The Decorator Design Pattern allows extending a class without changing its original implementation. The example shows decorating a TestString class that returns a string by wrapping it in a DecoratedTestString class that appends additional strings. This allows "ABC" to be decorated with "XYZ" and then "PQR" by chaining the decorators without changing the original TestString class.

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Decorator Design Pattern in C#
Author: Kasun Ranga Wijeweera
Email: krw19870829@gmail.com
Date: 2021 March 25
The purpose of the Decorator Design Pattern is to extend a class
without changing the original class.
Example
class Program
{
interface ITestString
{
string GetString();
}
class TestString : ITestString
{
public string GetString()
{
return "ABC";
}
}
class DecoratedTestString : ITestString
{
private ITestString t;
private string s="";
public DecoratedTestString(ITestString t)
{

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this.t = t;
}
public void Append(string s)
{
this.s += s;
}
public string GetString()
{
string s = t.GetString();
s += this.s;
return s;
}
}
static void Display(ITestString t)
{
Console.WriteLine(t.GetString());
}
static void Main(string[] args)
{
ITestString t = new TestString();
Display(t);
DecoratedTestString t1 = new DecoratedTestString(t);
t1.Append("XYZ");
Display(t1);
DecoratedTestString t2 = new DecoratedTestString(t1);
t2.Append("PQR");
Display(t2);

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Console.ReadLine();
}
}
The output of the above code is given below.
ABC
ABCXYZ
ABCXYZPQR