ADO.NET Interview Questions and Answers for 2 years experience

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ADO.NET Interview Questions & Answers

  1. What is ADO.NET?

    • Answer: ADO.NET is a set of .NET classes that allows developers to access and manipulate data from various data sources, such as relational databases (SQL Server, Oracle, MySQL), XML files, and other data providers. It provides a consistent programming model for interacting with different data sources.

  2. Explain the difference between Connected and Disconnected architecture in ADO.NET.

    • Answer: Connected architecture maintains a persistent connection to the database throughout the operation. Disconnected architecture retrieves data, closes the connection, and then allows manipulation of the data in memory before updating the database. Disconnected architecture is generally more efficient for large datasets or applications needing offline capabilities.

  3. What are the main components of ADO.NET?

    • Answer: The main components are `Connection`, `Command`, `DataReader`, `DataAdapter`, and `DataSet`. `Connection` establishes the link to the database. `Command` executes SQL queries or stored procedures. `DataReader` provides a forward-only, read-only stream of data. `DataAdapter` bridges the gap between the database and `DataSet`. `DataSet` represents a disconnected representation of data.

  4. What is a Connection object and its purpose?

    • Answer: The `Connection` object represents a physical connection to a data source. It's responsible for opening and closing the connection, and providing the context for executing commands.

  5. Explain the role of the Command object.

    • Answer: The `Command` object encapsulates a SQL query or stored procedure to be executed against the database. It allows setting parameters, specifying command type, and executing the command.

  6. What is a DataReader and when would you use it?

    • Answer: A `DataReader` provides a forward-only, read-only stream of data from a database. It's highly efficient for retrieving large datasets where only forward traversal is needed, minimizing memory usage.

  7. Describe the DataAdapter and its function.

    • Answer: The `DataAdapter` acts as a bridge between a `DataSet` and a database. It uses `SqlCommand` objects to execute SELECT, INSERT, UPDATE, and DELETE statements and fill or update the `DataSet`.

  8. What is a DataSet?

    • Answer: A `DataSet` is an in-memory representation of data. It can contain multiple `DataTable` objects, representing tables, and relationships between them. It facilitates disconnected data access.

  9. What is a DataTable?

    • Answer: A `DataTable` represents a single table of data within a `DataSet`. It contains rows and columns, similar to a database table.

  10. Explain the concept of parameterized queries. Why are they important?

    • Answer: Parameterized queries use parameters to represent values in SQL statements, preventing SQL injection vulnerabilities and improving performance by reusing query plans.

  11. How do you handle transactions in ADO.NET?

    • Answer: Transactions ensure data integrity by grouping multiple operations into a single unit of work. They are managed using the `Transaction` object, ensuring that either all operations succeed or none do.

  12. What are connection pooling and its benefits?

    • Answer: Connection pooling reuses existing database connections, reducing the overhead of repeatedly establishing new connections, improving performance and scalability.

  13. What are stored procedures and how do you use them with ADO.NET?

    • Answer: Stored procedures are pre-compiled SQL code stored on the database server. They are called using `SqlCommand` objects, specifying the stored procedure name and parameters.

  14. How do you handle exceptions in ADO.NET?

    • Answer: Exceptions are handled using `try-catch` blocks, catching specific ADO.NET exceptions (e.g., `SqlException`) to gracefully handle errors and prevent application crashes.

  15. Explain the difference between `AcceptChanges()` and `RejectChanges()` methods of a DataTable.

    • Answer: `AcceptChanges()` makes all changes permanent in the DataTable. `RejectChanges()` discards all pending changes and reverts to the original state.

  16. What is the purpose of the `Fill()` method of the DataAdapter?

    • Answer: `Fill()` populates a `DataSet` or `DataTable` with data retrieved from the database using the associated `SelectCommand`.

  17. How do you update a database using a DataAdapter?

    • Answer: After making changes to a `DataSet`, the `Update()` method of the `DataAdapter` synchronizes the changes back to the database using the `InsertCommand`, `UpdateCommand`, and `DeleteCommand`.

  18. What is optimistic concurrency and how is it implemented in ADO.NET?

    • Answer: Optimistic concurrency assumes that conflicts are rare. It checks for changes before updating, typically using a timestamp or version column. ADO.NET handles this through the `UpdateCommand` and error handling.

  19. What is pessimistic concurrency and how is it implemented in ADO.NET?

    • Answer: Pessimistic concurrency assumes conflicts are frequent. It locks records during updates, preventing concurrent modifications. This is usually handled through database-level locking mechanisms, rather than directly in ADO.NET.

  20. How do you handle null values in ADO.NET?

    • Answer: Null values are handled using `DBNull.Value` in ADO.NET. Check for `DBNull.Value` before accessing data to prevent exceptions.

  21. Explain different ways to retrieve data from a database using ADO.NET.

    • Answer: Data can be retrieved using `DataReader` for efficient forward-only access, `DataAdapter` and `DataSet` for disconnected access, or directly using `SqlCommand` and processing results.

  22. What are the advantages and disadvantages of using DataReader?

    • Answer: Advantages: High performance for read-only operations, low memory consumption. Disadvantages: Forward-only, read-only, requires a persistent connection.

  23. What are the advantages and disadvantages of using DataSet?

    • Answer: Advantages: Disconnected architecture, supports multiple tables and relationships. Disadvantages: Can consume more memory than DataReader, requires more overhead for updates.

  24. How can you improve the performance of ADO.NET applications?

    • Answer: Use parameterized queries, optimize SQL queries, use connection pooling, use appropriate data access methods (DataReader vs. DataSet), handle exceptions efficiently, and consider caching.

  25. What is SQL injection and how can you prevent it using ADO.NET?

    • Answer: SQL injection is a security vulnerability where malicious SQL code is injected into an application. Prevention: Use parameterized queries or stored procedures.

  26. Explain how to implement paging in ADO.NET.

    • Answer: Paging is implemented using SQL's `TOP` clause and `OFFSET`/`FETCH` clauses (or similar features depending on the database system) to retrieve subsets of data.

  27. How do you handle large datasets efficiently in ADO.NET?

    • Answer: Use DataReader for forward-only access, implement paging, process data in batches, and consider using streaming techniques.

  28. Describe different ways to handle data validation in ADO.NET.

    • Answer: Data validation can be performed at the database level (constraints, triggers), using ADO.NET's DataTable validation features, or in the application logic before sending data to the database.

  29. How do you work with different database systems using ADO.NET?

    • Answer: Use the appropriate provider for each database system (e.g., `System.Data.SqlClient` for SQL Server, `System.Data.OracleClient` for Oracle, etc.). The connection string specifies the provider and connection details.

  30. What are some common ADO.NET performance bottlenecks and how to fix them?

    • Answer: Inefficient queries, lack of indexing, excessive round trips to the database, improper use of DataReader/DataSet, improper exception handling.

  31. Explain the concept of a DataRelation in a DataSet.

    • Answer: A DataRelation defines a relationship between two DataTables in a DataSet, mimicking foreign key relationships in a database.

  32. How do you implement a one-to-many relationship using DataRelations?

    • Answer: Create a DataRelation between the parent and child DataTables, specifying the parent and child columns that form the relationship.

  33. How do you implement a many-to-many relationship using DataRelations?

    • Answer: A many-to-many relationship requires a junction table. Create DataRelations between each table and the junction table.

  34. What is the difference between `DataReader` and `SqlDataReader`?

    • Answer: `DataReader` is the base class; `SqlDataReader` is the specific implementation for SQL Server.

  35. How to handle concurrency issues when updating data using ADO.NET?

    • Answer: Use optimistic or pessimistic concurrency techniques, as discussed earlier. Check for conflicts and handle them appropriately.

  36. What are some best practices for using ADO.NET?

    • Answer: Use parameterized queries, handle exceptions gracefully, use appropriate data access methods, optimize SQL queries, and follow proper connection management.

  37. How would you debug an ADO.NET application?

    • Answer: Use debugging tools (like Visual Studio's debugger) to step through code, check values, examine exceptions, and use logging to track execution flow.

  38. Explain the role of the `CommandTimeout` property of the SqlCommand object.

    • Answer: Sets the timeout period (in seconds) for a command's execution. Prevents indefinite hangs if a query takes too long.

  39. How can you retrieve the number of rows affected by an INSERT, UPDATE, or DELETE operation?

    • Answer: The `ExecuteNonQuery()` method returns the number of rows affected.

  40. What is the purpose of the `TransactionScope` class?

    • Answer: Provides a simplified way to manage distributed transactions, encompassing multiple database operations.

  41. How do you handle errors during database operations?

    • Answer: Use `try-catch` blocks to handle exceptions, log errors for debugging, and present user-friendly error messages.

  42. What is the difference between `Close()` and `Dispose()` methods for a connection object?

    • Answer: `Close()` releases the connection back to the pool. `Dispose()` releases the connection and any associated resources, often preferred for resource management.

  43. How to retrieve the schema of a database table using ADO.NET?

    • Answer: Use `GetSchema()` method of the connection object or use metadata queries.

  44. How would you implement a simple CRUD (Create, Read, Update, Delete) operation using ADO.NET?

    • Answer: Use `SqlCommand` with appropriate SQL statements (INSERT, SELECT, UPDATE, DELETE) and handle parameters carefully.

  45. Explain how you would use ADO.NET with an Entity Framework Core.

    • Answer: While EF Core generally handles database interaction, you might use ADO.NET for specific low-level tasks or direct database queries not easily expressed in EF Core.

  46. What are some alternative technologies to ADO.NET for data access in .NET?

    • Answer: Entity Framework Core, Dapper, and other ORMs (Object-Relational Mappers).

  47. Have you worked with any asynchronous programming in ADO.NET? If so, how?

    • Answer: Use asynchronous methods like `ExecuteReaderAsync`, `ExecuteNonQueryAsync`, etc., for improved performance and responsiveness in applications handling many database interactions.

  48. How do you manage database connections efficiently in a multi-threaded application?

    • Answer: Use connection pooling, and manage connections carefully within each thread using `using` blocks to ensure proper disposal.

  49. Describe a situation where you had to troubleshoot a complex ADO.NET related issue.

    • Answer: *(This requires a personal anecdote. Describe a real or hypothetical scenario, highlighting the problem, your debugging steps, and the solution.)*

  50. What are your preferred methods for testing your ADO.NET code?

    • Answer: Use unit tests (e.g., xUnit, NUnit) to test individual methods and database interactions, possibly using mocking for database interactions in tests.

  51. How do you handle deadlocks in ADO.NET?

    • Answer: Deadlocks are primarily handled at the database level (e.g., proper indexing, transaction management). In ADO.NET, you'd catch the exception and potentially retry the operation after a delay.

  52. What are the security implications of using ADO.NET and how do you mitigate them?

    • Answer: SQL injection, improper error handling, and data exposure are key security concerns. Mitigate using parameterized queries, stored procedures, input validation, and secure connection strings.

  53. Explain the concept of database connection string and its importance.

    • Answer: A connection string contains the information needed to connect to a database, including server name, database name, username, password, and other connection parameters.

  54. How do you optimize database queries for performance?

    • Answer: Use proper indexing, avoid using `SELECT *`, use appropriate `JOIN` types, optimize `WHERE` clauses, and analyze query execution plans.

  55. What is the role of the `IsolationLevel` property in a transaction?

    • Answer: Specifies the level of isolation for a transaction, controlling how changes made by one transaction are visible to other concurrent transactions.

  56. Describe the different Isolation Levels and their implications.

    • Answer: Read Uncommitted, Read Committed, Repeatable Read, Serializable. Higher levels offer more data consistency but can reduce concurrency.

  57. How would you design a scalable database solution using ADO.NET?

    • Answer: Use connection pooling, optimize queries, consider database sharding or replication, and use a load balancer if necessary.

  58. How would you integrate ADO.NET with a web application framework like ASP.NET?

    • Answer: Use ADO.NET within the application logic to access the database. Handle connection management and data access appropriately within the framework's lifecycle.

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