Decorator Design Pattern - Example

Question

I am trying to understand Decorator Design Pattern.

Wonder what is the best way to decorate fields values in below example?

Am I using decorator pattern correctly? If not how can I improve it?

public class Fields
{
   string InvoiceNumber {get;set;}
   DateTime InvoiceDate {get;set;}
   DateTime DueDate {get;set;}
   string ReferenceNumber {get;set;}
   string OrganisationNumber {get;set;}
}



Fields fields = new Fields();
fields.InvoiceNumber = "1234";

Decorator1 decorator1 = new Decorator1(fields);
decorator1.Decorate(fields.InvoiceNumber);

Console.Write(fields.InvoiceNumber); //it changes InvoiceNumber to 1234++

Decorator2 decorator2 = new Decorator2(fields);
decorator2.Decorate(fields.InvoiceNumber); 

Console.Write(fields.InvoiceNumber); //it changes InvoiceNumber to 1234--

Answer 1

My take on a similar decorator pattern would be something like this:

public interface IFields
{
    string InvoiceNumber {get;set;}
}

public class Fields : IFields
{
   string InvoiceNumber {get;set;}
   // Let's dumb it down to one Field for brevity
}

public class FieldsPlusDecorator : IFields
{
    public static IFields Decorate(IFields decoratee) => new FieldsPlusDecorator (decoratee);
    
    private readonly IFields _subject:
    private FieldsPlusDecorator (IFields subject) => _subject = subject;
    
    public string InvoiceNumber
    {
       get { return $"{subject.InvoiceNumber}++"; }
       set { throw new NotSupportedException(); } // Again: For Brevity, maybe just write-through ...
    }
}

Usage:

IFields myFields = new Fields(){ InvoiceNumber = "1234" };
Console.WriteLine(myFields.InvoiceNumber); // 1234

myFields = FieldsPlusDecorator.Decorate(myFields);
Console.WriteLine(myFields.InvoiceNumber); // 1234++