.NET Core dependency injection - many databases, one DbContext

Question

I have an ASP.NET Core application using the DI from Microsoft.Extensions.DependencyInjection . The app connects to a stupid number of databases with an exact same interface, let's say a 100 of them. I want to facade them all with the same DbContext, say ExternalDbContext . The only difference between them is the connection string they're using. I need to instantiate the correct version of the context for a given request. So the chain of resolution would go like this:

1. User makes a request that has a provider = 'ABCD' parameter.
2. This is mapped to an Action in a Controller which has an ISomeService as a dependency.
3. That service has a method DoStuff(string provider) .
4. Here's the crucial part. The ISomeService needs a dependency on the data layer, but this cannot be a hard dependency on one injected ExternalDbContext , since that has to be resolved dynamically. My idea is to have an IExternalDbContextFactory which in turn can take an IServiceProvider . That factory would have a method GetExternalDbContext(string provider) and I would resolve the correct ExternalDbContext using the injected service provider.

To achieve that I would have to register the ExternalDbContexts in a way that would allow me to resolve them from an IServiceProvider based on a string parameter. For obvious reasons I don't want to have a 100 different useless marker classes inheriting from ExternalDbContext and then register them independently. Also I would prefer to use the handy AddDbContext method somehow.

I obviously could build my own resolution infrastructure entirely, but I'd rather reuse an existing solution than spend days writing and testing boilerplate for this particular use case.

Answer 1

You can manage connection strings using overload of AddDbContext which provides you access to IServiceProvider :

public void ConfigureServices(IServiceCollection services)
{
    services
        .AddEntityFrameworkSqlServer()
        .AddDbContext<ExternalDbContext>((serviceProvider, options) =>
        {
             var connectionString = serviceProvider.GetService // get service 
             // which can determine connection string
             .GetConnectionString();
              options.UseSqlServer(connectionString);
         });
       }
}

For example you can resolve IHttpContextAccessor there and check the request parameter.