Decorator Pattern in C#

I'm reading C# 3.0 Design Patterns by Judith Bisop
I've understood the decorator pattern.
Here is my little example for extending StreamReder class by password, when it reads.

using System;
using System.IO;

namespace Decorator
{
 public class LoginStream
 {
  private StreamReader  _streamReader;
  private string _password;

  public LoginStream (StreamReader streamReader, string password){
   _streamReader = streamReader;
   _password = password;
  }

  public string Read (string password){
   if (_password == password) {
    return _streamReader.ReadToEnd ();
   }
   throw new Exception ("Ivalid Password");
  }

 }
}

And the tests:

using System;
using System.IO;
using Decorator;
using NUnit.Framework;

namespace DecoratorTests
{
 [TestFixture()]
 public class Test
 {
  [Test()]
  public void LoginStream_Read_return_string_when_has_a_valid_password ()
  {
   var stream = new StreamReader("/home/luis/tmp/Prueba/Prueba.sln");
   var password = "password";
   var loginStream = new LoginStream(stream, password);

   var result = loginStream.Read(password);

   Assert.IsNotEmpty (result);
  }

  [Test()]
  [ExpectedException(typeof(Exception))]
  public void LoginStraeam_Read_return_exception_when_has_an_invalid_password ()
  {
   var stream = new StreamReader("path");
   var password = "password";
   var loginStream = new LoginStream(stream, password);

   loginStream.Read("invalidPassword");

   Assert.Fail ("Exception expected");
  }
 }
}