When are queries executed on DbContext

Question

When are queries executed on DbContext

asp.net asp.net-mvc-5 c# entity-framework entity-framework-6

Question

I am trying to understand the performance impact of having one DbContext class vs multiple when using EF6 framework.

For example, if we have a simple DbContext such as this:

public class MainDbContext : DbContext
{
    public DbSet<Car> Cars { get; set; }

    public void AddCar(Car car)
    {
        Cars.Add(car);
        SaveChanges();
    }
}

Now let us say I have a service that uses the said DbContext in the following way:

public class CarService
{
    public List<Car> Cars { get; private set; }
    public CarService()
    {
        var dbContext = new MainDbContext();
        Cars = dbContext.Cars.ToList();
    }
}

At what point in time did the DbContext go to the database and retrieved all cars that are stored in the database? Was it when I called var dbContext = new MainDbContext(); or was it when I called Cars = dbContext.Cars.ToList();?

If former, if I had a DbContext that contains 100 tables, will all 100 tables be queried when the DbContext is created?

Bojan

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Guilherme · Accepted Answer · 5/31/2017 11:56:11 PM

No. The query happens once you enumerate a table. Even in your second example, it still does not connect to the database.

It will connect when you enumerate, for example:

dbContext.Cars.ToList();

Or

foreach (Car c in dbContext.Cars)

Or bind the table to a UI control.

However when you make a where, order by, then by, join etc.. e.g.

var result = dbContext.Cars.Where(c => c.Id == 35).OrderBy(c => c.Name);

You will not connect to the database. You are only preparing the logical sequence of operations to translate into an SQL query.

AFTER THE QUESTION UPDATE

Now, your second example, with the ToList(), enumerate the results and connects to the database to get the data.

When are queries executed on DbContext

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