Tuesday, 4 February 2014

ADO.Net Interview Questions



ADO - ActiveX Data Object

Explain ADO.NET in brief?
ADO.NET is a very important feature of .NET Framework, which is used to work with data that is stored in structured data sources, such as databases and XML files. The following are some of the important features of ADO.NET:
1)    Contains a number of classes that provide you with various methods and attributes to manage the communication between your application and data source.
2)    Enables you to access different data sources, such as Microsoft SQL Server, and XML, as per your requirements.
3)    Provides a rich set of features, such as connection and commands that can be used to develop robust and highly efficient data services in .NET applications.
4)    Provides various data providers that are specific to databases produced by various vendors. For example, ADO.NET has a separate provider to access data from Oracle databases; whereas, another provider is used to access data from SQL databases.

What are major difference between classic ADO and ADO.NET?
Following are some major differences between both:
1)    In ADO we have recordset and in ADO.NET we have dataset.
2)    In recordset we can only have one table. If we want to accommodate more than one tables. We need to do inner join and fill the recordset. Dataset can have multiple tables.
3)    All data persist in XML as compared to classic ADO where data persisted in Binary format also.

What are the two fundamental objects in ADO.NET?
DataReader and DataSet are the two fundamental objects in ADO.NET.

What are the benefits of using of ADO.NET in .NET 4.0.
The following are the benefits of using ADO.NET in .NET 4.0 are as follows:
1)    Language-Integrated Query (LINQ) - Adds native data-querying capabilities to .NET languages by using syntax similar to that of SQL. This means that LINQ simplifies querying by eliminating the need to use a separate query language.
2)    LINQ to DataSet - Allows you to implement LINQ queries for disconnected data stored in a dataset. LINQ to DataSet enables you to query data that is cached in a DataSet object. DataSet objects allow you to use a copy of the data stored in the tables of a database, without actually getting connected to the database.
3)    LINQ to SQL - Allows you to create queries for data stored in SQL server database in your .NET application. You can use the LINQ to SQL technology to translate a query into a SQL query and then use it to retrieve or manipulate data contained in tables of an SQL Server database. LINQ to SQL supports all the key functions that you like to perform while working with SQL, that is, you can insert, update, and delete information from a table.
4)    SqlClient Support for SQL Server 2008 - Specifies that with the starting of .NET Framework version 3.5 Service Pack (SP) 1, .NET Framework Data Provider for SQL Server (System.Data.SqlClient namespace) includes all the new features that make it fully compatible with SQL Server 2008 Database Engine.
5)    ADO.NET Data Platform - Specifies that with the release of .NET Framework 3.5 Service Pack (SP) 1, an Entity Framework 3.5 was introduced that provides a set of Entity Data Model (EDM) functions. These functions are supported by all the data providers; thereby, reducing the amount of coding and maintenance in your application. In .NET Framework 4.0, many new functions, such as string, aggregate, mathematical, and date/time functions have been added.

Which namespaces are required to enable the use of databases in ASP.NET pages?
The following namespaces are required to enable the use of databases in ASP.NET pages:
1)    The System.Data namespace.
2)    The System.Data.OleDb namespace (to use any data provider, such as Access, Oracle, or SQL)
3)    The System.Data.SQLClient namespace (specifically to use SQL as the data provider)

Explain the DataAdapter.Update() and DataSet.AcceptChanges() methods.
The DataAdapter.Update() method calls any of the DML statements, such as the UPDATE, INSERT, or DELETE statements, as the case may be to update, insert, or delete a row in a DataSet.
The DataSet.Acceptchanges() method reflects all the changes made to the row since the last time the AcceptChanges() method was called.

What is the meaning of object pooling?
Object pooling is a concept of storing a pool (group) of objects in memory that can be reused later as needed.
Whenever, a new object is required to create, an object from the pool can be allocated for this request; thereby, minimizing the object creation. A pool can also refer to a group of connections and threads. Pooling, therefore, helps in minimizing the use of system resources, improves system scalability, and performance.

What is the difference between OLEDB Provider and SqlClient?
With respect to usage, there is no difference between OLEDB Provider and SqlClient. The difference lies in their performance. SqlClient is explicitly used to connect your application to SQL server directly; OLEDB Provider is generic for various databases, such as Oracle and MS-Access including SQL Server.
Therefore, there will be an overhead which leads to performance degradation.

Which architecture does Datasets follow?
Datasets follow the disconnected data architecture.

What is the role of the DataSet object in ADO.NET?
One of the major components of ADO.NET is the DataSet object, which always remains disconnected from the database and reduces the load on the database.

What is a DataReader object?
The DataReader object helps in retrieving the data from a database in a forward-only, read-only mode. The base class for all the DataReader objects is the DbDataReader class.
The DataReader object is returned as a result of calling the ExecuteReader() method of the Command object. The DataReader object enables faster retrieval of data from databases and enhances the performance of .NET applications by providing rapid data access speed. However, it is less preferred as compared to the DataAdapter object because the DataReader object needs an Open connection till it completes reading all the rows of the specified table.
An Open connection to read data from large tables consumes most of the system resources. When multiple client applications simultaneously access a database by using the DataReader object, the performance of data retrieval and other related processes is substantially reduced. In such a case, the database might refuse connections to other .NET applications until other clients free the resources.

How can you identify whether or not any changes are made to the DataSet object since it was last loaded?
The DataSet object provides the following two methods to track down the changes:
1)    The GetChanges() method - Returns the DataSet object, which is changed since it was loaded or since the AcceptChanges() method was executed.
2)    The HasChanges() method - Indicates if any changes occurred since the DataSet object was loaded or after a call to the AcceptChanges() method was made.
If you want to revert all changes since the DataSet object was loaded, use the RejectChanges() method.

Name the method that needs to be invoked on the DataAdapter control to fill the generated DataSet with data?
The Fill() method is used to fill the dataset with data.

What is the use of the CommandBuilder class?
The CommandBuilder class is used to automatically update a database according to the changes made in a DataSet.
This class automatically registers itself as an event listener to the RowUpdating event. Whenever data inside a row changes, the object of the CommandBuilder class automatically generates an SQL statement and uses the SelectCommand property to commit the changes made in DataSet.
OLEDB provider in .NET Framework has the OleDbCommandBuiider class; whereas, the SQL provider has the SqlCommandBuilder class.

Describe the disconnected architecture of ADO.NET's data access model.
ADO.NET maintains a disconnected database access model, which means, the application never remains connected constantly to the data source. Any changes and operations done on the data are saved in a local copy (dataset) that acts as a data source. Whenever, the connection to the server is re-established, these changes are sent back to the server, in which these changes are saved in the actual database or data source.

What are the usages of the Command object in ADO.NET?
The Command object in AD0.NET executes a command against the database and retrieves a DataReader or DataSet object.
1)    It also executes the INSERT, UPDATE, or DELETE command against the database.
2)    All the command objects are derived from the DbCommand class.
3)    The command object is represented by two classes: SqlCommand and OleDbCommand.
4)    The Command object provides three methods to execute commands on the database:
a)    The ExecuteNonQuery() method executes the commands and does not return any value.
b)    The ExecuteScalar() method returns a single value from a database query.
c)    The ExecuteReader() method returns a result set by using the DataReader object.

What are the pre-requisites for connection pooling?
The prerequisites for connection pooling are as follows:
  • There must be multiple processes to share the same connection describing the same parameters and security settings.
  • The connection string must be identical.
What is connection pooling?
Connection pooling refers to the task of grouping database connections in cache to make them reusable because opening new connections every time to a database is a time-consuming process. Therefore, connection pooling enables you to reuse already existing and active database connections, whenever required, and increasing the performance of your application.
You can enable or disable connection pooling in your application by setting the pooling property to either true or false in connection string. By default, it is enabled in an application.

What are the various methods provided by the DataSet object to generate XML?
The various methods provided by the DataSet object to generate XML are:
1)    ReadXml() - Reads XML document into a DataSet object.
2)    GetXml() - Returns a string containing an XML document.
3)    WriteXml() - Writes an XML data to disk.

Explain the new features in ADO.NET Entity Framework 4.0.
ADO.NET Entity Framework 4.0 is introduced in .NET Framework 4.0 and includes the following new features:
1)    Persistence Ignorance - Facilitates you to define your own Plain Old CLR Objects (POCO) which are independent of any specific persistence technology.
2)    Deferred or Lazy Loading - Specifies that related entities can be loaded automatically whenever required. You can enable lazy loading in your application by setting the DeferredLoadingEnabled property to true.
3)    Self-Tracking Entities - Refers to the entities that are able to track their own changes. These changes can be passed across process boundaries and saved to the database.
4)    Model-First Development - Allows you to create your own EDM and then generate relational model (database) from that EDM with matching tables and relations.
5)    Built-in Functions - Enables you to use built-in SQL Server functions directly in your queries.
6)    Model-Defined Functions - Enables you to use the functions that are defined in conceptual schema definition language (CSDL).

What is the difference between the Clone() and Copy() methods of the DataSet class?
The Clone() method copies only the structure of a DataSet. The copied structure includes all the relation, constraint, and DataTable schemas used by the DataSet. The Clone() method does not copy the data, which is stored in the DataSet.
The Copy() method copies the structure as well as the data stored in the DataSet.

What is the use of DataView?
User-defined view of a table is contained in a DataView. A complete table or a small section of table depending on some criteria can be presented by an object of the DataView class. You can use this class to sort and find data within DataTable.
The DataView class has the following methods:
  • Find() - Finds a row in a DataView by using sort key value.
  • FindRows() - Uses the sort key value to match it with the columns of DataRowView objects. It returns an array of all the corresponding objects of DataRowView whose columns match with the sort key value.
  • AddNew() - Adds a new row to the DataView object.
  • Delete() - Deletes the specified row from the DataView object according to the specified index.

What are the parameters that control most of connection pooling behaviors?
The parameters that control most of connection pooling behaviors are as follows:
  • Connect Timeout
  • Max Pool Size
  • Min Pool Size
  • Pooling

How can you add or remove rows from the DataTable object of DataSet?
The DataRowCollection class defines the collection of rows for the DataTable object in a DataSet. The DataTable class provides the NewRow() method to add a new DataRow to DataTable. The NewRow method creates a new row, which implements the same schema as applied to the DataTable. The following are the methods provided by the DataRowCollection object:
  • Add() - Adds a new row to DataRowCollection.
  • Remove()- Removes a DataRow object from DataRowCollection.
  • RemoveAt() - Removes a row whose location is specified by an index number.

Explain in brief DataAdapter class in ADO.NET.
The DataAdapter class retrieves data from the database, stores data in a dataset, and reflects the changes made in the dataset to the database. The DataAdapter class acts as an intermediary for all the communication between the database and the DataSet object. The DataAdapter Class is used to fill a DataTable or DataSet Object with data from the database using the Fill() method. The DataAdapter class applies the changes made in dataset to the database by calling the Update() method.
The DataAdapter class provides four properties that represent the database command:
SelectCommand, InsertCommand, DeleteCommand, and UpdateCommand.


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