1. How big is the datatype int in .NET?

32 bits.

2. How big is the char?

16 bits (Unicode).

3. How do you initiate a string without escaping each backslash?

Put an @ sign in front of the double-quoted string.

4. What are valid signatures for the Main function?

* public static void Main()
* public static int Main()
* public static void Main( string[] args )
* public static int Main(string[] args )

5. Does Main() always have to be public?

No.

6. How do you initialize a two-dimensional array that you don’t know the dimensions of?

* int [, ] myArray; //declaration
* myArray= new int [5, 8]; //actual initialization

7. What’s the access level of the visibility type internal?

Current assembly.

8. What’s the difference between struct and class in C#?

* Structs cannot be inherited.
* Structs are passed by value, not by reference.
* Struct is stored on the stack, not the heap.

9. Explain encapsulation.

The implementation is hidden, the interface is exposed.

10. What data type should you use if you want an 8-bit value that’s signed?

sbyte.

11. Speaking of Boolean data types, what’s different between C# and C/C++?

There’s no conversion between 0 and false, as well as any other number and true, like in C/C++.

12. Where are the value-type variables allocated in the computer RAM?

Stack.

13. Where do the reference-type variables go in the RAM?

The references go on the stack, while the objects themselves go on the heap. However, in reality things are more elaborate.

14. What is the difference between the value-type variables and reference-type variables in terms of garbage collection?

The value-type variables are not garbage-collected, they just fall off the stack when they fall out of scope, the reference-type objects are picked up by GC when their references go null.

15. How do you convert a string into an integer in .NET?

Int32.Parse(string), Convert.ToInt32()

16. How do you box a primitive data type variable?

Initialize an object with its value, pass an object, cast it to an object

17. Why do you need to box a primitive variable?

To pass it by reference or apply a method that an object supports, but primitive doesn’t.

18. What’s the difference between Java and .NET garbage collectors?

Sun left the implementation of a specific garbage collector up to the JRE developer, so their performance varies widely, depending on whose JRE you’re using. Microsoft standardized on their garbage collection.

19. How do you enforce garbage collection in .NET?

System.GC.Collect();

20. Can you declare a C++ type destructor in C# like ~MyClass()?

Yes, but what’s the point, since it will call Finalize(), and Finalize() has no guarantees when the memory will be cleaned up, plus, it introduces additional load on the garbage collector. The only time the finalizer should be implemented, is when you’re dealing with unmanaged code.

21. What’s different about namespace declaration when comparing that to package declaration in Java?

No semicolon. Package declarations also have to be the first thing within the file, can’t be nested, and affect all classes within the file.

22. What’s the difference between const and readonly?

You can initialize readonly variables to some runtime values. Let’s say your program uses current date and time as one of the values that won’t change. This way you declare
public readonly string DateT = new DateTime().ToString().

23. Can you create enumerated data types in C#?

Yes.

24. What’s different about switch statements in C# as compared to C++?

No fall-throughs allowed.

25. What happens when you encounter a continue statement inside the for loop?

The code for the rest of the loop is ignored, the control is transferred back to the beginning of the loop.

26. Is goto statement supported in C#?

How about Java? Gotos are supported in C#to the fullest. In Java goto is a reserved keyword that provides absolutely no functionality.

27. Describe the compilation process for .NET code?

Source code is compiled and run in the .NET Framework using a two-stage process. First, source code is compiled to Microsoft intermediate language (MSIL) code using a .NET Framework-compatible compiler, such as that for Visual Basic .NET or Visual C#. Second, MSIL code is compiled to native code.

28. Name any 2 of the 4 .NET authentification methods.

ASP.NET, in conjunction with Microsoft Internet Information Services (IIS), can authenticate user credentials such as names and passwords using any of the following authentication methods:
* Windows: Basic, digest, or Integrated Windows Authentication (NTLM or Kerberos).
* Microsoft Passport authentication
* Forms authentication
* Client Certificate authentication

29. How do you turn off SessionState in the web.config file?

In the system.web section of web.config, you should locate the httpmodule tag and you simply disable session by doing a remove tag with attribute name set to session.

30. What is main difference between Global.asax and Web.Config?

ASP.NET uses the global.asax to establish any global objects that your Web application uses. The .asax extension denotes an application file rather than .aspx for a page file. Each ASP.NET application can contain at most one global.asax file. The file is compiled on the first page hit to your Web application. ASP.NET is also configured so that any attempts to browse to the global.asax page directly are rejected. However, you can specify application-wide settings in the web.config file. The web.config is an XML-formatted text file that resides in the Web site’s root directory. Through Web.config you can specify settings like custom 404 error pages, authentication and authorization settings for the Web site, compilation options for the ASP.NET Web pages, if tracing should be enabled, etc.

ASP.NET interview questions

Q1 # Describe the role of inetinfo.exe, aspnet_isapi.dll andaspnet_wp.exe

in the page loading process. inetinfo.exe is theMicrosoft IIS server running,
handling ASP.NET requests among other things.When an ASP.NET request is received
(usually a file with .aspx extension),the ISAPI filter aspnet_isapi.dll takes
care of it by passing the request tothe actual worker process aspnet_wp.exe.

Q2 # What’s the difference between Response.Write() andResponse.Output.Write()?

The latter one allows you to write formattedoutput.

Q3 # What methods are fired during the page load?

nit() - when the page is instantiated, Load() - when the page is loaded into server memory, PreRender() - the brief moment before the page is displayed to the user asHTML,

Unload() - when page finishes loading.

Q4 # Where does the Web page belong in the .NET Framework class hierarchy?

System.Web.UI.Page

Q5 # Where do you store the information about the user’s locale?

System.Web.UI.Page.Culture

Q6 # What’s the difference between Codebehind="MyCode.aspx.cs" andSrc="MyCode.aspx.cs"?

CodeBehind is relevant to Visual Studio.NET only.

Q7 # What’s a bubbled event?

When you have a complex control, likeDataGrid, writing an event processing routine for each object (cell, button,row, etc.) is quite tedious. The controls can bubble up their eventhandlers, allowing the main DataGrid event handler to take care of itsconstituents.

Q8 # Suppose you want a certain ASP.NET function executed on MouseOver overa
certain button. Where do you add an event handler?

t’s the Attributesproperty, the Add function inside that property. So
btnSubmit.Attributes.Add("onMouseOver","someClientCode();")
A simple”Javascript:ClientCode();” in the button control of the .aspx page will attach the handler (javascript function)to the onmouseover event.

Q9 # What data type does the RangeValidator control support?

Integer,String and Date.

Q10 # Where would you use an iHTTPModule, and what are the limitations of any
approach you might take in implementing one?

One of ASP.NET’s most useful features is the extensibility of the HTTP pipeline, the path that data takes between client and server. You can use them to extend your ASP.NET applications by adding pre- and post-processing to each HTTP request coming into your application. For example, if you wanted custom authentication facilities for your application, the best technique
would be to intercept the request when it comes in and process the request in a custom HTTP module.

Q11 # Explain what a diffgram is, and a good use for one?

A DiffGram is
an XML format that is used to identify current and original versions of data elements. The DataSet uses the DiffGram format to load and persist its contents, and to serialize its contents for transport across a network connection. When a DataSet is written as a DiffGram, it populates the DiffGram with all the necessary information to accurately recreate the contents, though not the
schema, of the DataSet, including column values from both the Original and Current row versions, row error information, and row order.

ADO.NET

This lesson is an introduction to ADO.NET. It introduces primary ADO.NET concepts and objects that you will learn about in later lessons. Here are the objectives of this lesson:
· Learn what ADO.NET is.
· Understand what a data provider is.
· Understand what a connection object is.
· Understand what a command object is.
· Understand what a DataReader object is.
· Understand what a DataSet object is.
· Understand what a DataAdapter object is.
Introduction
ADO.NET is an object-oriented set of libraries that allows you to interact with data sources. Commonly, the data source is a data base, but it could also be a text file, an Excel spread sheet, or an XML file. For the purposes of this tutorial, we will look at ADO.NET as a way to interact with a data base.
As you are probably aware, there are many different types of data bases available. For example, there is Microsoft SQL Server, Microsoft Access, Oracle, Borland Interbase, and IBM DB2, just to name a few. To further refine the scope of this tutorial, all of the examples will use SQL Server.
Data Providers
We know that ADO.NET allows us to interact with different types of data sources and different types of data bases. However, there isn't a single set of classes that allow you to accomplish this universally. Since different data sources expose different protocols, we need a way to communicate with the right data source using the right protocol. Some older data sources use the ODBC protocol, many newer data sources use the OleDb protocol, and there are more data sources every day that allow you to communicate with them directly through .NET ADO.NET class libraries.
ADO.NET provides a relatively common way to interact with data sources, but comes in different sets of libraries for each way you can talk to a data source. These libraries are called Data Providers and are usually named for the protocol or data source type they allow you to interact with. table 1 lists some well known data providers, the API prefix they use, and the type of data source they allow you to interact with.
table 1. ADO.NET Data Providers are class libraries that allow a common way to interact with specific data sources or protocols. The library APIs have prefixes that indicate which provider they support.
Provider Name API prefix Data Source Description
ODBC Data Provider Odbc Data Sources with an ODBC interface. Normally older data bases.
OleDb Data Provider OleDb Data Sources that expose an OleDb interface, i.e. Access or Excel.
Oracle Data Provider Oracle For Oracle Data Bases.
SQL Data Provider Sql For interacting with Microsoft SQL Server.
Borland Data Provider Bdp Generic access to many data bases such as Interbase, SQL Server, IBM DB2, and Oracle.
An example may help you to understand the meaning of the API prefix. One of the first ADO.NET objects you'll learn about is the connection object, which allows you to establish a connection to a data source. If we were using the OleDb Data Provider to connect to a data source that exposes an OleDb interface, we would use a connection object named OleDbConnection. Similarly, the connection object name would be prefixed with Odbc or Sql for an OdbcConnection object on an Odbc data source or a SqlConnection object on a SQL Server data base, respectively. Since we are using MSDE in this tutorial (a scaled down version of SQL Server) all the API objects will have the Sql prefix. i.e. SqlConnection.
ADO.NET Objects
ADO.NET includes many objects you can use to work with data. This section introduces some of the primary objects you will use. Over the course of this tutorial, you'll be exposed to many more ADO.NET objects from the perspective of how they are used in a particular lesson. The objects below are the ones you must know. Learning about them will give you an idea of the types of things you can do with data when using ADO.NET.
The SqlConnection Object
To interact with a data base, you must have a connection to it. The connection helps identify the data base server, the data base name, user name, password, and other parameters that are required for connecting to the data base. A connection object is used by command objects so they will know which data base to execute the command on.
The SqlCommand Object
The process of interacting with a data base means that you must specify the actions you want to occur. This is done with a command object. You use a command object to send SQL statements to the data base. A command object uses a connection object to figure out which data base to communicate with. You can use a command object alone, to execute a command directly, or assign a reference to a command object to an SqlDataAdapter, which holds a set of commands that work on a group of data as described below.
The SqlDataReader Object
Many data operations require that you only get a stream of data for reading. The data reader object allows you to obtain the results of a SELECT statement from a command object. For performance reasons, the data returned from a data reader is a fast forward-only stream of data. This means that you can only pull the data from the stream in a sequential manner. This is good for speed, but if you need to manipulate data, then a DataSet is a better object to work with.
The DataSet Object
DataSet objects are in-memory representations of data. They contain multiple Datatable objects, which contain columns and rows, just like normal data base tables. You can even define relations between tables to create parent-child relationships. The DataSet is specifically designed to help manage data in memory and to support disconnected operations on data, when such a scenario make sense. The DataSet is an object that is used by all of the Data Providers, which is why it does not have a Data Provider specific prefix.
The SqlDataAdapter Object
Sometimes the data you work with is primarily read-only and you rarely need to make changes to the underlying data source. Some situations also call for caching data in memory to minimize the number of data base calls for data that does not change. The data adapter makes it easy for you to accomplish these things by helping to manage data in a disconnected mode. The data adapter fills a DataSet object when reading the data and writes in a single batch when persisting changes back to the data base. A data adapter contains a reference to the connection object and opens and closes the connection automatically when reading from or writing to the data base. Additionally, the data adapter contains command object references for SELECT, INSERT, UPDATE, and DELETE operations on the data. You will have a data adapter defined for each table in a DataSet and it will take care of all communication with the data base for you. All you need to do is tell the data adapter when to load from or write to the data base.
Summary
ADO.NET is the .NET technology for interacting with data sources. You have several Data Providers, which allow communication with different data sources, depending on the protocols they use or what the data base is. Regardless, of which Data Provider used, you'll use a similar set of objects to interact with a data source. The SqlConnection object lets you manage a connection to a data source. SqlCommand objects allow you to talk to a data source and send commands to it. To have fast forward-only read access to data, use the SqlDataReader. If you want to work with disconnected data, use a DataSet and implement reading and writing to/from the data source with a SqlDataAdapter.
Lesson 02: The SqlConnection Object
This lesson describes the SqlConnection object and how to connect to a data base. Here are the objectives of this lesson:
· Know what connection objects are used for.
· Learn how to instantiate a SqlConnection object.
· Understand how the SqlConnection object is used in applications.
· Comprehend the importance of effective connection lifetime management.
Introduction
The first thing you will need to do when interacting with a data base is to create a connection. The connection tells the rest of the ADO.NET code which data base it is talking to. It manages all of the low level logic associated with the specific data base protocols. This makes it easy for you because the most work you will have to do in code is instantiate the connection object, open the connection, and then close the connection when you are done. Because of the way that other classes in ADO.NET are built, sometimes you don't even have to do that much work.
Although working with connections is very easy in ADO.NET, you need to understand connections in order to make the right decisions when coding your data access routines. Understand that a connection is a valuable resource. Sure, if you have a stand-alone client application that works on a single data base one one machine, you probably don't care about this. However, think about an enterprise application where hundreds of users throughout a company are accessing the same data base. Each connection represents overhead and there can only be a finite amount of them. To look at a more extreme case, consider a Web site that is being hit with hundreds of thousands of hits a day. Applications that grab connections and don't let them go can have seriously negative impacts on performance and scalability.
Creating a SqlConnection Object
A SqlConnection is an object, just like any other C# object. Most of the time, you just declare and instantiate the SqlConnection all at the same time, as shown below:
SqlConnection conn = new SqlConnection(
"Data Source=(local);Initial Catalog=Northwind;Integrated Security=SSPI");
The SqlConnection object instantiated above uses a constructor with a single argument of type string. This argument is called a connection string. table 1 describes common parts of a connection string.
table 1. ADO.NET Connection Strings contain certain key/value pairs for specifying how to make a data base connection. They include the location, name of the database, and security credentials.
Connection String Parameter Name Description
Data Source Identifies the server. Could be local machine, machine domain name, or IP Address.
Initial Catalog Data base name.
Integrated Security Set to SSPI to make connection with user's Windows login
User ID Name of user configured in SQL Server.
Password Password matching SQL Server User ID.
Integrated Security is secure when you are on a single machine doing development. However, you will often want to specify security based on a SQL Server User ID with permissions set specifically for the application you are using. The following shows a connection string, using the User ID and Password parameters:
SqlConnection conn = new SqlConnection(
"Data Source=DatabaseServer;Initial Catalog=Northwind;User ID=YourUserID;Password=YourPassword");
Notice how the Data Source is set to DatabaseServer to indicate that you can identify a data base located on a different machine, over a LAN, or over the Internet. Additionally, User ID and Password replace the Integrated Security parameter.
Using a SqlConnection
The purpose of creating a SqlConnection object is so you can enable other ADO.NET code to work with a data base. Other ADO.NET objects, such as a SqlCommand and a SqlDataAdapter take a connection object as a parameter. The sequence of operations occurring in the lifetime of a SqlConnection are as follows:
1. Instantiate the SqlConnection.
2. Open the connection.
3. Pass the connection to other ADO.NET objects.
4. Perform data base operations with the other ADO.NET objects.
5. Close the connection.
We've already seen how to instantiate a SqlConnection. The rest of the steps, opening, passing, using, and closing are shown in Listing 1.
Listing 1. Using a SqlConnection
using System;
using System.Data;
using System.Data.SqlClient;

///
/// Demonstrates how to work with SqlConnection objects
///
class SqlConnectionDemo
{
static void Main()
{
// 1. Instantiate the connection
SqlConnection conn = new SqlConnection(
"Data Source=(local);Initial Catalog=Northwind;Integrated Security=SSPI");

SqlDataReader rdr = null;

try
{
// 2. Open the connection
conn.Open();

// 3. Pass the connection to a command object
SqlCommand cmd = new SqlCommand("select * from Customers", conn);

//
// 4. Use the connection
//

// get query results
rdr = cmd.ExecuteReader();

// print the CustomerID of each record
while (rdr.Read())
{
Console.WriteLine(rdr[0]);
}
}
finally{
// close the reader
if (rdr != null)
{
rdr.Close();
}

// 5. Close the connection
if (conn != null)
{
conn.Close();
}
}
}
}
As shown in Listing 1, you open a connection by calling the Open() method of the SqlConnection instance, conn. Any operations on a connection that was not yet opened will generate an exception. So, you must open the connection before using it.
Before using a connection, you must let the ADO.NET code know which connection it needs. In Listing 1, we set the second parameter to the SqlCommand object with the SqlConnection object, conn. Any operations performed with the SqlCommand will use that connection.
The code that uses the connection is a SqlCommand object, which performs a query on the Customers table. The result set is returned as a SqlDataReader and the while loop reads the first column from each row of the result set, which is the CustomerID column. We'll discuss the SqlCommand and SqlDataReader objects in later lessons. For right now, it is important for you to understand that these objects are using the SqlConnection object so they know what database to interact with.
When you are done using the connection object, you must close it. Failure to do so could have serious consequences in the performance and scalability of your application. There are a couple points to be made about how we closed the connection in Listing 1: the Close() method is called in a finally block and we ensure that the connection is not null before closing it.
Notice that we wrapped the ADO.NET code in a try/finally block. As described in Lesson 15: Introduction to Exception Handling of the C# Tutorial, finally blocks help guarantee that a certain piece of code will be executed, regardless of whether or not an exception is generated. Since connections are scarce system resources, you will want to make sure they are closed in finally blocks.
Another precaution you should take when closing connections is to make sure the connection object is not null. If something goes wrong when instantiating the connection, it will be null and you want to make sure you don't try to close an invalid connection, which would generate an exception.
This example showed how to use a SqlConnection object with a SqlDataReader, which required explicitly closing the connection. However, when using a disconnected data model, you don't have to open and close the connection yourself. We'll see how this works in a future lesson when we look at the SqlDataAdapter object.
Summary
SqlConnection objects let other ADO.NET code know what data base to connect to and how to make the connection. They are instantiated by passing a connection string with a set of key/value pairs that define the connection. The steps you use to manage the lifetime of a connection are create, open, pass, use, and close. Be sure to close your connection properly when you are done with it to ensure you don't have a connection resource leak.
Lesson 03: The SqlCommand Object
This lesson describes the SqlCommand object and how you use it to interact with a data base. Here are the objectives of this lesson:
· Know what a command object is.
· Learn how to use the ExecuteReader method to query data.
· Learn how to use the ExecuteNonQuery method to insert and delete data.
· Learn how to use the ExecuteScalar method to return a single value.
Introduction
A SqlCommand object allows you to specify what type of interaction you want to perform with a data base. For example, you can do select, insert, modify, and delete commands on rows of data in a data base table. The SqlCommand object can be used to support disconnected data management scenarios, but in this lesson we will only use the SqlCommand object alone. A later lesson on the SqlDataAdapter will explain how to implement an application that uses disconnected data. This lesson will also show you how to retrieve a single value from a data base, such as the number of records in a table.
Creating a SqlCommand Object
Similar to other C# objects, you instantiate a SqlCommand object via the new instance declaration, as follows:
SqlCommand cmd = new SqlCommand("select CategoryName from Categories", conn);
The line above is typical for instantiating a SqlCommand object. It takes a string parameter that holds the command you want to execute and a reference to a SqlConnection object. SqlCommand has a few overloads, which you will see in the examples of this tutorial.
Querying Data
When using a SQL select command, you retrieve a data set for viewing. To accomplish this with a SqlCommand object, you would use the ExecuteReader method, which returns a SqlDataReader object. We'll discuss the SqlDataReader in a future lesson. The example below shows how to use the SqlCommand object to obtain a SqlDataReader object:
// 1. Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand("select CategoryName from Categories", conn);

// 2. Call Execute reader to get query results
SqlDataReader rdr = cmd.ExecuteReader();
In the example above, we instantiate a SqlCommand object, passing the command string and connection object to the constructor. Then we obtain a SqlDataReader object by calling the ExecuteReader method of the SqlCommand object, cmd.
This code is part of the ReadData method of Listing 1 in the Putting it All Together section later in this lesson.
Inserting Data
To insert data into a data base, use the ExecuteNonQuery method of the SqlCommand object. The following code shows how to insert data into a data base table:
// prepare command string
string insertString = @"
insert into Categories
(CategoryName, Description)
values ('Miscellaneous', 'Whatever doesn''t fit elsewhere')";

// 1. Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand(insertString, conn);

// 2. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
The SqlCommand instantiation is just a little different from what you've seen before, but it is basically the same. Instead of a literal string as the first parameter of the SqlCommand constructor, we are using a variable, insertString. The insertString variable is declared just above the SqlCommand declaration.
Notice the two apostrophes ('') in the insertString text for the word "doesn''t". This is how you escape the apostrophe to get the string to populate column properly.
Another observation to make about the insert command is that we explicitly specified the columns CategoryName and Description. The Categories table has a primary key field named CategoryID. We left this out of the list because SQL Server will add this field itself. trying to add a value to a primary key field, such as CategoryID, will generate an exception.
To execute this command, we simply call the ExecuteNonQuery method on the SqlCommand instance, cmd.
This code is part of the Insertdata method of Listing 1 in the Putting it All Together section later in this lesson.
Updating Data
The ExecuteNonQuery method is also used for updating data. The following code shows how to update data:
// prepare command string
string updateString = @"
update Categories
set CategoryName = 'Other'
where CategoryName = 'Miscellaneous'";

// 1. Instantiate a new command with command text only
SqlCommand cmd = new SqlCommand(updateString);

// 2. Set the Connection property
cmd.Connection = conn;

// 3. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
Again, we put the SQL command into a string variable, but this time we used a different SqlCommand constructor that takes only the command. In step 2, we assign the SqlConnection object, conn, to the Connection property of the SqlCommand object, cmd.
This could have been done with the same constructor used for the insert command, with two parameters. It demonstrates that you can change the connection object assigned to a command at any time.
The ExecuteNonQuery method performs the update command.
This code is part of the UpdateData method of Listing 1 in the Putting it All Together section later in this lesson.
Deleting Data
You can also delete data using the ExecuteNonQuery method. The following example shows how to delete a record from a data base with the ExecuteNonQuery method:
// prepare command string
string deleteString = @"
delete from Categories
where CategoryName = 'Other'";

// 1. Instantiate a new command
SqlCommand cmd = new SqlCommand();

// 2. Set the CommandText property
cmd.CommandText = deleteString;

// 3. Set the Connection property
cmd.Connection = conn;

// 4. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
This example uses the SqlCommand constructor with no parameters. Instead, it explicity sets the CommandText and Connection properties of the SqlCommand object, cmd.
We could have also used either of the two previous SqlCommand constructor overloads, used for the insert or update command, with the same result. This demonstrates that you can change both the command text and the connection object at any time.
The ExecuteNonQuery method call sends the command to the data base.
This code is part of the DeleteData method of Listing 1 in the Putting it All Together section later in this lesson.
Getting Single values
Sometimes all you need from a data base is a single value, which could be a count, sum, average, or other aggregated value from a data set. Performing an ExecuteReader and calculating the result in your code is not the most efficient way to do this. The best choice is to let the data base perform the work and return just the single value you need. The following example shows how to do this with the ExecuteScalar method:
// 1. Instantiate a new command
SqlCommand cmd = new SqlCommand("select count(*) from Categories", conn);

// 2. Call ExecuteNonQuery to send command
int count = (int)cmd.ExecuteScalar();
The query in the SqlCommand constructor obtains the count of all records from the Categories table. This query will only return a single value. The ExecuteScalar method in step 2 returns this value. Since the return type of ExecuteScalar is type object, we use a cast operator to convert the value to int.
This code is part of the GetNumberOfRecords method of Listing 1 in the Putting it All Together section later in this lesson.
Putting it All Together
For simplicity, we showed snippets of code in previous sections to demonstrate the applicable techniques . It is also useful to have an entire code listing to see how this code is used in a working program. Listing 1 shows all of the code used in this example, along with a driver in the Main method to produce formatted output.
Listing 1. SqlConnection Demo
using System;
using System.Data;
using System.Data.SqlClient;

///
/// Demonstrates how to work with SqlCommand objects
///
class SqlCommandDemo
{
SqlConnection conn;

public SqlCommandDemo()
{
// Instantiate the connection
conn = new SqlConnection(
"Data Source=(local);Initial Catalog=Northwind;Integrated Security=SSPI");
}

// call methods that demo SqlCommand capabilities
static void Main()
{
SqlCommandDemo scd = new SqlCommandDemo();

Console.WriteLine();
Console.WriteLine("Ca tegories Before Insert");
Console.WriteLine("------------------------");

// use ExecuteReader method
scd.ReadData();

// use ExecuteNonQuery method for Insert
scd.Insertdata();
Console.WriteLine();
Console.WriteLi ne("Categories After Insert");
Console.WriteLine("------------------------------");
< BR>scd.ReadData();

// use ExecuteNonQuery method for Update
scd.UpdateData();

Console.WriteLine();
Console.Wri teLine("Categories After Update");
Console.WriteLine("------------------------------");
< BR>scd.ReadData();

// use ExecuteNonQuery method for Delete
scd.DeleteData();

Console.WriteLine();
Console.Wri teLine("Categories After Delete");
Console.WriteLine("------------------------------");
< BR>scd.ReadData();

// use ExecuteScalar method
int numberOfRecords = scd.GetNumberOfRecords();

Console.WriteLine();
Console.Write Line("Number of Records: {0}", numberOfRecords);
}

///
/// use ExecuteReader method
///
public void ReadData()
{
SqlDataReader rdr = null;

try
{
// Open the connection
conn.Open();

// 1. Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand("select CategoryName from Categories", conn);

// 2. Call Execute reader to get query results
rdr = cmd.ExecuteReader();

// print the CategoryName of each record
while (rdr.Read())
{
Console.WriteLine(rdr[0]);
}
}
finally{
// close the reader
if (rdr != null)
{
rdr.Close();
}

// Close the connection
if (conn != null)
{
conn.Close();
}
}
}

///
/// use ExecuteNonQuery method for Insert
///
public void Insertdata()
{
try
{
// Open the connection
conn.Open();

// prepare command string
string insertString = @"
insert into Categories
(CategoryName, Description)
values ('Miscellaneous', 'Whatever doesn''t fit elsewhere')";

// 1. Instantiate a new command with a query and connection
SqlCommand cmd = new SqlCommand(insertString, conn);

// 2. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
}
finally
{
// Close the connection
if (conn != null)
{
conn.Close();
}
}
}

///
/// use ExecuteNonQuery method for Update
///
public void UpdateData()
{
try
{
// Open the connection
conn.Open();

// prepare command string
string updateString = @"
update Categories
set CategoryName = 'Other'
where CategoryName = 'Miscellaneous'";

// 1. Instantiate a new command with command text only
SqlCommand cmd = new SqlCommand(updateString);

// 2. Set the Connection property
cmd.Connection = conn;

// 3. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
}
finally
{
// Close the connection
if (conn != null)
{
conn.Close();
}
}
}

///
/// use ExecuteNonQuery method for Delete
///
public void DeleteData()
{
try
{
// Open the connection
conn.Open();

// prepare command string
string deleteString = @"
delete from Categories
where CategoryName = 'Other'";

// 1. Instantiate a new command
SqlCommand cmd = new SqlCommand();

// 2. Set the CommandText property
cmd.CommandText = deleteString;

// 3. Set the Connection property
cmd.Connection = conn;

// 4. Call ExecuteNonQuery to send command
cmd.ExecuteNonQuery();
}
finally
{
// Close the connection
if (conn != null)
{
conn.Close();
}
}
}

///
/// use ExecuteScalar method
///
/// number of records
public int GetNumberOfRecords()
{
int count = -1;

try
{
// Open the connection
conn.Open();

// 1. Instantiate a new command
SqlCommand cmd = new SqlCommand("select count(*) from Categories", conn);

// 2. Call ExecuteNonQuery to send command
count = (int)cmd.ExecuteScalar();
}
finally
{
// Close the connection
if (conn != null)
{
conn.Close();
}
}
return count;
}
}
In Listing 1, the SqlConnection object is instantiated in the SqlCommandDemo structure. This is okay because the object itself will be cleaned up when the CLR garbage collector executes. What is important is that we close the connection when we are done using it. This program opens the connection in a try block and closes it in a finally block in each method.
The ReadData method displays the contents of the CategoryName column of the Categories table. We use it several times in the Main method to show the current status of the Categories table, which changes after each of the insert, update, and delete commands. Because of this, it is convenient to reuse to show you the effects after each method call.
Summary
A SqlCommand object allows you to query and send commands to a data base. It has methods that are specialized for different commands. The ExecuteReader method returns a SqlDataReader object for viewing the results of a select query. For insert, update, and delete SQL commands, you use the ExecuteNonQuery method. If you only need a single aggregate value from a query, the ExecuteScalar is the best choice.


Lesson 04: Reading Data with the SqlDataReader
This lesson explains how to read data with a SqlDataReader object. Here are the objectives of this lesson:
· Learn what a SqlDataReader is used for.
· Know how to read data using a SqlDataReader.
· Understand the need to close a SqlDataReader.
Introduction
A SqlDataReader is a type that is good for reading data in the most efficient manner possible. You can *not* use it for writing data. SqlDataReaders are often described as fast-forward firehose-like streams of data.
You can read from SqlDataReader objects in a forward-only sequential manner. Once you've read some data, you must save it because you will not be able to go back and read it again.
The forward only design of the SqlDataReader is what enables it to be fast. It doesn't have overhead associated with traversing the data or writing it back to the data source. Therefore, if your only requirement for a group of data is for reading one time and you want the fastest method possible, the SqlDataReader is the best choice. Also, if the amount of data you need to read is larger than what you would prefer to hold in memory beyond a single call, then the streaming behavior of the SqlDataReader would be a good choice.
Note: Observe that I used the term "one time" in the previous paragraph when discussing the reasons why you would use a SqlDataReader. As with anything, there are exceptions. In many cases, it is more efficient to use a cached DataSet. While caching is outside the scope of this tutorial, we will discuss using DataSet objects in the next lesson.
Creating a SqlDataReader Object
Getting an instance of a SqlDataReader is a little different than the way you instantiate other ADO.NET objects. You must call ExecuteReader on a command object, like this:
SqlDataReader rdr = cmd.ExecuteReader();
The ExecuteReader method of the SqlCommand object, cmd , returns a SqlDataReader instance. Creating a SqlDataReader with the new operator doesn't do anything for you. As you learned in previous lessons, the SqlCommand object references the connection and the SQL statement necessary for the SqlDataReader to obtain data.
Reading Data
previous lessons contained code that used a SqlDataReader, but the discussion was delayed so we could focus on the specific subject of that particular lesson. This lesson builds from what you've seen and explains how to use the SqlDataReader.
As explained earlier, the SqlDataReader returns data via a sequential stream. To read this data, you must pull data from a table row-by-row. Once a row has been read, the previous row is no longer available. To read that row again, you would have to create a new instance of the SqlDataReader and read through the data stream again.
The typical method of reading from the data stream returned by the SqlDataReader is to iterate through each row with a while loop. The following code shows how to accomplish this:
while (rdr.Read())
{
// get the results of each column
string contact = (string)rdr["ContactName"];
string company = (string)rdr["CompanyName"];
string city = (string)rdr["City"];

// print out the results
Console.Write("{0,-25}", contact);
Console.Write("{0,-20}", city);
Console.Write("{0,-25}", company);
Console.WriteLine();
}
Notice the call to Read on the SqlDataReader, rdr, in the while loop condition in the code above. The return value of Read is type bool and returns true as long as there are more records to read. After the last record in the data stream has been read, Read returns false.
In previous lessons, we extracted the first column from the row by using the SqlDataReader indexer, i.e. rdr[0]. You can extract each column of the row with a numeric indexer like this, but it isn't very readable. The example above uses a string indexer, where the string is the column name from the SQL query (the table column name if you used an asterisk, *. String indexers are much more readable, making the code easier to maintain.
Regardless of the type of the indexer parameter, a SqlDataReader indexer will return type object. This is why the example above casts results to a string. Once the values are extracted, you can do whatever you want with them, such as printing them to output with Console type methods.
Finishing Up
Always remember to close your SqlDataReader, just like you need to close the SqlConnection. Wrap the data access code in a try block and put the close operation in the finally block, like this:
try
{
// data access code
}
finally
{
// 3. close the reader
if (rdr != null)
{
rdr.Close();
}

// close the connection too
}
The code above checks the SqlDataReader to make sure it isn't null. After the code knows that a good instance of the SqlDataReader exists, it can close it. Listing 1 shows the code for the previous sections in its entirety.
Listing 1: Using the SqlDataReader
using System;
using System.Data;
using System.Data.SqlClient;

namespace Lesson04
{
class ReaderDemo
{
static void Main()
{
ReaderDemo rd = new ReaderDemo();
rd.SimpleRead();
}

public void SimpleRead()
{
// declare the SqlDataReader, which is used in
// both the try block and the finally block
SqlDataReader rdr = null;

// create a connection object
SqlConnection conn = new SqlConnection(
"Data Source=(local);Initial Catalog=Northwind;Integrated Security=SSPI");

// create a command object
SqlCommand cmd = new SqlCommand(
"select * from Customers", conn);

try
{
// open the connection
conn.Open();

// 1. get an instance of the SqlDataReader
rdr = cmd.ExecuteReader();

// print a set of column headers
Console.WriteLine(
"Contact Name City Company Name");
Console.WriteLine(
"------------ ------------ ------------");

// 2. print necessary columns of each record
while (rdr.Read())
{
// get the results of each column
string contact = (string)rdr["ContactName"];
string company = (string)rdr["CompanyName"];
string city = (string)rdr["City"];

// print out the results
Console.Write("{0,-25}", contact);
Console.Write("{0,-20}", city);
Console.Write("{0,-25}", company);
Console.WriteLine();
}
}
finally
{
// 3. close the reader
if (rdr != null)
{
rdr.Close();
}

// close the connection
if (conn != null)
{
conn.Close();
}
}
}
}
}
Summary
SqlDataReader objects allow you to read data in a fast forward-only manner. You obtain data by reading each row from the data stream. Call the Close method of the SqlDataReader to ensure there are not any resource leaks.
Lesson 05: Working with Disconnected Data - The DataSet and SqlDataAdapter
This lesson explains how to work with disconnected data, using the DataSet and SqlDataAdapter objects. Here are the objectives of this lesson:
· Understand the need for disconnected data.
· Obtain a basic understanding of what a DataSet is for.
· Learn to use a SqlDataAdapter to retrieve and update data.
Introduction
In Lesson 3, we discussed a fully connected mode of operation for interacting with a data source by using the SqlCommand object. In Lesson 4, we learned about how to read data quickly an let go of the connection with the SqlDataReader. This Lesson shows how to accomplish something in-between SqlConnection and SqlDataReader interaction by using the DataSet and SqlDataAdapter objects.
A DataSet is an in-memory data store that can hold numerous tables. DataSets only hold data and do not interact with a data source. It is the SqlDataAdapter that manages connections with the data source and gives us disconnected behavior. The SqlDataAdapter opens a connection only when required and closes it as soon as it has performed its task. For example, the SqlDataAdapter performs the following tasks when filling a DataSet with data:
1. Open connection
2. Retrieve data into DataSet
3. Close connection
and performs the following actions when updating data source with DataSet changes:
1. Open connection
2. Write changes from DataSet to data source
3. Close connection
In between the Fill and Update operations, data source connections are closed and you are free to read and write data with the DataSet as you need. These are the mechanics of working with disconnected data. Because the applications holds on to connections only when necessary, the application becomes more scalable.
A couple scenarios illustrate why you would want to work with disconnected data: people working without network connectivity and making Web sites more scalable. Consider sales people who need customer data as they travel. At the beginning of the day, they'll need to sync up with the main data base to have the latest information available. During the day, they'll make modifications to existing customer data, add new customers, and input new orders. This is okay because they have a given region or customer base where other people won't be changing the same records. At the end of the day, the sales person will connect to the network and update changes for overnight processing.
Another scenario is making a Web site more scalable. With a SqlDataReader, you have to go back to the data base for records every time you show a page. This requires a new connection for each page load, which will hurt scalability as the number of users increase. One way to relieve this is to use a DataSet that is updated one time and stored in cache. Ev