What are the commonly used classes in System.Data.SqlClient?

Here are the commonly used classes in System.Data.SqlClient:

SqlCommand:
Represents a Transact-SQL statement or stored procedure to execute against a SQL Server database.

SqlCommandBuilder:
Automatically generates single-table commands used to reconcile changes made to a DataSet with the associated SQL Server database.

SqlConnection:
Represents an open connection to a SQL Server database.

SqlDataAdapter:
Represents a set of data commands and a database connection that are used to fill the DataSet and update a SQL Server database.

SqlDataReader:
Provides a means of reading a forward-only stream of rows from a SQL Server database.

SqlTransaction:
Represents a Transact-SQL transaction to be made in a SQL Server database.

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What is the Namespace for using SQL Server database in Dotnet Application?

System.Data.SqlClient namespace is the .NET Framework Data Provider for SQL Server.

The .NET Framework Data Provider for SQL Server describes a collection of classes used to access a SQL Server database in the managed space. Using the SqlDataAdapter, you can fill a memory-resident DataSet, which you can use to query and update the database.

For the commonly used classes in System.Data.SqlClient, click here.

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What is QueryString? What are the advantages and disadvantages of QueryString?

QueryString is a property of Request object in ASP.Net that provides a mechanism to send information to the server in the form of Sting in the URL.

Example: http://www.testsite.com/Webform1.aspx?fname=Alok&lname=Ranjan

In the above example, the destination webform name is Webform1.aspx and the information passed in QueryString is fname=Alok&lname=Ranjan. fname is a variable whose value is Alok, and similarly lname variable has value as Ranjan.

Advantages of QueryString:

• It's very easy to implement
• Every browser works with QueryString.
• It doesn't require server resources. Hence, doesn't put any burden on Server.

Disadvantages of QueryString:

• There is a limitation in the length of the QueryString as URL length has a limit. If you have to send lot of information, QueryString approach doesn't work.
• The information passed through QueryString is publicly visible. Hence, it poses security issue. So, QueryString can't be used while sending sensitive info. 
• QueryString can not be used to send & and space characters.