用实际的软件编程示例解释C＃中的装饰器模式

// This is the base abstract class which is inherited 
    // by all concrete and decorator classes
    abstract class InvoiceBase
    {
        // string data stores the content which it to be printed in the invoice.
        // This variable will be altered by the subclasses. Thus, it is marked protected.
        protected static string data;
        
        // This function will be implemented in all the concrete classes as well as decorators
        public abstract void CreateInvoice();

        public void PrintInvoice()
        {
            Console.WriteLine(data);
        }

        //This function clear the variable value after invoice is printed.
        public void Reset()
        {
            data = string.Empty;
        }
    }

    public class Invoice : InvoiceBase
    {
        public override void CreateInvoice()
        {
            data += "\n";
            data += "\tCustomer Name : Chris\n";
            data += "\tCustomer Address : Edmond Road, NJ\n";
            data += "\tOrder No : 1254158\n";
            data += "\tOrder Amount : Rs. 2540/- \n";
            data += "\tOrder Date : 09-Apr-2020 \n";
            data += "\n";
        }
    }

以下是Decorator与分离的具体实现类。我们可以添加任意数量的Decorator来扩展功能。另外，请注意，我已经按照SOLID原则为一个职责创建了一个类，即颜色处理由ColorDecorater类负责，页眉信息处理由HeaderDecorator类负责。

// This is the base Decorator class which has the composition i.e. InvoiceBase object
// AttachTo method is used to attach responsibility dynamically through client code.
abstract class InvoiceDecorator : InvoiceBase
{
    InvoiceBase invoiceBase;
    public void AttachTo(InvoiceBase invoice)
    {
        this.invoiceBase = invoice;
    }
    public override void CreateInvoice()
    {
        invoiceBase.CreateInvoice();
    }
}

//These are individual decorator classes.
class ColorDecorator : InvoiceDecorator
{
    public override void CreateInvoice()
    {
        AddColor();
        base.CreateInvoice();
    }
    private void AddColor()
    {
        Console.ForegroundColor = ConsoleColor.Green;
    }
}

class HeaderDecorator : InvoiceDecorator
{
    public override void CreateInvoice()
    {
        AddHeader();
        base.CreateInvoice();
    }
    private void AddHeader()
    {
        string header = "\n\tBlue Heaven Inc.\n"
                        + "\tBay Area, NC\n"
                        + "\t+1 784251485\n\n";
        data = header + data;
    }
}

class FooterDecorator : InvoiceDecorator
{
    public override void CreateInvoice()
    {
        base.CreateInvoice();
        AddFooter();
    }
    void AddFooter()
    {
        string footer = "\n\tCopyright @ 2020.All rights reserved\n";
        data += footer;
    }
}

客户端代码如下所示：

// CASE 1: This is the existing implementation to print a simple invoice.
InvoiceBase invoice = new Invoice();
invoice.CreateInvoice();
invoice.PrintInvoice();

// CASE 2: Add color to the invoice
InvoiceBase invoice = new Invoice();
InvoiceDecorator colorDecorator = new ColorDecorator();

colorDecorator.AttachTo(invoice);
colorDecorator.CreateInvoice();

invoice.PrintInvoice();

// CASE 3: Add Color, Header and Footer to the invoice
InvoiceBase invoice = new Invoice();
InvoiceDecorator colorDecorator = new ColorDecorator();
InvoiceDecorator headerDecorator = new HeaderDecorator();
InvoiceDecorator footerDecorator = new FooterDecorator();

colorDecorator.AttachTo(invoice);
footerDecorator.AttachTo(colorDecorator);
headerDecorator.AttachTo(footerDecorator);
headerDecorator.CreateInvoice();

invoice.PrintInvoice();