Working with DbContext in EF Core

We created our entities and DbContext class in the previous chapters. Here you will learn the overview of working with the DbContext to create a database and interact with it using EF Core 7 on .NET 7 platform.

The following is our .NET 7 console project along with entities and a context (SchoolDbContext) class.

The following are Student and Grade classes.

public class Student
{
    public int StudentId { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }

    public int GradeId { get; set; }
    public Grade Grade { get; set; }
}
       
public class Grade
{
     public Grade()
     {
         Students = new List<Student>();
     }

    public int GradeId { get; set; }
    public string GradeName { get; set; }

    public IList<Student> Students { get; set; }
}

The following is our context class SchoolDbContext created in the Create DbContext chapter.

public class SchoolContext : DbContext
{       
    //entities
    public DbSet<Student> Students { get; set; }
    public DbSet<Grade> Grades { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("Server=(localdb)\\mssqllocaldb;Database=SchoolDb;Trusted_Connection=True;");
    }
} 

The above context class includes two DbSet<TEntity> properties, for Student and Grade, type which will be mapped to the Students and Grades tables in the underlying database. In the OnConfiguring() method, an instance of DbContextOptionsBuilder is used to specify a database connection string.

Database Schema Creation in EF Core

After creating the context and entity classes, it's time to start interacting with our underlying database using the context class. However, before we save or retrieve data from the database, we first need to ensure that the database schema is created as per our entities and configurations.

There are two ways using which you can create a database and schema:

1. Using the EnsureCreated() method
2. Using Migration API

Creating a database using the EnsureCreated() method is common to use following by EnsureDeleted() method when testing or prototyping using Entity Framework Core framework.

Migrations API allows you to create an initial database schema based on your entities and then as and when you add/delete/modify your entities and config, it will sync to the corresponding schema changes to your database so that it remains compatible with your EF Core model (entities and other configurations).

Let's use the EnsureCreated() method to create a database and use the context class to save student and grade data in the database.

using EFCoreTutorialsConsole;

using (var context = new SchoolDbContext())
{
    //creates db if not exists 
    context.Database.EnsureCreated();

    //create entity objects
    var grd1 = new Grade() { GradeName = "1st Grade" };
    var std1 = new Student() {  FirstName = "Yash", LastName = "Malhotra", Grade = grd1};

    //add entitiy to the context
    context.Students.Add(std1);

    //save data to the database tables
    context.SaveChanges();

    //retrieve all the students from the database
    foreach (var s in context.Students) {
        Console.WriteLine($"First Name: {s.FirstName}, Last Name: {s.LastName}");
    }
}

Let's understand the above example code:

• The using(){ .. } statement creates a scope for a SchoolDbContext instance called context. It ensures that the context is properly disposed of when it's no longer needed.
• The context.Database.EnsureCreated(); statement checks if the database exists. If it doesn't exist, it creates the database based on the entity classes defined as DbSet<TEntity> properties in the SchoolDbContext class. This is a simple way to create the database schema based on your entity model.
• Next, two entity objects are created: grd1 of type Grade and std1 of type Student. These objects are entity objects that represent records in the database.
• The std1 student entity is added to the Students property of the context. It also assigns grd1 to its Grade property. This prepares the Student entity to be saved to the database along with the Grade entity.
• The context.SaveChanges(); is called to persist the changes to the database. It effectively inserts a new student record into the "Students" table and a new grade record into the "Grades" table.
• After saving changes, a foreach loop retrieves all the students from the "Students" table in the database using the context.Students property. It then prints the first name and last name of each student to the console.

Now, run the project by pressing Ctrl + F5. It will display the following output.

As you can see, it displays the output. EF Core API has created the "SchoolDB" database and inserted records in the "Students" and "Grades" tables. You can see that in the SQL Server Explorer in your visual studio from View -> SQL Server Object Explorer. If it is not connected to your local db then click on the + sign to connect to your db, as shown below.

In the popup, select "MSSQLLocalDB" and click on the "Connect" button, as shown below.

Now, you can see the database, as shown below.

Expand (localdb) > Database > SchoolDB node in SQL Server Object Explorer. You can also see tables and their columns by expanding it further, as shown below.

To see the records, right-click on the table name and click "View Data". It will display records in Visual Studio as below.

In this way, we have successfully used EF Core API to save and retrieve data to the SQL Server database. Visit Saving Data and Querying chapters to learn more about saving and retrieving data in EF Core.