Entity FrameworkEntity Framework Code First

Connect to an existing database

To achieve the simplest task in Entity Framework - to connect to an existing database ExampleDatabase on your local instance of MSSQL you have to implement two classes only.

First is the entity class, that will be mapped to our database table dbo.People.

    class Person
    {
        public int PersonId { get; set; }
        public string FirstName { get; set; }
    }

The class will use Entity Framework's conventions and map to table dbo.People which is expected to have primary key PersonId and a varchar(max) property FirstName.

Second is the context class which derives from System.Data.Entity.DbContext and which will manage the entity objects during runtime, pupulate them from database, handle concurrency and save them back to the database.

    class Context : DbContext
    {
        public Context(string connectionString) : base(connectionString)
        {
            Database.SetInitializer<Context>(null);
        }

        public DbSet<Person> People { get; set; }
    }

Please mind, that in the constructor of our context we need to set database initializer to null - we don't want Entity Framework to create the database, we just want to access it.

Now you are able manipulate data from that table, e.g. change the FirstName of first person in the database from a console application like this:

    class Program
    {
        static void Main(string[] args)
        {
            using (var ctx = new Context("DbConnectionString"))
            {
                var firstPerson = ctx.People.FirstOrDefault();
                if (firstPerson != null) {
                    firstPerson.FirstName = "John";
                    ctx.SaveChanges();
                }
            }
        }
    }

In the code above we created instance of Context with an argument "DbConnectionString". This has to be specified in our app.config file like this:

  <connectionStrings>
    <add  name="DbConnectionString" 
    connectionString="Data Source=.;Initial Catalog=ExampleDatabase;Integrated Security=True" 
    providerName="System.Data.SqlClient"/>
  </connectionStrings>