Using NHibernate in C# for Object-Relational Mapping (ORM)

Learn how to use NHibernate, a powerful Object-Relational Mapper (ORM) for .NET, to simplify database interactions in your C# applications. This tutorial covers NHibernate's core components, mapping strategies (XML, Fluent NHibernate), and how it facilitates working with objects instead of writing raw SQL queries.

Using NHibernate in C# for Object-Relational Mapping (ORM)

Introduction

NHibernate is a powerful Object-Relational Mapper (ORM) for .NET. ORMs simplify database interactions by letting you work with objects instead of writing raw SQL queries. This article introduces NHibernate and its core components.

What is NHibernate?

NHibernate is an ORM framework that bridges the gap between your C# objects (your application's domain model) and a relational database. It handles the complexities of translating object operations into SQL, allowing developers to focus on business logic rather than database specifics.

NHibernate Architecture

NHibernate's architecture involves several key components:

• SessionFactory: A thread-safe object created once at application startup. It's the factory for creating `Session` objects.
• Session: A lightweight object representing a single unit of work (a series of database operations). It's used to interact directly with the database.
• Configuration: Used to set up NHibernate at startup. This includes specifying database connection details, dialect (for SQL type mapping), and mapping files.
• Mapping Files: Define how your C# classes (objects) map to database tables and columns. NHibernate supports XML-based mapping files and Fluent Mapping (using C# code).

Mapping with NHibernate

NHibernate uses mapping files to understand how your objects correspond to the database structure. Let's look at both XML and Fluent mapping approaches.

Example: Customer Class

public class Customer {
    public virtual int Id { get; set; }
    public virtual string FirstName { get; set; }
    public virtual string LastName { get; set; }
    public virtual string Email { get; set; }
}

XML Mapping

(An example of an XML mapping file for the Customer class would be included here. This XML would show how the class properties map to database table columns.)

Fluent Mapping

public class CustomerMap : ClassMap<Customer> {
    public CustomerMap() {
        Table("Customers");
        Id(x => x.Id, "CustomerId").GeneratedBy.Identity();
        Map(x => x.FirstName).Column("FirstName");
        Map(x => x.LastName).Column("LastName");
        Map(x => x.Email).Column("Email");
    }
}

Fluent mapping uses C# code to define the mapping, offering a more code-centric approach.

The NHibernate `Session` Object

The `Session` object is crucial for interacting with the database. It's used for creating, retrieving, updating, and deleting objects. Sessions are lightweight and should be created and disposed of as needed within a `using` statement to manage resources effectively.

Conclusion

NHibernate significantly simplifies database interactions in C# by providing an object-oriented approach. Understanding its architecture and mapping mechanisms is key to building data-driven applications efficiently.