Wednesday, May 8, 2013

Constructor and Destructor

A constructor is a specialized function that is used to initialize fields. A constructor has the same name as the class. Instance constructors are invoked with the new operator and can't be called in the same manner as other member functions. There are some important rules pertaining to constructors as in the following;

Constructor and Destructor

A constructor is a specialized function that is used to initialize fields. A constructor has the same name as the class. Instance constructors are invoked with the new operator and can't be called in the same manner as other member functions. There are some important rules pertaining to constructors as in the following;

Encapsulation

Encapsulation binds together code and the data it manipulates and keeps them both safe from outside interference and misuse. Encapsulation is a protective container that prevents code and data from being accessed by other code defined outside the container.

Inheritance

Inheritance is the process by which one object acquires the properties of another object. A type derives from a base type, taking all the base type members fields and functions. Inheritance is most useful when you need to add functionality to an existing type. For example all .NET classes inherit from the System.Object class, so a class can include new functionality as well as use the existing object's class functions and properties as well.

Polymorphism

Polymorphism is a feature that allows one interface to be used for a general class of action. This concept is often expressed as "one interface, multiple actions". The specific action is determined by the exact nature of circumstances.

Sealed class
A sealed class is a class that does not allow inheritance. Some object model designs need to allow the creation of new instances but not inheritance, if this is the case, the class should be declared as sealed.
To create a sealed class in C#, the class declaration should be done as:
sealed class Shape To create a sealed class in VB.NET, the class declaration should be done as:
NonInheritable Class Shape Abstraction
Abstraction is "the process of identifying common patterns that have systematic variations; an abstraction represents the common pattern and provides a means for specifying which variation to use" (Richard Gabriel).
An abstract class is a parent class that allows inheritance but can never be instantiated. Abstract classes contain one or more abstract methods that do not have implementation. Abstract classes allow specialization of inherited classes.

Abstraction:

Abstraction is a process of hiding the implementation details and displaying the essential features.

Example1: A Laptop consists of many things such as processor, motherboard, RAM, keyboard, LCD screen, wireless antenna, web camera, usb ports, battery, speakers etc. To use it, you don't need to know how internally LCD screens, keyboard, web camera, battery, wireless antenna, speaker’s works. You just need to know how to operate the laptop by switching it on. Think about if you would have to call to the engineer who knows all internal details of the laptop before operating it. This would have highly expensive as well as not easy to use everywhere by everyone.

Encapsulation

Encapsulation is the mechanism that binds together code and the data if manipulates, and keeps both safe from outside interference and misuse. One way to think about encapsulation is as a protective wrapper that prevents the code and data from being arbitrarily accessed by other code defined outside the wrapper. Access to the code and data inside the wrapper is tightly controlled through a well-defined interface. Conclusion: The wrapping up of data and methods into a single unit (called class) is known as encapsulation.

Inheritance

Inheritance is the process by which object of one class acquires the properties of another class. Inheritance supports the concept of hierarchical classification. For example, the atlas is a part of class bicycle, which is again a part of the class cycle. As illustrated in the principal behind this sort of division is that each derived class shares common characteristics with the class from which it is derived. In OOP, the concept of inheritance provides the idea of reusability. This means that we can add additional features to an existing class without modifying it. This is possible deriving a new class from the existing one. The new class will have the combined features of both the classes. Thus the real appeal and power of the inheritance mechanism is that allows the programmer to reuse a class that is almost, but not exactly, what he wants, and to tailor the class is such a way that is does not introduce any undesirable side effects into the rest of the class. The drive class is knows as 'subclass'.

Polymorphism

Polymorphism is a feature that allows one interface to be used for a general class of actions. The specific action is determined by the exact nature of the solution. Polymorphism means the ability to take more than one form. For example, consider the operation of addition. For two numbers, the operation will generate the sum. If the operands are three numbers, then the operation would produce the product of them. That a single function name can be used to handle different number and different arguments. This is something similar to a particular word having several different meanings depending on the context. Polymorphism plays an important role in allowing objects having different internal structures to share the same external interface. This means that a general class of operations may be accessed in the same manner even though specific actions associated with each operation may differ. Polymorphism is extensively used in implementing inheritance.

Monday, May 6, 2013

lustered

Clustered indexes sort and store the data rows in the table or view based on their key values. These are the columns included in the index definition. There can be only one clustered index per table, because the data rows themselves can be sorted in only one order.
The only time the data rows in a table are stored in sorted order is when the table contains a clustered index. When a table has a clustered index, the table is called a clustered table. If a table has no clustered index, its data rows are stored in an unordered structure called a heap.

Nonclustered

Nonclustered indexes have a structure separate from the data rows. A nonclustered index contains the nonclustered index key values and each key value entry has a pointer to the data row that contains the key value.
The pointer from an index row in a nonclustered index to a data row is called a row locator. The structure of the row locator depends on whether the data pages are stored in a heap or a clustered table. For a heap, a row locator is a pointer to the row. For a clustered table, the row locator is the clustered index key.
You can add nonkey columns to the leaf level of the nonclustered index to by-pass existing index key limits, 900 bytes and 16 key columns, and execute fully covered, indexed, queries. For more information, see Create Indexes with Included Columns

Index Types
There are two main index types; Clustered index and Non-Clustered index.

A clustered index alters the way that the rows are stored. When you create a clustered index on a column (or a number of columns), SQL server sorts the table’s rows by that column(s). It is like a dictionary, where all words are sorted in alphabetical order in the entire book. Since it alters the physical storage of the table, only one clustered index can be created per table. In the above example the entire rows are sorted by computer_id since a clustered index on computer_id column has been created.
```
CREATE CLUSTERED INDEX [IX_CLUSTERED_COMPUTER_ID] 
ON [dbo].[nics] ([computer_id] ASC)
```
A non-clustered index, on the other hand, does not alter the way the rows are stored in the table. It creates a completely different object within the table that contains the column(s) selected for indexing and a pointer back to the table’s rows containing the data. It is like an index in the last pages of a book, where keywords are sorted and contain the page number to the material of the book for faster reference. A non-clustered index on the computer_id in the previous example would look like the table below:
```
CREATE NONCLUSTERED INDEX [IX_NONCLUSTERED_COMPUTER_ID] 
ON [dbo].[nics] ([computer_id] ASC)
```
```
 
```
```
 
```
Here is the common misconception prevailing in the industry.

Primary Key has to be Clustered Index.

In reality the statement should be corrected as follows:

Primary Key can be Clustered or Non-clustered but it is a common best practice to create a Primary Key as Clustered Index.

Scenario 1 : Primary Key will default to Clustered Index

In this case we will create only Primary Key and when we check the kind of index created on the table we will notice that it has created clustered index automatically over it.
-- Case 1 Primary Key Defaults to Clustered Index USE TempDB GO -- Create table CREATE TABLE TestTable (ID INT NOT NULL PRIMARY KEY, Col1 INT NOT NULL) GO -- Check Indexes SELECT OBJECT_NAME(OBJECT_ID) TableObject, [name] IndexName, [Type_Desc] FROM sys.indexes WHERE OBJECT_NAME(OBJECT_ID) = 'TestTable' GO -- Clean up DROP TABLE TestTable GO
```
 
```

Tuesday, April 23, 2013

State Management

State Management in ASP.NET - Introduction

By Sachin Gargava, 11 Nov 2012

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Download source code - 6.5 KB

Introduction

This article does an overview of state management techniques in ASP.NET. I will be discussing about the various types of state management techniques both client side and server side.

Background

State management means to preserve state of a control, web page, object/data, and user in the application explicitly because all ASP.NET web applications are stateless, i.e., by default, for each page posted to the server, the state of controls is lost. Nowadays all web apps demand a high level of state management from control to application level.

Using the code

Types of state management

There are two types of state management techniques: client side and server side.

Client side

Hidden Field
View State
Cookies
Control State
Query Strings

Server side

Session
Application

Levels of state management

Control level: In ASP.NET, by default controls provide state management automatically.
Variable or object level: In ASP.NET, member variables at page level are stateless and thus we need to maintain state explicitly.
Single or multiple page level: State management at single as well as multiple page level i.e., managing state between page requests.
User level: State should be preserved as long as a user is running the application.
Application level: State available for complete application irrespective of the user, i.e., should be available to all users.
Application to application level: State management between or among two or more applications.

Client side methods

1. Hidden field

Hidden field is a control provided by ASP.NET which is used to store small amounts of data on the client. It store one value for the variable and it is a preferable way when a variable's value is changed frequently. Hidden field control is not rendered to the client (browser) and it is invisible on the browser. A hidden field travels with every request like a standard control’s value.
Let us see with a simple example how to use a hidden field. These examples increase a value by 1 on every "No Action Button" click. The source of the hidden field control is.

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<asp:HiddenField ID="HiddenField1" runat="server"  />

In the code-behind page:

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protected void Page_Load(object sender, EventArgs e)
{
   if (HiddenField1.Value != null)
   {
    int val= Convert.ToInt32(HiddenField1.Value) + 1;
    HiddenField1.Value = val.ToString();
    Label1.Text = val.ToString();
   }
}
protected void Button1_Click(object sender, EventArgs e)
{
  //this is No Action Button Click
}

2. View state

View state is another client side state management mechanism provided by ASP.NET to store user's data, i.e., sometimes the user needs to preserve data temporarily after a post back, then the view state is the preferred way for doing it. It stores data in the generated HTML using hidden field not on the server.
View State provides page level state management i.e., as long as the user is on the current page, state is available and the user redirects to the next page and the current page state is lost. View State can store any type of data because it is object type but it is preferable not to store a complex type of data due to the need for serialization and deserilization on each post back. View state is enabled by default for all server side controls of ASP.NET with a property EnableviewState set to true.
Let us see how ViewState is used with the help of the following example. In the example we try to save the number of postbacks on button click.

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protected void Page_Load(object sender, EventArgs e)
{
    if (IsPostBack)
    {
        if (ViewState["count"] != null)
        {
            int ViewstateVal = Convert.ToInt32(ViewState["count"]) + 1;
            Label1.Text = ViewstateVal.ToString();
            ViewState["count"]=ViewstateVal.ToString();
        }
        else
        {
            ViewState["count"] = "1";
        }
    }
}
protected void Button1_Click(object sender, EventArgs e)
{
       Label1.Text=ViewState["count"].ToString();
}

3. Cookies

Cookie is a small text file which is created by the client's browser and also stored on the client hard disk by the browser. It does not use server memory. Generally a cookie is used to identify users.
A cookie is a small file that stores user information. Whenever a user makes a request for a page the first time, the server creates a cookie and sends it to the client along with the requested page and the client browser receives that cookie and stores it on the client machine either permanently or temporarily (persistent or non persistence). The next time the user makes a request for the same site, either the same or another page, the browser checks the existence of the cookie for that site in the folder. If the cookie exists it sends a request with the same cookie, else that request is treated as a new request.

Types of Cookies

1. Persistence Cookie: Cookies which you can set an expiry date time are called persistence cookies. Persistence cookies are permanently stored till the time you set.
Let us see how to create persistence cookies. There are two ways, the first one is:

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Response.Cookies["nameWithPCookies"].Value = "This is A Persistance Cookie";
Response.Cookies["nameWithPCookies"].Expires = DateTime.Now.AddSeconds(10);

And the second one is:

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HttpCookie aCookieValPer = new HttpCookie("Persistance");
aCookieValPer.Value = "This is A Persistance Cookie";
aCookieValPer.Expires = DateTime.Now.AddSeconds(10);
Response.Cookies.Add(aCookieValPer);

2. Non-Persistence Cookie: Non persistence cookies are not permanently stored on the user client hard disk folder. It maintains user information as long as the user accesses the same browser. When user closes the browser the cookie will be discarded. Non Persistence cookies are useful for public computers.
Let us see how to create a non persistence cookies. There are two ways, the first one is:

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Response.Cookies["nameWithNPCookies"].Value = "This is A Non Persistance Cookie";

And the second way is:

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HttpCookie aCookieValNonPer = new HttpCookie("NonPersistance");
aCookieValNonPer.Value = "This is A Non Persistance Cookie;
Response.Cookies.Add(aCookieValNonPer);how to create cookie :

How to read a cookie:

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if (Request.Cookies["NonPersistance"] != null)
Label2.Text = Request.Cookies["NonPersistance"].Value;

Let's understand persistence and non persistence cookies more clearly with a diagram:

Limitation of cookies: The number of cookies allowed is limited and varies according to the browser. Most browsers allow 20 cookies per server in a client's hard disk folder and the size of a cookie is not more than 4096 bytes or 4 KB of data that also includes name and value data.

4. Control State

Control State is another client side state management technique. Whenever we develop a custom control and want to preserve some information, we can use view state but suppose view state is disabled explicitly by the user, the control will not work as expected. For expected results for the control we have to use Control State property. Control state is separate from view state.
How to use control state property: Control state implementation is simple. First override the OnInit() method of the control and add a call for the Page.RegisterRequiresControlState() method with the instance of the control to register. Then override LoadControlState and SaveControlState in order to save the required state information.

Server side

1. Session

Session management is a very strong technique to maintain state. Generally session is used to store user's information and/or uniquely identify a user (or say browser). The server maintains the state of user information by using a session ID. When users makes a request without a session ID, ASP.NET creates a session ID and sends it with every request and response to the same user.
How to get and set value in Session:

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Session["Count"] = Convert.ToInt32(Session["Count"]) + 1;//Set Value to The Session
Label2.Text = Session["Count"].ToString(); //Get Value from the Sesion

Let us see an example where we save the count of button clicks in a session, and save the “number of redirects to the same page” button click in a query string. Here I have set the expiry to 10 minutes. After starting the application, the application variable exists till the end of the application. A session variable will expire after 10 minutes (if it is idle). A query string contains the value in URL so it won’t depend on the user idle time and could be used by the server anytime it is passed with a request.

Session Events in ASP.NET

To manage a session, ASP.NET provides two events: session_start and session_end that is written in a special file called Global.aspx in the root directory of the project.
Session_Start: The Session_start event is raised every time a new user makes a request without a session ID, i.e., new browser accesses the application, then a session_start event raised. Let's see the Global.asax file.

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void Session_Start(object sender, EventArgs e)
{
   Session["Count"] = 0;  // Code that runs when a new session is started
}

Session_End: The Session_End event is raised when session ends either because of a time out expiry or explicitly by using Session.Abandon(). The Session_End event is raised only in the case of In proc mode not in the state server and SQL Server modes.
There are four session storage mechanisms provided by ASP.NET:

In Proc mode
State Server mode
SQL Server mode
Custom mode

In Process mode: In proc mode is the default mode provided by ASP.NET. In this mode, session values are stored in the web server's memory (in IIS). If there are more than one IIS servers then session values are stored in each server separately on which request has been made. Since the session values are stored in server, whenever server is restarted the session values will be lost.

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<configuration>
 <sessionstate mode="InProc" cookieless="false" timeout="10" 
    stateConnectionString="tcpip=127.0.0.1:80808" 
    sqlConnectionString="Data Source=.\SqlDataSource;User ID=userid;Password=password"/>
</configuration>

In State Server mode: This mode could store session in the web server but out of the application pool. But usually if this mode is used there will be a separate server for storing sessions, i.e., stateServer. The benefit is that when IIS restarts the session is available. It stores session in a separate Windows service. For State server session mode, we have to configure it explicitly in the web config file and start the aspnet_state service.

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<configuration><sessionstate mode="stateserver" cookieless="false" 
   timeout="10"  stateConnectionString="tcpip=127.0.0.1:42424"  
   sqlConnectionString="Data Source=.\SqlDataSource;User ID=userid;Password=password"/> </configuration>

In SQL Server mode: Session is stored in a SQL Server database. This kind of session mode is also separate from IIS, i.e., session is available even after restarting the IIS server. This mode is highly secure and reliable but also has a disadvantage that there is overhead from serialization and deserialization of session data. This mode should be used when reliability is more important than performance.

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<configuration>
    <sessionstate mode="sqlserver" cookieless="false" timeout="10" 
       stateConnectionString="tcpip=127.0.0.1:4  2424" 
       sqlConnectionString="Data Source=.\SqlDataSource;User ID=userid;Password=password"/>
</configuration>

Custom Session mode: Generally we should prefer in proc state server mode or SQL Server mode but if you need to store session data using other than these techniques then ASP.NET provides a custom session mode. This way we have to maintain everything customized even generating session ID, data store, and also security.

Attributes	Description
`Cookieless` true/false	Indicates that the session is used with or without cookie. `cookieless` set to true indicates sessions without cookies is used and `cookieless` set to false indicates sessions with cookies is used. `cookieless` set to false is the default set.
`timeout`	Indicates the session will abound if it is idle before session is abounded explicitly (the default time is 20 min).
`StateConnectionString`	Indicates the session state is stored on the remote computer (server). This attribute is required when session mode is `StateServer`
`SqlConnectionString`	Indicates the session state is stored in the database. This attribute is required when session mode is `SqlServer`.

2. Application

Application state is a server side state management technique. The date stored in application state is common for all users of that particular ASP.NET application and can be accessed anywhere in the application. It is also called application level state management. Data stored in the application should be of small size.
How to get and set a value in the application object:

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Application["Count"] = Convert.ToInt32(Application["Count"]) + 1; //Set Value to The Application Object
Label1.Text = Application["Count"].ToString(); //Get Value from the Application Object

Application events in ASP.NET

There are three types of events in ASP.NET. Application event is written in a special file called Global.asax. This file is not created by default, it is created explicitly by the developer in the root directory. An application can create more than one Global.asax file but only the root one is read by ASP.NET.
Application_start: The Application_Start event is raised when an app domain starts. When the first request is raised to an application then the Application_Start event is raised. Let's see the Global.asax file.

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void Application_Start(object sender, EventArgs e)
{
    Application["Count"] = 0;
}

Application_Error: It is raised when an unhandled exception occurs, and we can manage the exception in this event.
Application_End: The Application_End event is raised just before an application domain ends because of any reason, may IIS server restarting or making some changes in an application cycle.
So we have talked about various types of state management techniques in this article. I have tried to touch several topics in this article but the main intention for this article was to get the user familiar with the various state management techniques that exist in ASP.NET. The details for all these techniques will make a complete article by itself which I will try to post in future.

Point of Interest

In this article we saw how State management is a very essential means to preserve the state of a control, web page, object/data and user. Since all web apps demand a high level of state management, I tried to put together an overview article of the various state management techniques.

History

08 Nov 2012: First version.

License

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)

About the Author

Sachin Gargava

Software Developer

India

Member

He has done Master Degree in Computer Application (MCA) and currently working as a S/W Developer and having more than 3 years exp. He has worked with all the .NET framework and good ex-poser in Web and Windows based development using Microsoft Technology.

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