Hooking IDbInterceptor to EntityFramework DbContext only once

Question

The IDbCommandInterceptor interface is not very well documented. And I've only found a few scarce tutorials on it:

• http://www.entityframeworktutorial.net/entityframework6/database-command-interception.aspx
• https://msdn.microsoft.com/en-us/data/jj556606%28v=vs.113%29.aspx
• https://entityframework.codeplex.com/wikipage?title=Interception
• https://www.tutorialspoint.com/entity_framework/entity_framework_command_interception.htm
• https://msdn.microsoft.com/en-us/data/dn469464%28v=vs.113%29.aspx

And a few SO questions:

• Entity Framework 6 - Timing queries
• Getting DbContext from implementation of IDbCommandInterceptor

---

These are the suggestions on hooking I've found:

1 - The static DbInterception class:

DbInterception.Add(new MyCommandInterceptor());

2 - Doing the above suggestion in a DbConfiguration class

public class MyDBConfiguration : DbConfiguration {
    public MyDBConfiguration() {
        DbInterception.Add(new MyCommandInterceptor());
    }
}

3 - Using the config file:

<entityFramework>
  <interceptors>
    <interceptor type="EFInterceptDemo.MyCommandInterceptor, EFInterceptDemo"/>
  </interceptors>
</entityFramework>

---

Although I couldn't figure out how to hook the DbConfiguration class to the DbContext, and neither what to put in the type part of the config method. Another example I found seemed to suggest that you write the namespace of a logger:

type="System.Data.Entity.Infrastructure.Interception.DatabaseLogger, EntityFramework"

I noted that DataBaseLogger implements IDisposable, IDbConfigurationInterceptor and
IDbInterceptor. IDbCommandInterceptor also implements IDbInterceptor, so I tried (without success) to format it like this:

type="DataLayer.Logging.MyCommandInterceptor, DataLayer"

And when I called the static DbInterception class directly, it added another interceptor every call. So my quick and dirty solution was to utilize static constructors:

//This partial class is a seperate file from the Entity Framework auto-generated class,
//to allow dynamic connection strings
public partial class MyDbContext // : DbContext
{
    public Guid RequestGUID { get; private set; }

    public MyDbContext(string nameOrConnectionString) : base(nameOrConnectionString)
    {
        DbContextListeningInitializer.EnsureListenersAdded();

        RequestGUID = Guid.NewGuid();
        //Database.Log = m => System.Diagnostics.Debug.Write(m);
    }

    private static class DbContextListeningInitializer
    {
        static DbContextListeningInitializer() //Threadsafe
        {
            DbInterception.Add(new MyCommandInterceptor());
        }
        //When this method is called, the static ctor is called the first time only
        internal static void EnsureListenersAdded() { }
    }
}

But what are the proper/intended ways to do it?

Loupgarou

Answer 1

I figured out that my DbContext class just needed to have the DbConfigurationType attribute, to attach a configuration at runtime:

[DbConfigurationType(typeof(MyDBConfiguration))]
public partial class MyDbContext // : DbContext
{
    public MyDbContext(string nameOrConnectionString) : base(nameOrConnectionString)
    { }
}

public class MyDBConfiguration : DbConfiguration {
    public MyDBConfiguration() {
        this.AddInterceptor(new MyCommandInterceptor());
    }
}

Loupgarou

I don't think this is thread-safe. What happens if another thread has MyDBConfiguration2 and a similar constructor body? Then both DbContext's will have MyCommandInterceptor registered twice, right? – Antipus

In your MyDBConfiguration constructor, you dont have to use that static class, you have this.AddInterceptor() method. – Quirk

@JohnZabroski Do you mean that DbInterception.Add() isn't thread-safe? The documentation doesn't say so, so I'd assume you're right. Jan'splite'K.'s suggestion looks like the logical answer - and I tested it to confirm that it works. Updated answer accordingly. – Loupgarou

Thread-safe may have been a poor/lazy choice of words. Really, the behavior of your code was not idempotent. – Antipus

I agree with @JohnZabroski, I have two Contexts in different parts of the application and I had errors depending on which context was created first - the second complained that the DbConfigurationType was not the same, so all of this config is global – Stonechat

The documentation says that DbConfiguration is applied at the App-Domain level. It also says that DbConfigurationType attribute should be used when your DbContext is in a separate assembly from your configuration: learn.microsoft.com/en-us/ef/ef6/fundamentals/configuring/… – Scyphus

Answer 2

The docs suggests that you can just put it in Application_Start:

protected void Application_Start()
{
    AreaRegistration.RegisterAllAreas();
    FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
    RouteConfig.RegisterRoutes(RouteTable.Routes);
    BundleConfig.RegisterBundles(BundleTable.Bundles);
    DbInterception.Add(new SchoolInterceptorTransientErrors());
    DbInterception.Add(new SchoolInterceptorLogging());
}

The important part is that it only gets called once.

Ophthalmia