Yet another Stack Overflow question has sparked a heated discussion and got us thinking whether we can do better.

In a nutshell, the question was about finding a way to query EF Core model metadata without directly referencing the assembly that defines it. Think MsBuild Task that needs to check if your model is following your company standards. Or a test of some sort.

First stab at it

We were able to help the OP by quickly whipping up the following loader code:

var assembly = Assembly.LoadFrom(@"C:\OnlineShoppingStore\bin\Debug\netcoreapp2.2\OnlineShoppingStore.dll");
var contextType = assembly.GetTypes().First(d => d.Name == "OnlineStoreDbContext");
var ctx = Activator.CreateInstance(contextType) as DbContext; // instantiate your context. this will effectively build your model, so you must have all required EF references in your project
var p = ctx.Model.FindEntityType(assembly.GetTypes().First(d => d.Name == "Product")); // get the type from loaded assembly
//var p = ctx.Model.FindEntityType("OnlineStoreDbContext.Product"); // querying model by type name also works, but you'd need to correctly qualify your type names
var pk = p.FindPrimaryKey().Properties.First().Name; // your PK property name as built by EF model

The answer ended up being accepted, but the OP had a bit of an issue with instantiating the Context:

System.InvalidOperationException: 'No database provider has been configured for this DbContext. 
A provider can be configured by overriding the DbContext.OnConfiguring method or by using AddDbContext on the application service provider. 
If AddDbContext is used, then also ensure that your DbContext type accepts a DbContextOptions object in its constructor and passes it to the base constructor for DbContext.

This is kind of expected: when EF creates the context it will invoke OnConfiguring override and set up DB provider with connection strings and so on and so forth. It all is necessary for the actual thing to run, but for the OP it meant having to drag all DB providers into the test harness. Not ideal.

The idea

After a bit back and forth I’ve got an idea. What if we subclass the Context yet again and override the OnConfiguring with a predefined Provider (say, InMemory)?

IL Emit all things

We don’t get to use IL Emit often – it’s meant for pretty specific use cases and I think this is one. The key to getting it right in our case was finding the correct overload of UseInMemoryDatabase. There’s a chance however, that you might need to tweak it to suit your needs. It is pretty trivial once you know what you’re looking for.

public static MethodBuilder OverrideOnConfiguring(this TypeBuilder tb)
        {
            MethodBuilder onConfiguringMethod = tb.DefineMethod("OnConfiguring",
                MethodAttributes.Public
                | MethodAttributes.HideBySig
                | MethodAttributes.NewSlot
                | MethodAttributes.Virtual,
                CallingConventions.HasThis,
                null,
                new[] { typeof(DbContextOptionsBuilder) });

            // the easiest method to pick will be .UseInMemoryDatabase(this DbContextOptionsBuilder optionsBuilder, string databaseName, Action<InMemoryDbContextOptionsBuilder> inMemoryOptionsAction = null)
            // but since constructing generic delegate seems a bit too much effort we'd rather filter everything else out
            var useInMemoryDatabaseMethodSignature = typeof(InMemoryDbContextOptionsExtensions)
                .GetMethods()
                .Where(m => m.Name == "UseInMemoryDatabase")
                .Where(m => m.GetParameters().Length == 3)
                .Where(m => m.GetParameters().Select(p => p.ParameterType).Contains(typeof(DbContextOptionsBuilder)))
                .Where(m => m.GetParameters().Select(p => p.ParameterType).Contains(typeof(string)))
                .Single();
            
            // emits the equivalent of optionsBuilder.UseInMemoryDatabase("test");
            var gen = onConfiguringMethod.GetILGenerator();
            gen.Emit(OpCodes.Ldarg_1);
            gen.Emit(OpCodes.Ldstr, Guid.NewGuid().ToString());
            gen.Emit(OpCodes.Ldnull);
            gen.Emit(OpCodes.Call, useInMemoryDatabaseMethodSignature);
            gen.Emit(OpCodes.Pop);
            gen.Emit(OpCodes.Ret);

            return onConfiguringMethod;
        }

with the above out of the way we now can build our dynamic type and plug it into our test harness!

class Program
    {
        static void Main(string[] args)
        {
            // load assembly under test
            var assembly = Assembly.LoadFrom(@"..\ef-metadata-query\OnlineShoppingStore\bin\Debug\netcoreapp3.1\OnlineShoppingStore.dll");
            var contextType = assembly.GetTypes().First(d => d.Name == "OnlineStoreDbContext");

            // create yet another assembly that will hold our dynamically generated type
            var typeBuilder = AssemblyBuilder
                                .DefineDynamicAssembly(new AssemblyName(Guid.NewGuid().ToString()), AssemblyBuilderAccess.RunAndCollect)
                                .DefineDynamicModule(Guid.NewGuid() + ".dll")
                                .DefineType("InheritedDbContext", TypeAttributes.Public, contextType); // make new type inherit from DbContext under test!

            // this is the key here! now our dummy implementation will kick in!
            var onConfiguringMethod = typeBuilder.OverrideOnConfiguring();
            typeBuilder.DefineMethodOverride(onConfiguringMethod, typeof(DbContext).GetMethod("OnConfiguring", BindingFlags.Instance | BindingFlags.NonPublic));
            
            var inheritedDbContext = typeBuilder.CreateType(); // enough config, let's get the type and roll with it

            // instantiate inheritedDbContext with default OnConfiguring implementation
            var context = Activator.CreateInstance(inheritedDbContext) as DbContext; // instantiate your context. this will effectively build your model, so you must have all required EF references in your project
            var p = context?.Model.FindEntityType(assembly.GetTypes().First(d => d.Name == "Product")); // get the type from loaded assembly
            
            //query the as-built model
            //var p = ctx.Model.FindEntityType("OnlineStoreDbContext.Product"); // querying model by type name also works, but you'd need to correctly qualify your type names
            var pk = p.FindPrimaryKey().Properties.First().Name; // your PK property name as built by EF model
            
            Console.WriteLine(pk);
        }
    }

This is runnable

Source code is available on GitHub in case you want to check it out and play a bit