Interview Q&A

DO.NET (Active Data Objects .NET) is a data access technology in the .NET framework

that enables applications to interact with databases and other data sources. It provides a set

of classes for connecting to databases, retrieving data, manipulating data, and updating

data. ADO.NET is designed for disconnected data access, meaning that data can be

retrieved, modified, and worked with without maintaining an ongoing connection to the

database.

Real-Time Example:

In a C# application, ADO.NET is used to retrieve a list of products from a database and

display it in a UI like a GridView. The data is fetched from the database, stored in a DataSet

or DataTable, and then bound to the UI controls.

nd forth between rows (using DataRow and DataColumn).

• DataReader: A DataReader is a forward-only, read-only data cursor. It provides

faster, streaming access to the data from the database but doesn’t allow

modifications. It maintains an open connection while reading data.

Example:

• DataSet: If you're working on a report with multiple tables, such as Customers,

Orders, and Products, you'd use a DataSet.

• DataReader: If you're fetching customer details one by one for a quick operation, a

DataReader would be used.

• DataSet: It is a disconnected, in-memory data structure that can hold multiple tables.

It can also be updated and later written back to the database. You can move back

and forth between rows (using DataRow and DataColumn).

• DataReader: A DataReader is a forward-only, read-only data cursor. It provides

faster, streaming access to the data from the database but doesn’t allow

modifications. It maintains an open connection while reading data.

Example:

• DataSet: If you're working on a report with multiple tables, such as Customers,

Orders, and Products, you'd use a DataSet.

• DataReader: If you're fetching customer details one by one for a quick operation, a

DataReader would be used.

DataAdapter serves as a bridge between a DataSet/DataTable and the database. It is

used to fill a DataSet with data and to update changes made in the DataSet back to the

database.

Example:

SqlDataAdapter adapter = new SqlDataAdapter("SELECT * FROM

Customers", connection);

DataSet dataset = new DataSet();

dapter.Fill(dataset, "Customers"); // Fills DataSet with data from

the Customers table

The Command object is used to execute SQL queries or stored procedures against a

database. It encapsulates the SQL statement or stored procedure and returns results.

Example:

SqlCommand command = new SqlCommand("SELECT * FROM Customers",

connection);

SqlDataReader reader = command.ExecuteReader();

Follow:

• ExecuteNonQuery: Executes SQL commands that do not return data (e.g., INSERT,

UPDATE, DELETE).

• Example: command.ExecuteNonQuery(); (used to insert a record).
• ExecuteReader: Executes SQL commands that return rows (e.g., SELECT queries).

It returns a DataReader.

• Example: command.ExecuteReader(); (used to select records).
• ExecuteScalar: Executes SQL commands and returns a single value (e.g., a single

cell of data).

• Example: command.ExecuteScalar(); (used for retrieving the count or

aggregate values).

The ConnectionString contains information required to connect to the database, such as

the database server, database name, credentials, and other configurations.

Example:

string connectionString = "Data Source=server_name;Initial
Catalog=database_name;User ID=user_name;Password=password;";

• SqlConnection: Used specifically for connecting to SQL Server databases.
• OleDbConnection: A more general connection class that can connect to a variety of

data sources, such as MS Access, Excel, Oracle, etc.

Example:

Use SqlConnection for SQL Server:

SqlConnection sqlConnection = new SqlConnection(connectionString);

• Use OleDbConnection for Access or other databases:

OleDbConnection oleDbConnection = new

OleDbConnection(connectionString);

• SQLCommand: A SQL command is a string of SQL code (like SELECT, INSERT,

etc.) that is executed directly against the database.

• Stored Procedure: A precompiled collection of SQL statements that can be

executed as a unit. It can contain more complex logic, including control-of-flow and

error handling.

Example:

• SQL Command: SELECT * FROM Customers;
• Stored Procedure: EXEC GetCustomerDetails;

Connection Pooling allows multiple applications or threads to reuse existing database

connections instead of opening a new connection each time. It improves performance by

reducing the overhead of opening and closing connections repeatedly.

Example: When using SQL Server, the connection pool automatically manages the reuse of

connections.

The CommandTimeout property specifies the amount of time (in seconds) before a

command is considered to have timed out. If the command execution takes longer than the

specified time, an error is raised.

Example:

SqlCommand command = new SqlCommand("SELECT * FROM Customers",

connection);

command.CommandTimeout = 30; // Timeout after 30 seconds
Intermediate ADO.NET Questions

Answer: The SqlDataAdapter serves as a bridge between a DataSet (or DataTable) and the database. It is responsible for:

What interviewers expect

• A clear definition tied to ADO.NET in ADO.NET projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production ADO.NET application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in ADO.NET architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

mount of data quickly and do not need to modify the data. It requires an open connection to

the database and reads data sequentially, one row at a time.

Example:

SqlCommand command = new SqlCommand("SELECT CustomerName FROM

Customers", connection);

SqlDataReader reader = command.ExecuteReader();

while (reader.Read())

{

Console.WriteLine(reader["CustomerName"]);

}

reader.Close();

The SqlDataReader is used to retrieve data from the database in a forward-only and

read-only manner. It is more efficient than a DataSet when you need to retrieve a large

amount of data quickly and do not need to modify the data. It requires an open connection to

the database and reads data sequentially, one row at a time.

Example:

SqlCommand command = new SqlCommand("SELECT CustomerName FROM

Customers", connection);

SqlDataReader reader = command.ExecuteReader();

while (reader.Read())

Console.WriteLine(reader["CustomerName"]);

reader.Close();

Answer: dapter.Fill(dataset, "Customers"); // Fills the DataSet with data from the "Customers" table

What interviewers expect

• A clear definition tied to ADO.NET in ADO.NET projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production ADO.NET application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in ADO.NET architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• DataSet is a collection of DataTable objects, and it can hold multiple tables and

relationships between them. It is used for handling disconnected data and can work

offline, and it can also support complex structures, such as parent-child relationships.

• DataTable represents a single table in-memory and contains rows and columns. It

can be used for simpler scenarios where only one table of data is needed.

Example:

• DataSet: Holds multiple tables, such as Customers and Orders.
• DataTable: Holds data from a single table like Customers.

The SqlConnection object is responsible for opening a connection to a SQL Server

database. It represents the physical connection to the data source and must be open before

executing commands (like SqlCommand) or reading data (using SqlDataReader or

SqlDataAdapter).

Example:

SqlConnection connection = new SqlConnection(connectionString);

connection.Open();

// Execute database commands

connection.Close();

• Strongly Typed DataSet: A DataSet that is generated from an XSD (XML Schema

Definition) file, which defines the structure of the data (tables, columns,

relationships). It provides type safety and compile-time checking.

• Loosely Typed DataSet: A DataSet that does not have predefined schemas,

meaning you access tables and columns by name at runtime. This offers flexibility but

no compile-time checking.

Example:

• Strongly Typed: You get intellisense and type safety when accessing columns.
• Loosely Typed: You need to access tables and columns by name as strings, without

intellisense.

Answer: transaction in ADO.NET is a sequence of database operations that are executed as a single unit. If one operation fails, all previous operations are rolled back. You can manage transactions using the SqlTransaction object. Steps:

What interviewers expect

• A clear definition tied to ADO.NET in ADO.NET projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production ADO.NET application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in ADO.NET architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

way to safely and securely inject data into queries, reducing the risk of SQL injection

ttacks.

In ADO.NET, you handle parameters using the Parameters collection of a SqlCommand

object.

Example:

SqlCommand command = new SqlCommand("SELECT * FROM Customers WHERE

CustomerID = @CustomerID", connection);

command.Parameters.AddWithValue("@CustomerID", customerId);

Parameters are used to pass values to SQL commands or stored procedures. They provide

a way to safely and securely inject data into queries, reducing the risk of SQL injection

attacks.

In ADO.NET, you handle parameters using the Parameters collection of a SqlCommand

object.

Example:

SqlCommand command = new SqlCommand("SELECT * FROM Customers WHERE

CustomerID = @CustomerID", connection);

command.Parameters.AddWithValue("@CustomerID", customerId);

Answer: snapshot of the data and remains unchanged even if the data in the database changes during the operation. It is slower and consumes more memory compared to forward-only cursor.

What interviewers expect

• A clear definition tied to ADO.NET in ADO.NET projects
• Trade-offs (performance, maintainability, security, cost)
• When you would and would not use it in production

Real-world example

In a production ADO.NET application, teams apply this when handling user-facing features or integration boundaries. For example, you might use it during a sprint where reliability and observability matter—logging metrics, validating edge cases, and documenting the decision in an ADR so future developers understand why the approach was chosen.

How to explain in the interview

1. Define the concept in one or two sentences.
2. Context — where it fits in ADO.NET architecture.
3. Example — a specific project, bug, or performance win.
4. Trade-off — what you gain vs what you sacrifice.

Tip: Practice aloud on Toolliyo mock interview or the Interview Q&A section before your real interview.

• Forward-only cursor: A cursor that allows you to read rows sequentially in a forward

direction only. It is fast and lightweight, but you cannot go back to previous rows.

• Static cursor: A cursor that allows both forward and backward navigation. It provides

a snapshot of the data and remains unchanged even if the data in the database

changes during the operation. It is slower and consumes more memory compared to

a forward-only cursor.

n ADO.NET DataProvider is a set of classes used to interact with different types of data

sources (such as SQL Server, Oracle, etc.). It provides methods for opening connections,

executing commands, and retrieving data.

The main types of DataProviders are:

• SqlClient (for SQL Server)
• OleDb (for OLE DB-compatible data sources)
• Odbc (for ODBC-compatible data sources)
• Oracle (for Oracle databases)

llows you to define the name, data type, size, and value of a parameter. SqlParameter is

used to protect against SQL injection and to pass data to SQL queries safely.

Example:

SqlCommand command = new SqlCommand("SELECT * FROM Customers WHERE

CustomerID = @CustomerID", connection);

SqlParameter parameter = new SqlParameter("@CustomerID",

SqlDbType.Int);

parameter.Value = customerId;

command.Parameters.Add(parameter);