c# readonly vs constant in different assembly

In C#/.NET we can declare a constant value by either using keyword 'const' or 'readonly' . Using const keyword will define compile time constant and readonly will define runtime. Only the C# built-in types can be declared using 'const' for user-defined types like class, struct or array use 'readonly'. Compiler will have a literal value for all the fields that declared const, so if you decompile the code you will find no reference to the constant but actual value. Compile time constant are faster then readonly but are less flexible and can create issues if not used properly. As a general rule one should strictly use compile time constant only for values that are never going to change for example defining value of PI, any value that might change in future use readonly.

During developing a large application there are numerous scenario where you might have to choose between compile-time and run-time constant's. Compile time constants are faster then run-time, although in certain conditions to avoid potential problems you might want to consider using run-time constants . The difference between the two is more clearly explained in this post [ linky ]

Here is an practical example, two teams are working on a same project one team develops a external class library and other team develops the main application. Team one has developed the class library that has some const and readonly variables, these values are consumed in the application developed by the team two. If in future team one updates the constant value in the external class library and the application is not recompiled it would not reflect the new value. This issue is only created if your are using constant values form external assemblies. If a const value changes in a assembly then you need to rebuild all the clients applications dependent on it.

   1:  using System;

   2:  using ExternalLibrary;

   3:   

   4:  namespace ExternalLibrary

   5:  {

   6:      public class ConstantLib

   7:      {

   8:          public static readonly int StartValue = 105;

   9:          public const int EndValue = 120;

  10:          public readonly int ReadonlyValue = 555;

  11:      }

  12:  }

  13:   

  14:   

  15:   

  16:  namespace CTvsRT

  17:  {

  18:     class Program

  19:     {

  20:        static void Main(string[] args)

  21:        {

  22:         ConstantLib cl = new ConstantLib();

  23:   

  24:         Console.WriteLine("ConstantLib.StartValue {0}", ConstantLib.StartValue.ToString());

  25:         Console.WriteLine("ConstantLib.EndValue {0}", ConstantLib.EndValue.ToString());

  26:         Console.WriteLine("ConstantLib.readonlyValue {0}", cl.ReadonlyValue.ToString());

  27:   

  28:         Console.Read();

  29:        }

  30:     }

  31:  }

Visual Studio Templates for Silverlight

For Visual Studio and Silverlight there are nice templates available online. If you are a new developers this templates will be very useful, also for experience developers they should be a handy tool in their toolbox. In this post we will learn how to install the project item templates and use them. I assume every .NET developer should have used the inbuilt visual studio template's and understand how helpful they can be.

The five template are :-
1) Silverlight Client Access Policy File:-
This templates adds a file named clientaccesspolicy.xml in the root folder of the Silverlight project.Silverlight 4 supports two different mechanisms for services to enable cross-domain access clientaccesspolicy.xml or crossdomain.xml file. This file need to be at the root of the domain where the service is hosted, I tend to use clientaccesspolicy.xml file because it provides more granular control over allowed domains and can also be used to set configuration for Sockets.A very good explanation can be found here http://www.devtoolshed.com/explanation-cross-domain-and-client-access-policy-files-silverlight

Either way, when you are done  the policy file needs to go in the ROOT of the domain. This is important as it is not the application root, but the root web. Even if your app is located at   foo.com/myapp, the policy file needs to be at  foo.com/clientaccesspolicy.xml.

2) Value Converter:-
This templates adds a file for converter.

3) C# Trigger Template for Silverlight:-
This templates adds a file for trigger.

4) C# Behavior Template for Silverlight:-
This templates adds a file for behavior.

5) C# Action Template for Silverlight:-
This templates adds a file for action.

How to get the templates ?
Open Visual Studio, open an existing silverlight project or create new one.
Right click on the Silverlight Project –> Add -> New Item ->(left side) select Online Templates -> Silverlight


Organizing the templates
This templates will create files and place them under the root folder. What I do is usually create folders in the silverlight project named as behaviors,triggers,actions,converters. How this helps? eventually and gradually when the projects files increase its easy to locate code-files and manage them effectively.

Code Snippets
I know there are code snippets available for the same, I like the templates because a single click will generate the file with basic code and it opens the popular open-file dialog where you can name the file.

Silverlight Cookie Manager

Cookies are popular in web programming for state management, mostly used for persisting small data on client computer. Each cookie is a name-value pairs, this information is stored as small files on client hard-disk. You can also set expiration  date and time of a cookie. If no expiration is provided cookie will be discarded when user closes the browser, this type of non-persistent cookies is useful to store secure or short time data.

In Silverlight, one can access cookies through the 'HtmlPage' class. This class provides browser functionalities and other details like Cookie data, Browser name and version, Popup Window, Platform, UserAgent, Product name and version.  Below is a diagram illustrating how the Silverlight plug-in access cookies through the browser. In traditional web programming, the Response Object or java script is used to set a cookie. A user can disable cookies on his browser, so while using the response object  we should check if cookies are enabled or not. To verify if cookies are enabled, set a cookie and try to read it back, if you can't read it means cookies are disabled. Silverlight code executes on client PC, so we can directly check if cookies are enabled or not just by using 'HtmlPage.BrowserInformation.CookiesEnabled'.

Securing cookie data is very important and should be take in account during application design. To prevent unauthorized access of cookies, combination of tricks can be used like encryption, expiration time, HttpOnly,etc. Silverlight code executes on client PC and cannot access HttpOnly cookies. Cookie class is a part of 'System.Net' and is used to retrieve information about cookies that are received with Http responses. Cookies are stored in a CookieContainer on a Web request, and a CookieCollection on a Web response. You must always create a CookieContainer to send with a request if you want cookies to be returned on the response, also for HTTPOnly cookies.

Limitation if a cookie is that it can store only string data types.
1) Some user may disable cookies on their browser in some case user may manually delete cookies
2) Size limitations Most browsers place a 4096-byte limit on the size of a cookie, although support for 8192-byte cookies is becoming more common in newer browser and client-device versions.
3) User-configured refusal Some users disable their browser or client device's ability to receive cookies, thereby limiting this functionality.
4) Potential security risks Cookies are subject to tampering. Users can manipulate cookies on their computer, which can potentially cause a security risk.

I have created a simple class that aggregates common functionality related to cookies. All members in the class are static so there is no need to create new instances every time you read or write a cookie. The overloaded method SetCookie() will create a new cookie or reset the value of an existing cookie, you can also set the expiration time, path, domain, security using the same method. GetAllCookieList() method will return a list of all available cookies. DeleteCookie() method will delete the cookie by setting the expiration time to yesterday. GetCookieAsString() will return the full cookie data as a string.

1. using System;
2. using System.Net;
3. using System.Text;
4. using System.Linq;
5. using System.Collections.Generic;
6. using System.Windows.Browser;
7.  
8. namespace Utilities
9. {
10.     public static class CookieManager
11.     {
12.         // 1) If same name cooke exist, SetCookie() will over-ride value.
13.         // 2) Exception handling should be done in user code
14.         // 3) - expireDays = 0, indicates a session cookie that will not be written to disk
15.         //    - expireDays = -1, indicates that the cookie will not expire and will be permanent
16.         //    - expireDays = n, indicates that the cookie will expire in n days
17.  
18.  
19.         public static bool IsCookieEnabled()
20.         {
21.             return HtmlPage.BrowserInformation.CookiesEnabled;
22.         }
23.  
24.         public static bool SetCookie(string key, string value)
25.         {
26.             return SetCookie(key, value, null, null, null, false);
27.         }
28.  
29.         public static bool SetCookie(string key, string value, TimeSpan? expiry)
30.         {
31.             return SetCookie(key, value, expiry, null, null, false);
32.         }
33.  
34.         public static bool SetCookie(string key, string value, TimeSpan? expiry, string path, string domain, bool secure)
35.         {
36.             if (!IsCookieEnabled()) return false; //If cookies not enabled return false.
37.  
38.             StringBuilder sbCookie = new StringBuilder();
39.             sbCookie.Append(string.Concat(key, "=", value));
40.  
41.             if (expiry.HasValue)
42.             {
43.                 DateTime expire = DateTime.UtcNow + expiry.Value; sbCookie.Append(string.Concat(";expires=", expire.ToString("R")));
44.             }
45.  
46.             if (path != null)
47.             {
48.                 sbCookie.Append(string.Concat(";path=", path));
49.             }
50.  
51.             if (domain != null)
52.             {
53.                 sbCookie.Append(string.Concat(";domain=", domain));
54.             }
55.  
56.             if (secure)
57.             {
58.                 sbCookie.Append(";secure");
59.             }
60.  
61.             HtmlPage.Document.SetProperty("cookie", sbCookie.ToString());  // User should handle exceptions if any while writing cookie.
62.  
63.             return true;
64.         }
65.  
66.         public static List<Cookie> GetAllCookieList()
67.         {
68.             string[] cookies = HtmlPage.Document.Cookies.Split(';');
69.             List<Cookie> cookieList = new List<Cookie>();
70.             foreach (string cookie in cookies)
71.             {
72.                 string[] cookieParts = cookie.Split('=');
73.                 if (cookieParts.Count() >= 1)
74.                 {
75.                     cookieList.Add(new Cookie(cookieParts[0].Trim(), cookieParts[1].Trim()));
76.                 }
77.             }
78.  
79.             return cookieList; //User should check for count to know how many cookies are retrieved.
80.  
81.             ////LINQ code
82.             //return (from cookie in cookies
83.             //        select cookie.Split('=')
84.             //            into cookieParts
85.             //            where cookieParts.Count() >= 1
86.             //            select new Cookie(cookieParts[0].Trim(), cookieParts[1].Trim())).ToList(); //User should check for count to know how many cookeies are retrieved.
87.         }
88.  
89.         public static CookieCollection GetAllCookieCollection()
90.         {
91.             string[] cookies = HtmlPage.Document.Cookies.Split(';');
92.             CookieCollection cookieCollection = new CookieCollection();
93.             foreach (string cookie in cookies)
94.             {
95.                 string[] cookieParts = cookie.Split('=');
96.                 if (cookieParts.Count() >= 1)
97.                 {
98.                     cookieCollection.Add(new Cookie(cookieParts[0].Trim(), cookieParts[1].Trim()));
99.                 }
100.             }
101.             return cookieCollection; //User should check for count to know how many cookies are retrieved.
102.  
103.             //LINQ code
104.             //foreach (string[] cookieParts in
105.             //   cookies.Select(cookie => cookie.Split('=')).Where(cookieParts => cookieParts.Count() >= 1))
106.             //{
107.             //    cookieCollection.Add(new Cookie(cookieParts[0].Trim(), cookieParts[1].Trim()));
108.             //}
109.             //return cookieCollection; //User should check for count to know how many cookies are retrieved.
110.         }
111.  
112.         public static string GetCookieAsString(string key)
113.         {
114.             string[] cookies = HtmlPage.Document.Cookies.Split(';');
115.  
116.             foreach (string cookie in cookies)
117.             {
118.                 string[] keyValue = cookie.Split('=');
119.  
120.                 if (keyValue.Length == 2)
121.                 {
122.                     if (keyValue[0].ToString().Trim() == key) //sometime we get one space infront of the cookie so need to Trim()
123.                     {
124.                         return cookie;
125.                     }
126.                 }
127.             }
128.             return null;
129.             //LINQ code
130.             //return (from cookie in cookies
131.             //        let keyValue = cookie.Split('=')
132.             //        where keyValue.Length == 2
133.             //        where keyValue[0].ToString().Trim() == key
134.             //        select cookie).FirstOrDefault();
135.         }
136.  
137.         public static string GetValue(string key)
138.         {
139.             string[] cookies = HtmlPage.Document.Cookies.Split(';');
140.  
141.             foreach (string cookie in cookies)
142.             {
143.                 string[] keyValue = cookie.Split('=');
144.  
145.                 if (keyValue.Length == 2)
146.                 {
147.                     if (keyValue[0].ToString().Trim() == key) //sometime we get one space infront of the cookie so need to Trim()
148.                     {
149.                         return keyValue[1]; //this will return only Value                        
150.                     }
151.                 }
152.             }
153.  
154.             return null;
155.             //LINQ code
156.             //return (from cookie in cookies
157.             //        select cookie.Split('=')
158.             //            into keyValue
159.             //            where keyValue.Length == 2
160.             //            where keyValue[0].ToString().Trim() == key
161.             //            select keyValue[1]).FirstOrDefault();
162.         }
163.  
164.         public static bool DeletCookie(string key)
165.         {
166.  
167.             if (Exists(key, ""))// check if cookie is present or not
168.             {
169.                 DateTime expireDate = DateTime.Now - TimeSpan.FromDays(1); // yesterday
170.                 string expires = ";expires=" + expireDate.ToString("R");
171.                 string cookie = key + "=" + expires;
172.                 HtmlPage.Document.SetProperty("cookie", cookie);
173.                 return true;
174.             }
175.             else
176.             {
177.                 return false;
178.             }
179.         }
180.  
181.         public static bool Exists(string key, string value)
182.         {
183.             if (string.IsNullOrEmpty(key))
184.                 return false; //If key not provided, return false
185.  
186.             return string.IsNullOrEmpty(value)
187.                        ? HtmlPage.Document.Cookies.Contains(key + "=")
188.                        : HtmlPage.Document.Cookies.Contains(key + "=" + value);
189.         }
190.  
191.     }
192. }

Cookie FAQ
http://www.cookiecentral.com/faq/

Silverlight cookies
http://msdn.microsoft.com/en-us/library/dd920298%28v=VS.95%29.aspx

HttpOnly cookies
http://msdn.microsoft.com/en-us/library/system.web.httpcookie.httponly.aspx