Proper way to profile a DbContext using MiniProfiler and EF 5 and Autofac

Question

The MiniProfiler site gives the following code for generating an Entity Framework ObjectContext:

public static MyModel Get()
{
  var conn =  new StackExchange.Profiling.Data.EFProfiledDbConnection(GetConnection(), MiniProfiler.Current);
  return ObjectContextUtils.CreateObjectContext<MyModel>(conn); // resides in the MiniProfiler.EF nuget pack
}

However, using Entity Framework 5, I am not using an ObjectContext - rather I am using a DbContext. I cannot plug the model name in here, since the CreateObjectContext<T>() method expects T to be of type ObjectContext. (For the same reason, the code given in this answer also doesn't work).

Additionally, I am using autofac to initialize my Db connections. This is being registered with the following (MyData = the name of my EF DataContext):

Builder.RegisterType<MyData>().As<DbContext>().InstancePerHttpRequest();

So combining two parts: how can I use autofac to initialize my DbContext tied into MiniProfiler.EF? And if that is not possible, at least how can I do the first part (create a factory method for MiniProfiler.EF to return a DbContext)?

Answer 1

I just got this working:

public static class DbContextUtils
{
    private const BindingFlags PrivateInstance = BindingFlags.NonPublic | BindingFlags.Instance;

    public static T CreateDbContext<T>() where T : DbContext
    {
        return CreateDbContext<T>(GetProfiledConnection<T>());
    }

    public static T CreateDbContext<T>(this DbConnection connection) where T : DbContext
    {
        var workspace = new MetadataWorkspace(new[] { "res://*/" }, new[] { typeof(T).Assembly });
        var factory = DbProviderServices.GetProviderFactory(connection);

        var itemCollection = workspace.GetItemCollection(DataSpace.SSpace);
        var providerFactoryField = itemCollection.GetType().GetField("_providerFactory", PrivateInstance);
        if (providerFactoryField != null) providerFactoryField.SetValue(itemCollection, factory);

        var ec = new EntityConnection(workspace, connection);

        return CtorCache<T, DbConnection>.Ctor(ec);
    }

    public static DbConnection GetProfiledConnection<T>() where T : DbContext
    {
        var dbConnection = ObjectContextUtils.GetStoreConnection("name=" + typeof(T).Name);
        return new EFProfiledDbConnection(dbConnection, MiniProfiler.Current);
    }

    internal static class CtorCache<TType, TArg> where TType : class
    {
        public static readonly Func<TArg, TType> Ctor;
        static CtorCache()
        {
            var argTypes = new[] { typeof(TArg) };
            var ctor = typeof(TType).GetConstructor(argTypes);
            if (ctor == null)
            {
                Ctor = x => { throw new InvalidOperationException("No suitable constructor defined"); };
            }
            else
            {
                var dm = new DynamicMethod("ctor", typeof(TType), argTypes);
                var il = dm.GetILGenerator();
                il.Emit(OpCodes.Ldarg_0);
                il.Emit(OpCodes.Newobj, ctor);
                il.Emit(OpCodes.Ret);
                Ctor = (Func<TArg, TType>)dm.CreateDelegate(typeof(Func<TArg, TType>));
            }
        }
    }
}

It is based on the code in MiniProfiler's ObjectContextUtils.

You use it like this:

builder.Register(c => DbContextUtils.CreateDbContext<MyData>()).As<DbContext>().InstancePerHttpRequest();

This solution REQUIRES your DbContext to have a constructor which takes a DbConnection and passes it to base, like this:

public MyData(DbConnection connection)
    : base(connection, true)
{
}

Answer 2

There is a constructor of the DbContext class which takes an existing DbConnection

So you need a new contructor on your MyData which just calls the base

public class MyData : DbContext
{
    public MyData(DbConnection existingConnection, bool contextOwnsConnection)
        : base(existingConnection, contextOwnsConnection)
    {
    }

    //..
}

Then you register your MyData with Register:

builder.Register(c => 
{
   var conn =  new EFProfiledDbConnection(GetConnection(), MiniProfiler.Current);
   return new MyData(conn, true);
}).As<DbContext>().InstancePerHttpRequest();