	}
}

public class YearExpression : AbstractExpression
{
	private string expression;

	public void Evaluate(Context context)
	{
		expression = context.expression;
		context.expression = expression.Replace("YYYY", context.date.Year.ToString());
    	}
}

public class SeparatorExpression : AbstractExpression
{
	private string expression;
	public void Evaluate(Context context)
	{
		expression = context.expression;
		context.expression = expression.Replace(" ", "-");
	}
}

#2 Web Application Project

Step 1 Create the interpreter pattern interface class files and the interpreter pattern-related interface class files in the Core project. One interface class file name sets “IInterpreter.” Other interface class file name sets “IAbstractExpression.”

Step 2 Edit the interface class file from “IAbstractExpression” to start. Create the class files which need to inherit the “IAbstractExpression” interface class file.

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