DbContextCaching.cs

using System;
using System.Collections.Generic;
using System.ComponentModel.DataAnnotations;
using System.Data;
using System.Data.Entity;
using System.Data.Entity.Infrastructure;
using System.Diagnostics;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace DbContextCaching {
  public class EntityBase {
    [Key]
    public int Id { get; set; }

    public DateTime DataCadastro { get; set; }
    public DateTime DataAlteracao { get; set; }
  }

  public class Usuario : EntityBase {
    [StringLength(50)]
    public string Nome { get; set; }

    public override string ToString() {
      return string.Format("{0} -{1}", Id, Nome);
    }
  }

  public class TesteContextInitializer : CreateDatabaseIfNotExists<TesteContext> {
    protected override void Seed(TesteContext context) {
      var usuario = new Usuario {
        Nome = "Teste"
      };

      context.Usuarios.Add(usuario);

      base.Seed(context);
    }
  }

  public class TesteContext : DbContext {
    static readonly TesteContext _InternalCache = new TesteContext();

    public static TesteContext InternalCache {
      get { return _InternalCache; }
    }

    static TesteContext() {
      Database.SetInitializer(new TesteContextInitializer());
    }

    public DbSet<Usuario> Usuarios { get; set; }

    public override int SaveChanges() {

      var entities = ChangeTracker.Entries<EntityBase>();
      foreach (var entry in entities) {
        switch (entry.State) {
          case EntityState.Added: {
              entry.Entity.DataCadastro = DateTime.Now;
              entry.Entity.DataAlteracao = DateTime.Now;
              break;
            }
          case EntityState.Modified: {
              entry.Property(x => x.DataCadastro).IsModified = false;
              entry.Entity.DataAlteracao = DateTime.Now;
              break;
            }
        }
      }

      //cache para usuários
      foreach (var dbEntityEntry in ChangeTracker.Entries<Usuario>()) {
        DbEntityEntry<Usuario> entry = dbEntityEntry;
        var cachedEntry = _InternalCache.ChangeTracker.Entries<Usuario>().FirstOrDefault(x => x.Entity.Id == entry.Entity.Id);
        if (cachedEntry != null)
          cachedEntry.State = EntityState.Detached;
      }

      return base.SaveChanges();
    }

  }

  class Program {
    static void Main(string[] args) {
      using (var ctx = new TesteContext()) {
        ctx.Database.Initialize(false);
      }

      using (var ctx = new TesteContext()) {
        foreach (var usuario in ctx.Usuarios) {
          Console.WriteLine(usuario);
          Console.WriteLine("Data de Modificação: {0}", usuario.DataAlteracao);
        }
      }

      //coloca o usuário no cache
      var cachedUsuario = TesteContext.InternalCache.Usuarios.FirstOrDefault();
      Console.WriteLine("Colocar o usuário no cache: {0} ", cachedUsuario);

      //verifica se está no cache realmente
      var testeCachedUsuario = TesteContext.InternalCache.Usuarios.Local.FirstOrDefault();
      Console.WriteLine("Usuário verificado no cache: {0} ", testeCachedUsuario);

      //compara as instâncias
      Debug.Assert(cachedUsuario == testeCachedUsuario);
      Console.WriteLine("{0} comparado com {1} ", cachedUsuario, testeCachedUsuario);

      //para remover do cache, basta alterar o usuário
      using (var ctx = new TesteContext()) {
        var usuario = ctx.Usuarios.First();
        usuario.Nome = Guid.NewGuid().ToString().Substring(0, 10);
        ctx.SaveChanges();
        Console.WriteLine("Usuário alterado normalmente: {0} / {1}", usuario, usuario.DataAlteracao);
      }

      //testa novamente o usuário no cache
      var novoTesteCachedUsuario = TesteContext.InternalCache.Usuarios.Local.FirstOrDefault();
      Debug.Assert(novoTesteCachedUsuario == null);
      Console.WriteLine("Usuário saiu do cache {0}", novoTesteCachedUsuario);

      var normal =
        TesteContext.InternalCache.Usuarios.Local.FirstOrDefault() ??
        TesteContext.InternalCache.Usuarios.FirstOrDefault();

      Console.WriteLine("Usuário recuperado {0}", normal);

      Console.ReadLine();
    }
  }
}