functionality classes ?
Following are the namespaces provided by .NET for data management :-
System.data
This contains the basic objects used for accessing and storing relational data, such as DataSet,DataTable, and DataRelation. Each of these is independent of the type of data source and the way we connect to it.
System.Data.OleDB
Contains the objects that we use to connect to a data source via an OLE-DB provider, such as OleDbConnection, OleDbCommand, etc. These objects inherit from the common base classes, and so have the same properties, methods, and events as the SqlClient equivalents.
System.Data.SqlClient:
This Contains the objects that we use to connect to a data source via the Tabular Data Stream (TDS) interface of Microsoft SQL Server (only). This can generally provide better performance as it removes some of the intermediate layers required by an OLE-DB connection.
System.XML
This Contains the basic objects required to create, read, store, write, and manipulate
XML documents according to W3C recommendations.
(B) Can you give a overview of ADO.NET architecture ?
The most important section in ADO.NET architecture is “Data Provider”.Data Provider provides access to datasource (SQL SERVER , ACCESS ,ORACLE).In short it provides object to achieve functionalities like opening and closing connection , retrieve data and update data.In the below figure you can see the four main sections of a data provider :-
Ö Connection.
* 70 percent of IT projects are database projects,so ADO.NET forms the heart of major
.NET interviews.
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Ö Command object (This is the responsible object to use stored procedures)
Ö Data Adapter (This object acts as a bridge between datastore and dataset).
Ö Datareader (This object reads data from data store in forward only mode). Dataset object represents disconnected and cached data.If you see the diagram it is not in
direct connection with the data store (SQL SERVER , ORACLE etc) rather it talks with Data adapter , who is responsible for filling the dataset.Dataset can have one or more Datatable and relations.
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Figure :- 9.1 ADO.NET Architecture
* Not answering simple ADO.NET questions can lead to negative results.
“DataView” object is used to sort and filter data in Datatable.
Note:- This is one of the favorite questions in .NET.Just paste the picture in your mind and during interview try to refer that image.
(B)What are the two fundamental objects in ADO.NET ? Datareader and Dataset are the two fundamental objects in ADO.NET. (B)What is difference between dataset and datareader ? Following are some major differences between dataset and datareader :-
Ö DataReader provides forward-only and read-only access to data , while the
DataSet object can hold more than one table (in other words more than one rowset) from the same data source as well as the relationships between them.
Ö Dataset is a disconnected architecture while datareader is connected architecture.
Ö Dataset can persists contents while datareader can not persist contents , they are forward only.
(I)What are major difference between classic ADO and
ADO.NET ?
Following are some major differences between both
Ö As in classic ADO we had client and server side cursors they are no more present in ADO.NET.Note it's a disconnected model so they are no more applicable.
Ö Locking is not supported due to disconnected model.
Ö All data is persisted in XML as compared to classic ADO where data was persisted in Binary format also.
(B)What is the use of connection object ?
They are used to connect a data to a Command object.
Ö An OleDbConnection object is used with an OLE-DB provider
* Why do you want to leave this company ? (Never say anything negative about your past company)
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Ö A SqlConnection object uses Tabular Data Services (TDS) with MS SQL Server
(B)What is the use of command objects and what are the methods provided by the command object ?
They are used to connect connection object to Datareader or dataset.Following are the methods provided by command object :-
Ö ExecuteNonQuery :- Executes the command defined in the CommandText property against the connection defined in the Connection property for a query that does not return any rows (an UPDATE, DELETE or INSERT).Returns an Integer indicating the number of rows affected by the query.
Ö ExecuteReader :- Executes the command defined in the CommandText property against the connection defined in the Connection property. Returns a "reader" object that is connected to the resulting rowset within the database, allowing the rows to be retrieved.
Ö ExecuteScalar :- Executes the command defined in the CommandText property against the connection defined in the Connection property. Returns only a single value (effectively the first column of the first row of the resulting rowset). Any other returned columns and rows are discarded. Fast and efficient when only a "singleton" value is required
(B)What is the use of dataadapter ?
These are objects that connect one or more Command objects to a Dataset object..They provide logic that gets the data from the data store and populates the tables in the DataSet, or pushes the changes in the DataSet back into the data store.
Ö An OleDbDataAdapter object is used with an OLE-DB provider
Ö A SqlDataAdapter object uses Tabular Data Services with MS SQL Server.
(B)What are basic methods of Dataadapter ?
There are three most commonly used methods of Dataadapter :-
Fill :- Executes the SelectCommand to fill the DataSet object with data from the data source. Can also be used to update (refresh) an existing table in a DataSet with changes
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* Author has Intentionally marked all Database questions as Basic belive me not answering them will not give a good impression during interviews.
made to the data in the original datasource if there is a primary key in the table in the
DataSet.
FillSchema :- Uses the SelectCommand to extract just the schema for a table from the data source, and creates an empty table in the DataSet object with all the corresponding constraints.
Update:- Calls the respective InsertCommand, UpdateCommand, or DeleteCommand for each inserted, updated,or deleted row in the DataSet so as to update the original data source with the changes made to the content of the DataSet. This is a little like the UpdateBatch method provided by the ADO Recordset object, but in the DataSet it can be used to update more than one table.
(B)What is Dataset object?
The DataSet provides the basis for disconnected storage and manipulation of relational data. We fill it from a data store,work with it while disconnected from that data store, then reconnect and flush changes back to the data store if required.
(B)What are the various objects in Dataset ?
Dataset has a collection of DataTable object within the Tables collection. Each DataTable object contains a collection of DataRow objects and a collection of DataColumn objects. There are also collections for the primary keys,constraints, and default values used in this table which is called as constraint collection, and the parent and child relationships between the tables.Finally, there is a DefaultView object for each table. This is used to create a DataView object based on the table, so that the data can be searched, filtered or otherwise manipulated while displaying the data.
Note :- Look back again to the main diagram for ADO.NET architecture for visualizing this answer in pictorial form
(B) How can we connect to Microsoft Access , Foxpro , Oracle etc ?
Microsoft provides System.Data.OleDb namespace to communicate with databases like Access , oracle etc.In short any OLE DB-Compliant database can be connected using System.Data.OldDb namespace.
* No one can be perfect in .NET as whole , so when you think you do not know the answer admit it , it put’s lot of + effect.
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Note :- Small sample of OLEDB is provided in “WindowsAppOleDb” which uses
“Nwind.mdb” in bin directory to display data in Listbox.
Private Sub loadData()
Dim strPath As String
strPath = AppDomain.CurrentDomain.BaseDirectory
Dim objOLEDBCon As New OleDbConnection(“Provider=Microsoft.Jet.OLEDB.4.0;Data Source =” & strPath & “Nwind.mdb”)
Dim objOLEDBCommand As OleDbCommand Dim objOLEDBReader As OleDbDataReader Try
objOLEDBCommand = New OleDbCommand(“Select FirstName from Employees”)
objOLEDBCon.Open() objOLEDBCommand.Connection = objOLEDBCon objOLEDBReader = objOLEDBCommand.ExecuteReader() Do While objOLEDBReader.Read()
lstNorthwinds.Items.Add(objOLEDBReader.GetString(0)) Loop
Catch ex As Exception
Throw ex
Finally objOLEDBCon.Close()
End Try
End Sub
The main heart is the “Loaddata()” method which actually loads the data in listbox.
Note:- This source code has the connectionstring hard coded in the program itself which is not a good programming practice.For windows application the best place to store connectionstring is “App.config”.Also note “AppDomain.CurrentDomain.BaseDirectory” function this gives the current path of the running exe which is “BIN” and the MDB file is in that directory.Also note the finally block which executes irrespective that there is error or not.Thus ensuring that all the connection to the datastore is freed.It’s best practice to put all clean up statements in finally block thus ensuring that the resources are deallocated properly.
(B) How do we connect to SQL SERVER , which namespace do we use ?
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* Where do you see yourself after three years ?
First below is the code , after the code i have given the explanation for it.For this sample we will also need a SQL Table setup which i have imported using the DTS wizard.
Private Sub LoadData()
‘ note :- with and end with makes your code more readable
Dim strConnectionString As String
Dim objConnection As New SqlConnection
Dim objCommand As New SqlCommand Dim objReader As SqlDataReader Try
file.
‘ this gets the connectionstring from the app.config
‘ note if this gives error see where the MDB file is
stored in your pc and point to that strConnectionString =
AppSettings.Item(“ConnectionString”)
‘ take the connectiostring and initialize the connection
object
Employees”)
With objConnection
.ConnectionString = strConnectionString
.Open() End With
objCommand = New SqlCommand(“Select FirstName from
With objCommand
.Connection = objConnection objReader = .ExecuteReader()
End With
‘ looping through the reader to fill the list box
Do While objReader.Read()
lstData.Items.Add(objReader.Item(“FirstName”)) Loop
Catch ex As Exception
Throw ex
Finally objConnection.Close()
End Try
* What are your negative points ? (Careful guy’s)
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Note:- The above code is provided in CD in folder WindowsAppSqlClient”.Comments in the code do explain a lot but we will again iterate through the whole code
later..”LoadData” is the main method which loads the data from SQL SERVER.Before running this code you have to install SQL SERVER in your machine.As we are dealing with SQLCLIENT we need to setup database in SQL SERVER.For this sample i have imported access “Nwind.mdb” in “SampleAccessDatabase” folder in CD in to SQlSERVER.Depending on computer you will also have to change the connectionstring in Web.config file.
For setting up the sample SQL table we can use the DTS import wizard to import.See the below figure which is using data source as Microsoft Access.While importing the database author had give the database name as “Employees”.
Figure:- 9.2 Loading “Nwind.mdb” in SQL SERVER for the sample
180 * Are you comfortable with location shift ? If you have personal problems say no right at the first stage.... or else after joining surely you will repent.
Figure :- 9.3 Load only the Employee table.
For simplicity sake we will only import the employee table as thats the only thing needed in our sample code.
* Do you work during late night’s ?.Best answer if there is project deadline yes.Do not sho1w81
that it’s your culture to work during nights.
Figure :- 9.4 View of loaded Employee table
Now from interview point of view definitely you are not going to say the whole source code which is given in book.Interviewer expects only the broader answer of what are the steps needed to connect to SQL SERVER.For fundamental sake author has explained the whole source code.In short you have to explain the “LoadData” method in broader way.Following are the steps to connect to SQL SERVER :-
Ö First is import the namespace “System.Data.SqlClient”.
Ö Create a connection object as shown in “LoadData” method.
With objConnection
.ConnectionString = strConnectionString
.Open() End With
Ö Create the command object with the SQL.Also assign the created connection object to command object. and execute the reader.
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* Any special acheivements in your life till now...tell your best project which you have done best in your career .
objCommand = New SqlCommand(“Select FirstName from Employees”) With objCommand
.Connection = objConnection objReader = .ExecuteReader()
End With
Ö Finally loop through the reader and fill the list box.If old VB programmers are
expecting the movenext command it’s replaced by Read() which returns true if there is any data to be read.If the .Read() return’s false that means that it’s end of datareader and there is no more data to be read.
Do While objReader.Read() lstData.Items.Add(objReader.Item(“FirstName”)) Loop
Ö Finally do not forget to close the connection object.
Note:- In “LoadData” you will see that connectionstring is stored in Web.config file and is loaded using “AppSettings.Item(“ConnectionString”)”.While running this sample live on your database do not forget to change this connectionstring accordingly to your machine name and SQL SERVER or else the source code will not run.
(B) How do we use stored procedure in ADO.NET and how do we provide parameters to the stored procedures?
ADO.NET provides the SqlCommand object which provides the functionality of executing stored procedures.
Note :- Sample code is provided in folder “WindowsSqlClientCommand”.There are two stored procedures created in same database “Employees” which was created for the previous question.
CREATE PROCEDURE SelectByEmployee @FirstName nvarchar(200) AS Select FirstName from Employees where FirstName like @FirstName + '%' CREATE PROCEDURE SelectEmployee AS
Select FirstName from Employees
If txtEmployeeName.Text.Length = 0 Then
objCommand = New SqlCommand(“SelectEmployee”)
Else
objCommand = New SqlCommand(“SelectByEmployee”)
* Are you looking for onsite oppurtunites ? (Be careful do not show your desperation of abroad journeys)
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objCommand.Parameters.Add(“@FirstName”, Data.SqlDbType.NVarChar, 200)
objCommand.Parameters.Item(“@FirstName”).Value =
txtEmployeeName.Text.Trim() End If
In the above sample not lot has been changed only that the SQL is moved to the stored
procedures.There are two stored procedures one is “SelectEmployee” which selects all the employees and the other is “SelectByEmployee” which returns employee name starting with a specific character.As you can see to provide parameters to the stored procedures we are using the parameter object of the command object.In such question interviewer expects two simple answers one is that we use command object to execute stored procedures and the parameter object to provide parameter to the stored procedure.Above sample is provided only for getting the actual feel of it.Be short be nice and get a job.
(B) How can we force the connection object to close after my datareader is closed ?
Command method Executereader takes a parameter called as CommandBehavior where in we can specify saying close connection automatically after the Datareader is close.
pobjDataReader = pobjCommand.ExecuteReader(CommandBehavior.CloseConnection)
(B) I want to force the datareader to return only schema of the datastore rather than data ?
pobjDataReader = pobjCommand.ExecuteReader(CommandBehavior.SchemaOnly)
(B) How can we fine tune the command object when we are expecting a single row or a single value ?
Again CommandBehaviour enumeration provides two values SingleResult and SingleRow.If you are expecting a single value then pass “CommandBehaviour.SingleResult” and the query is optimized accordingly , if you are expecting single row then pass “CommandBehaviour.SingleRow” and query is optimized according to single row.
(B) Which is the best place to store connectionstring in .NET
projects ?
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* Why have you changed so many jobs ? (Prepare a decent answer do not blame companies and individiuals for your frequent change)
Config files are the best place to store connection strings.If it’s a web-based application “Web.config” file will be used and if it’s a windows application “App.config” files will be used.
(B) What are steps involved to fill a dataset ?
Twist :- How can we use dataadapter to fill a dataset ?
Sample code is provided in “WindowsDataSetSample” folder in CD.”LoadData” has all the implementation of connecting and loading to dataset.This dataset is finally binded to a ListBox.Below is the sample code.
Private Sub LoadData()
Dim strConnectionString As String
strConnectionString = AppSettings.Item(“ConnectionString”) Dim objConn As New SqlConnection(strConnectionString) objConn.Open()
Dim objCommand As New SqlCommand(“Select FirstName from
Employees”)
objCommand.Connection = objConn
Dim objDataAdapter As New SqlDataAdapter() objDataAdapter.SelectCommand = objCommand Dim objDataSet As New DataSet
End Sub
In such type of question’s interviewer is looking from practical angle , that have you worked with dataset and datadapters.Let me try to explain the above code first and then we move to what steps to say suring interview.
Dim objConn As New SqlConnection(strConnectionString)
objConn.Open()
First step is to open the connection.Again note the connection string is loaded from config file.
Dim objCommand As New SqlCommand(“Select FirstName from Employees”)
objCommand.Connection = objConn
Second step is to create a command object with appropriate SQL and set the connection object to this command.
Dim objDataAdapter As New SqlDataAdapter()
* Any plans of opening your own software company...Beware do not start pouring your billgate’s dream to him.....can lead to a serious situation.
185
objDataAdapter.SelectCommand = objCommand
Third steps is to create the Adapter object and pass the command object to the adapter object.
objDataAdapter.Fill(objDataSet)
Fourth step is to load the dataset using the “Fill” method of the dataadapter. lstData.DataSource = objDataSet.Tables(0).DefaultView lstData.DisplayMember = “FirstName”
lstData.ValueMember = “FirstName”
Fifth step is to bind to the loaded dataset with the GUI.At this moment sample has listbox as the UI.Binding of the UI is done by using DefaultView of the dataset.Just to revise every dataset has tables and every table has views.In this sample we have only loaded one table i.e. Employees table so we are referring that with a index of zero.
Just say all the five steps during interview and you will see the smile in the interviewer’s face.....Hmm and appointment letter in your hand.
(B)What are the various methods provided by the dataset object to generate XML?
Note:- XML is one of the most important leap between classic ADO and ADO.NET.So this question is normally asked more generally how can we convert any data to XML format.Best answer is convert in to dataset and use the below methods.
Ö ReadXML
Read’s a XML document in to Dataset.
Ö GetXML
This is function’s which return’s a string containing XML document.
Ö WriteXML
This write’s a XML data to disk.
(B) How can we save all data from dataset ?
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Dataset has “AcceptChanges” method which commits all the changes since last time
“Acceptchanges” has been executed.
Note :- This book does not have any sample of Acceptchanges.I leave that to reader’s as homework sample.But yes from interview aspect that will be enough.
(B) How can we check that some changes have been made to dataset since it was loaded ?
Twist :- How can we cancel all changes done in dataset ? , How do we get values which are changed in a dataset ?
For tracking down changes Dataset has two methods which comes as rescue “GetChanges
“and “HasChanges”.
GetChanges
Return’s dataset which are changed since it was loaded or since Acceptchanges was executed.
HasChanges
This property indicate’s has any change’s been made since the dataset was loaded or acceptchanges method was executed.
If we want to revert or abandon all change’s since the dataset was loaded use
“RejectChanges”.
Note:- One of the most misunderstood things about these properties is that it tracks the changes of actual database.That’s a fundamental mistake , actually the changes are related to only changes with dataset and has nothing to with changes happening in actual database.As dataset are disconnected and do not know anything about the changes happening in actual database.
(B) How can we add/remove row’s in “DataTable” object of
“DataSet” ?
“Datatable” provides “NewRow” method to add new row to “DataTable”.”DataTable” has “DataRowCollection” object which has all rows in a “DataTable” object.Following are the methods provided by “DataRowCollection” object :-
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Add
Add’s a new row in DataTable
Remove
Remove’s a “DataRow” object from “DataTable”
RemoveAt
Remove’s a “DataRow” object from “DataTable” depending on index position of the
“DataTable”.
(B) What’s basic use of “DataView” ?
“DataView” represent’s a complete table or can be small section of row’s depending on some criteria.It’s best used for sorting and finding data with in “datatable”.
Dataview has the following method’s :-
Find
Take’s a array of value’s and return’s the index of the row.
FindRow
This also takes array of values but returns a collection of “DataRow”.
If we want to manipulate data of “DataTable” object create “DataView” (Using the “DefaultView” we can create “DataView” object) of the “DataTable” object. and use the following functionalities :-
AddNew
Add’s a new row to the “DataView” object.
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Delete
Deletes the specified row from “DataView” object.
(B) What’s difference between “DataSet” and “DataReader”
?
Twist :- Why is DataSet slower than DataReader ? Fourth point is the answer to the twist.
Note:- This is my best question and i expect everyone to answer it.It’s asked almost 99% in all companies....Basic very Basic cram it.
Following are the major difference between “DataSet” and “DataReader” :-
Ö “DataSet” is a disconnected architecture , while “DataReader” has live connection while reading data.So if we want to cache data and pass to a different tier “DataSet” forms the best choice and it has decent XML support.
Ö When application needs to access data from more than one table “DataSet”
forms the best choice.
Ö If we need to move back while reading record’s , “datareader” does not support this functionality.
Ö But one of the biggest drawbacks of DataSet is speed.As “DataSet” carry considerable overhead because of relations,multiple tables etc speed is slower than “DataReader”.Always try to use “DataReader” whereever possible. , as it’s meant specially for speed performance.
(B) How can we load multiple tables in a DataSet ? objCommand.CommandText = "Table1" objDataAdapter.Fill(objDataSet, "Table1") objCommand.CommandText = "Table2" objDataAdapter.Fill(objDataSet, "Table2")
Above is a sample code which shows how to load multiple “DataTable” object’s in one
“DataSet” object.Sample code shows two tables “Table1” and “Table2” in object
ObjDataSet.
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lstdata.DataSource = objDataSet.Tables("Table1").DefaultView
In order to refer “Table1” DataTable , use Tables collection of DataSet and the Defaultview object will give you the necessary output.
(B) How can we add relation’s between table in a DataSet ?
Dim objRelation As DataRelation objRelation=New DataRelation("CustomerAddresses",objDataSet.Tables("Customer").Columns("Custid")
,objDataSet.Tables("Addresses").Columns("Custid_fk"))
objDataSet.Relations.Add(objRelation)
Relation’s can be added between “DataTable” object’s using the “DataRelation” object.Above sample code is trying to build a relationship between “Customer” and “Addresses” “Datatable” using “CustomerAddresses” “DataRelation” object.
(B) What’s the use of CommandBuilder ?
CommandBuilder builds “Parameter” objects automatically.Below is a simple code which uses commandbuilder to load its parameter object’s.
Dim pobjCommandBuilder As New OleDbCommandBuilder(pobjDataAdapter)
pobjCommandBuilder.DeriveParameters(pobjCommand)
Be careful while using “DeriveParameters” method as it needs a extra trip to the Datastore which can be very inefficient.
(B) What’s difference between “Optimistic” and
“Pessimistic” locking ?
In pessimistic locking when user wants to update data it locks the record and till then no one can update data.Other user’s can only view the data when there is pessimistic locking.
In optimistic locking multiple user’s can open the same record for updating , thus increase maximum concurrency.Record is only locked when updating the record.This is the most
190
preferred way of locking practically.Now a days browser based application are very common and having pessimistic locking is not a practical solution.
(A) How many way’s are there to implement locking in
ADO.NET ?
Following are the ways to implement locking using ADO.NET :-
Ö When we call “Update” method of DataAdapter it handles locking internally.If the DataSet values are mot matching with current data in Database it raises Concurrency exception error.We can easily trap this error using Try..Catch block and raise appropriate error message to the user.
Ö Define a Datetime stamp field in the table.When actually you are firing the UPDATE SQL statements compare the current timestamp with one existing in the database.Below is a sample SQL which checks for timestamp before updating and any mismatch in timestamp it will not update the records.This is the best practice used by industries for locking.
Update table1 set field1=@test where LastTimeStamp=@CurrentTimeStamp
Ö Check for original values stored in SQL SERVER and actual changed values.In stored procedure check before updating that the old data is same as the current.Example in the below shown SQL before updating field1 we check that is the old field1 value same.If not then some one else has updated and necessary action has to be taken.
Update table1 set field1=@test where field1 = @oldfield1value
Locking can be handled at ADO.NET side or at SQL SERVER side i.e. in stored procedures.for more details of how to implementing locking in SQL SERVER read “What are different locks in SQL SERVER ?” in SQL SERVER chapter.
Note:- This is one of the favorite question’s of interviewer , so cram it....When i say cram it i do not mean it.... i mean understand it.This book has tried to cover ADO.NET as much as possible , but indeterminist nature of ADO.NET interview question’s makes it difficult to make full justice.But hope so that the above question’s will make you quiet confident during interviews.
(A)How can we perform transactions in .NET?
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The most common sequence of steps that would be performed while developing a transactional application is as follows:
Ö Open a database connection using the Open method of the connection object.
Ö Begin a transaction using the Begin Transaction method of the connection object. This method provides us with a transaction object that we will use later to commit or rollback the transaction. Note that changes caused by any queries executed before calling the Begin Transaction method will be committed to the database immediately after they execute.Set the Transaction property of the command object to the above mentioned transaction object.
Ö Execute the SQL commands using the command object. We may use one or more command objects for this purpose, as long as the Transaction property of all the objects is set to a valid transaction object.
Ö Commit or roll back the transaction using the Commit or Rollback method of the transaction object.
Ö Close the database connection.
(I)What’s difference between Dataset. clone and Dataset. copy ?
Clone: - It only copies structure, does not copy data. Copy: - Copies both structure and data.
(A)Can you explain the difference between an ADO.NET Dataset and an ADO Recordset?
There two main basic differences between recordset and dataset :-
Ö With dataset you an retrieve data from two databases like oracle and sql server and merge them in one dataset , with recordset this is not possible
Ö All representation of Dataset is using XML while recordset uses COM.
Ö Recordset can not be transmitted on HTTP while Dataset can be.
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