Top Dapper ORM Questions for .NET Developers
Q1. What is Dapper?
Answer
Dapper is a lightweight, high-performance micro-ORM (Object-Relational Mapper) for .NET that simplifies data access by mapping database queries to .NET objects.
Q2. What are the advantages of using Dapper?
Answer
Dapper offers high performance, simplicity, and flexibility. It is easy to use, supports various databases, and provides a straightforward way to execute queries and map results to .NET objects.
Q3. How does Dapper differ from Entity Framework?
Answer
Dapper is a micro-ORM that focuses on performance and simplicity, requiring manual SQL queries. Entity Framework is a full-featured ORM that provides LINQ support, change tracking, and more, but can be slower and more complex.
Q4. How do you install Dapper in a .NET project?
Answer
Dapper can be installed via NuGet Package Manager using the command `Install-Package Dapper` or by adding it through the NuGet Package Manager UI in Visual Studio.
Q5. What is the basic syntax for querying data with Dapper?
Answer
Dapper uses the `Query` method to execute SQL queries and map the results to .NET objects. For example:
var users = connection.Query("SELECT * FROM Users").ToList();
Q6. How do you execute a stored procedure with Dapper?
Answer
Stored procedures can be executed using the `Execute` method for non-query operations or the `Query` method for queries that return results.
var result = connection.Query("sp_GetUsers", commandType: CommandType.StoredProcedure).ToList();
Q7. What is the purpose of the `Query` method in Dapper?
Answer
The `Query` method is used to execute a SQL query and map the results to a list of .NET objects.
Looking for more Microservice resources? Read our Master Microservices: Top 30 Interview Questions and Answers for 2025 to build a strong foundation!
Q8. How do you handle SQL parameters in Dapper?
Answer
SQL parameters are handled using anonymous objects or `DynamicParameters` to pass values to queries, preventing SQL injection attacks.
var user = connection.QuerySingle("SELECT * FROM Users WHERE Id = @Id", new { Id = 1 });
Q9. What is the `QuerySingle` method in Dapper?
Answer
The `QuerySingle` method is used to execute a query and return a single result, throwing an exception if no results are found.
Q10. How do you perform an insert operation with Dapper?
Answer
Insert operations are performed using the `Execute` method with an `INSERT` SQL statement.
connection.Execute("INSERT INTO Users (Name, Email) VALUES (@Name, @Email)", new { Name = "John Doe", Email = "john@example.com" });
Q11. How do you perform an update operation with Dapper?
Answer
Update operations are performed using the `Execute` method with an `UPDATE` SQL statement.
connection.Execute("UPDATE Users SET Name = @Name WHERE Id = @Id", new { Name = "Jane Doe", Id = 1 });
Q12. How do you perform a delete operation with Dapper?
Answer
Delete operations are performed using the `Execute` method with a `DELETE` SQL statement.
connection.Execute("DELETE FROM Users WHERE Id = @Id", new { Id = 1 });
Q13. What is the `Execute` method in Dapper?
Answer
The `Execute` method is used to execute a SQL command that does not return any results, such as `INSERT`, `UPDATE`, or `DELETE` operations.
Q14. How do you handle transactions in Dapper?
Answer
Transactions are handled using the `BeginTransaction` method of the `IDbConnection` object, allowing multiple operations to be executed within a single transaction scope.
Q15. What is the purpose of the `QueryMultiple` method in Dapper?
Answer
The `QueryMultiple` method is used to execute multiple queries in a single database call, returning a `GridReader` object that allows reading multiple result sets.
Q16. How do you map query results to complex objects in Dapper?
Answer
Complex objects can be mapped using the `Query` method with multi-mapping, specifying how to split the result set into multiple objects.
var userPosts = connection.Query("SELECT * FROM Users u INNER JOIN Posts p ON u.Id = p.UserId", (user, post) => { userPost.User = user; userPost.Post = post; return userPost; }).ToList();
Q17. What is the `DynamicParameters` class in Dapper?
Answer
The `DynamicParameters` class is used to create a dynamic parameter object that can be used to pass parameters to SQL queries, supporting various data types and operations.
Q18. How do you use `DynamicParameters` in Dapper?
Answer
`DynamicParameters` is used to define and pass parameters to SQL queries, allowing for dynamic and flexible parameter handling.
var parameters = new DynamicParameters();
parameters.Add("@Name", "John Doe");
parameters.Add("@Email", "john@example.com");
connection.Execute("INSERT INTO Users (Name, Email) VALUES (@Name, @Email)", parameters);
Q19. What is the purpose of the `QueryFirst` method in Dapper?
Answer
The `QueryFirst` method is used to execute a query and return the first result, returning a default value if no results are found.
Q20. How do you handle nullable types in Dapper?
Answer
Nullable types are handled automatically by Dapper, mapping database `NULL` values to nullable .NET types (e.g., `int?`, `DateTime?`).
Q21. What is the purpose of the `QueryAsync` method in Dapper?
Answer
The `QueryAsync` method is used to execute a query asynchronously, returning a task that represents the operation, allowing for non-blocking database operations.
Q22. How do you perform asynchronous operations with Dapper?
Answer
Asynchronous operations are performed using the `Async` suffix methods, such as `QueryAsync`, `ExecuteAsync`, and `QuerySingleAsync`, allowing for non-blocking database interactions.
Q23. What is the purpose of the `ExecuteScalar` method in Dapper?
Answer
The `ExecuteScalar` method is used to execute a query and return the first column of the first row in the result set, typically used for aggregate functions like `COUNT`.
Q24. How do you handle connection management in Dapper?
Answer
Connection management is handled using the `IDbConnection` interface, ensuring that connections are opened and closed properly, typically using a `using` statement.
Q25. What is the purpose of the `GridReader` class in Dapper?
Answer
The `GridReader` class is used to read multiple result sets returned by a `QueryMultiple` operation, allowing for sequential access to each result set.
Q26. How do you map columns to properties in Dapper?
Answer
Columns are mapped to properties automatically by matching names, but custom mappings can be defined using attributes or custom type handlers.
Q27. What is a custom type handler in Dapper?
Answer
A custom type handler is a class that implements the `SqlMapper.ITypeHandler` interface, allowing for custom mapping logic between database columns and .NET properties.
Q28. How do you create a custom type handler in Dapper?
Answer
A custom type handler is created by implementing the `SqlMapper.ITypeHandler` interface and registering it with Dapper using the `SqlMapper.AddTypeHandler` method.
Q29. What is the purpose of the `SqlMapper.AddTypeHandler` method?
Answer
The `SqlMapper.AddTypeHandler` method is used to register a custom type handler with Dapper, allowing for custom mapping logic to be applied globally.
Looking for more ASP.NET Core resources? Read our ASP.NET Core Identity, Authentication, and JWT Interview Questions to build a strong foundation!
Q30. How do you handle enumerations in Dapper?
Answer
Enumerations are handled automatically by Dapper, mapping database values to .NET enum types, but custom type handlers can be used for more complex mappings.
Q31. What is the purpose of the `QueryFirstOrDefault` method in Dapper?
Answer
The `QueryFirstOrDefault` method is used to execute a query and return the first result or a default value if no results are found, avoiding exceptions.
Q32. How do you handle one-to-many relationships in Dapper?
Answer
One-to-many relationships are handled using multi-mapping, querying related data in a single query and mapping it to a list of child objects.
Q33. How do you handle many-to-many relationships in Dapper?
Answer
Many-to-many relationships are handled using multi-mapping and join queries, mapping the results to a collection of related objects.
Q34. What is the purpose of the `QuerySingleOrDefault` method in Dapper?
Answer
The `QuerySingleOrDefault` method is used to execute a query and return a single result or a default value if no results are found, avoiding exceptions.
Q35. How do you perform batch operations with Dapper?
Answer
Batch operations are performed using transactions and the `Execute` method, executing multiple commands within a single transaction scope.
Q36. What is the purpose of the `ExecuteReader` method in Dapper?
Answer
The `ExecuteReader` method is used to execute a command and return a data reader, allowing for efficient data retrieval and processing.
Q37. How do you handle large result sets in Dapper?
Answer
Large result sets are handled using paging techniques, such as limiting the number of rows returned per query and using offsets to retrieve subsequent pages.
Q38. What is the purpose of the `SqlBuilder` class in Dapper?
Answer
The `SqlBuilder` class is used to construct dynamic SQL queries programmatically, allowing for flexible and reusable query generation.
Q39. How do you use `SqlBuilder` in Dapper?
Answer
`SqlBuilder` is used to create dynamic SQL queries by adding conditions, joins, and other SQL elements programmatically, generating a SQL string and parameters.
Q40. What is the purpose of the `AddTemplate` method in `SqlBuilder`?
Answer
The `AddTemplate` method is used to add a SQL template to the `SqlBuilder` instance, allowing for dynamic SQL generation based on the template.
Q41. How do you handle dynamic queries in Dapper?
Answer
Dynamic queries are handled using the `SqlBuilder` class or by constructing SQL strings with parameters, ensuring that queries are flexible and secure.
Q42. What is the purpose of the `AddParameters` method in `SqlBuilder`?
Answer
The `AddParameters` method is used to add parameters to the `SqlBuilder` instance, allowing for dynamic and secure parameter handling in generated SQL queries.
Q43. How do you handle stored procedure parameters in Dapper?
Answer
Stored procedure parameters are handled using anonymous objects or `DynamicParameters`, mapping .NET properties to SQL parameters.
Q44. What is the purpose of the `CommandDefinition` class in Dapper?
Answer
The `CommandDefinition` class is used to define a SQL command with parameters, allowing for reusable and configurable command definitions.
Q45. How do you use `CommandDefinition` in Dapper?
Answer
`CommandDefinition` is used to create a reusable command definition with SQL and parameters, executing it using Dapper's query and execute methods.
Q46. What is the purpose of the `QueryMultipleAsync` method in Dapper?
Answer
The `QueryMultipleAsync` method is used to execute multiple queries asynchronously, returning a `GridReader` object that allows for non-blocking processing of multiple result sets.
Q47. How do you handle database schema changes in Dapper?
Answer
Database schema changes are handled by updating the SQL queries and mappings in the application code, ensuring that the queries match the current database schema.
Q48. What is the purpose of the `SplitOn` parameter in Dapper?
Answer
The `SplitOn` parameter is used in multi-mapping to specify the column or columns that Dapper should use to split the result set into multiple objects.
Q49. How do you handle complex joins in Dapper?
Answer
Complex joins are handled using multi-mapping and the `Query` method, mapping the results to multiple .NET objects based on the join conditions.
Q50. What are the best practices for using Dapper in a .NET application?
Answer
Best practices include using parameterized queries to prevent SQL injection, managing connections efficiently with `using` statements, handling transactions for data integrity, and leveraging asynchronous methods for improved performance.
.png)
