Frequently asked ADO.NET Interview Questions

How do you create an instance of SqlDataReader class?
To create an instance of SqlDataReader class, you must call the ExecuteReader method of the SqlCommand object, instead of directly using a constructor.
//Error! Cannot use SqlDataReader() constructor
//to create an instance of SqlDataReader class
SqlDataReader ReaderObject = new SqlDataReader();

//Call the ExecuteReader method of the SqlCommand object
SqlCommand CommandObject = new SqlCommand();
SqlDataReader ReaderObject = CommandObject.ExecuteReader();

Creating an instance of SqlDataReader class using SqlDataReader() constructor generates a compile time error - The type 'System.Data.SqlClient.SqlDataReader' has no constructors defined.

How do you programatically check if a specified SqlDataReader instance has been closed?
Use the IsClosed property of SqlDataReader to check if a specified SqlDataReader instance has been closed. If IsClosed property returns true, the SqlDataReader instance has been closed else not closed.

How do you get the total number of columns in the current row of a SqlDataReader instance?
FieldCount property can be used to get the total number of columns in the current row of a SqlDataReader instance.

Give an example for executing a stored procedure with parameters?
//Create the Connection Object
SqlConnection ConnectionObject = new SqlConnection(ConnectionString);
//Create the Command Object
SqlCommand CommandObject = new SqlCommand("StoredProcedureName", ConnectionObject);
//Specify to CommandObject that you intend to execute a Stored Procedure
CommandObject.CommandType = CommandType.StoredProcedure;
//Create an SQL Parameter object
SqlParameter ParameterObject = new SqlParameter();
//Specify the name of the SQL Parameter
ParameterObject.ParameterName = "Parameter1";
//Assign the Parameter value
ParameterObject.Value = "Some Value";
//Specify the Database DataType of the Parameter
ParameterObject.DbType = DbType.String;
//Specify the type of parameter - input-only, output-only, bidirectional
ParameterObject.Direction = ParameterDirection.Input;
//Associate the Parameter to the Command Object
CommandObject.Parameters.Add(ParameterObject);
//Open the connection
ConnectionObject.Open();
//Execute the command
int Records_Affected = CommandObject.ExecuteNonQuery();
//Close the Connection
ConnectionObject.Close();

What is the use of SqlParameter.Direction Property?
SqlParameter.Direction Property is used to specify the Sql Parameter type - input-only, output-only, bidirectional, or a stored procedure return value parameter. The default is Input.

How do you retrieve two tables of data at the same time by using data reader?
Include 2 select statements either in a stored procedure or in a select command and call the ExecuteReader() method on the command object. This will automatically fill the DataReader with 2 Tables of data.

The datareader will always return the data from first table only. If you want to get the second table then you need to use ReaderObject.NextResult() method. The NextResult() method will return true if there is another table. The following code shows you how do it.
//Create the SQL Query with 2 Select statements
string SQLQuery = "Select * from Customers;Select * from Employees;";
//Create the Connection Object
SqlConnection ConnectionObject = new SqlConnection(ConnectionString);
//Create the Command Object
SqlCommand CommandObject = new SqlCommand(SQLQuery, ConnectionObject);
//Open the connection
ConnectionObject.Open();
//Execute the command. Now reader object will have 2 tables of data.
SqlDataReader ReaderObject = CommandObject.ExecuteReader();
//Loop thru the tables in the DataReader object
while (ReaderObject.NextResult())
{
while (ReaderObject.Read())
{
//Do Something
}
}
//Close the Reader
ReaderObject.Close();
//Close the Connection
ConnectionObject.Close();

What are the advantages of using SQL stored procedures instead of adhoc SQL queries in an ASP.NET web application?
Better Performance : As stored procedures are precompiled objects they execute faster than SQL queries. Every time we run a SQL query, the query has to be first compiled and then executed where as a stored procedure is already compiled. Hence executing stored procedures is much faster than executing SQL queries.
Better Security : For a given stored procedure you can specify who has the rights to execute. You cannot do the same for an SQL query. Writing the SQL statements inside our code is usually not a good idea. In this way you expose your database schema (design) in the code which may be changed. Hence most of the time programmers use stored procedures instead of plain SQL statements.
Reduced Network Traffic : Stored Procedures reside on the database server. If you have to execute a Stored Procedure from your ASP.NET web application, you just specify the name of the Stored Procedure. So over the network you just send the name of the Stored Procedure. With an SQL query you have to send all the SQL statements over the network to the database server which could lead to increased network traffic.

Can you update the database using DataReader object?
No, You cannot update the database using DataReader object. DataReader is read-only, foward only. It reads one record at atime. After DataReader finishes reading the current record, it moves to the next record. There is no way you can go back to the previous record.

What is the difference between a DataReader and a DataSet?
DataReader
1. DatReader works on a Connection oriented architecture.
2. DataReader is read only, forward only. It reads one record at atime. After DataReader finishes reading the current record, it moves to the next record. There is no way you can go back to the previous record. So using a DataReader you read in forward direction only.
3. Updations are not possible with DataReader.
4. As DataReader is read only, forward only it is much faster than a DataSet.
DataSet
1. DataSet works on disconnected architecture.
2. Using a DataSet you can move in both directions. DataSet is bi directional.
3. Database can be updated from a DataSet.
4. DataSet is slower than DataReader.

Give an example scenario of using a DataSet and a DataReader?
If you want to just read and display the data(No updates, deletes, or inserts) then use a DataReader.
If you want to do a batch inserts, updates and deletes then use a DataSet.