ASP.Net Core – Resolving types from dynamic assemblies

It is not a secret that ASP.NET core comes with dependency injection support out of the box. And we don’t remember ever feeling it lacks features. All we have to do is register a type in Startup.cs and it’s ready to be consumed in our controllers:

public class Startup
    public void ConfigureServices(IServiceCollection services)
        services.AddScoped<IDBLogger, IdbLogger>();
public class HomeController : Controller
    private readonly IDBLogger _idbLogger;
    public HomeController(IDBLogger idbLogger)     
        _idbLogger = idbLogger; // all good here!

What if it’s a plugin?

Now imagine we’ve got a Order type that we for whatever strange reason load at runtime dynamically.

public class Order
     private readonly IDBLogger _logger; // suppose we've got the reference from common assembly that both our main application and this plugin are allowed to reference
     public Order(IDBLogger logger)
         _logger = logger; // will it resolve?
     public void GetOrderDetail()
        _logger.Log("Inside GetOrderDetail"); // do we get a NRE here?

Load it in the controller

External assembly being external kind of implies that we want to load it at the very last moment – right in our controller where we presumably need it. If we try explore this avenue, we immediately see the issue:

public HomeController(IDBLogger idbLogger)
     _idbLogger = idbLogger;
     var assembly = Assembly.LoadFrom(Path.Combine("..\Controllers\bin\Debug\netcoreapp3.1", "Orders.dll"));
     var orderType = assembly.ExportedTypes.First(t => t.Name == "Order");
     var order = Activator.CreateInstance(orderType); //throws System.MissingMethodException: 'No parameterless constructor defined for type 'Orders.Order'.'
     orderType.InvokeMember("GetOrderDetail", BindingFlags.Public | BindingFlags.Instance|BindingFlags.InvokeMethod, null, order, new object[] { });

The exception makes perfect sense – we need to inject dependencies! Making it so:

public HomeController(IDBLogger idbLogger)
     _idbLogger = idbLogger;
     var assembly = Assembly.LoadFrom(Path.Combine("..\Controllers\bin\Debug\netcoreapp3.1", "Orders.dll"));
     var orderType = assembly.ExportedTypes.First(t => t.Name == "Order");
     var order = Activator.CreateInstance(orderType, new object[] { _idbLogger }); // we happen to know what the constructor is expecting
     orderType.InvokeMember("GetOrderDetail", BindingFlags.Public | BindingFlags.Instance|BindingFlags.InvokeMethod, null, order, new object[] { });

Victory! or is it?

The above exercise is nothing new of exceptional – the point we are making here is – dependency injection frameworks were invented so we don’t have to do this manually. In this case it was pretty easy but more compex constructors can many dependencies. What’s worse – we may not be able to guarantee we even know all dependencies we need. If only there was a way to register dynamic types with system DI container…

Yes we can

The most naive solution would be to load our assembly on Startup.cs and register needed types along with our own:

public void ConfigureServices(IServiceCollection services)
     // load assembly and register with DI  
     var assembly = Assembly.LoadFrom(Path.Combine("..\\Controllers\\bin\\Debug\\netcoreapp3.1", "Orders.dll")); 
    var orderType = assembly.ExportedTypes.First(t => t.Name == "Order");
    services.AddScoped(orderType); // this is where we would make our type known to the DI container  
    var loadedTypesCache = new LoadedTypesCache(); // this step is optional - i chose to leverage the same DI mechanism to avoid having to load assembly in my controller for type definition.  
    loadedTypesCache.LoadedTypes.Add("order", orderType);
    services.AddSingleton(loadedTypesCache); // singleton seems like a good fit here            

And that’s it – literally no difference where the type is coming from! In controller, we’d inject IServiceProvider and ask to hand us an instance of type we cached earlier:

public HomeController(IServiceProvider serviceProvider, LoadedTypesCache cache)
     var order = serviceProvider.GetService(cache.LoadedTypes["order"]); // leveraging that same loaded type cache to avoid having to load assembly again
 // following two lines are just to call the method 
    var m = cache.LoadedTypes["order"].GetMethod("GetOrderDetail", BindingFlags.Public | BindingFlags.Instance); 
    m.Invoke(order, new object[] { }); // Victory!

Attaching debugger to dynamically loaded assembly with Reflection.Emit

Imagine a situation where you’d like to attach a debugger to an assembly that you have loaded dynamically? To make it a bit more plausible let us consider a scenario. Our client has a solution where they maintain extensive plugin ecosystem. Each plugin is a class library built with .net 4.5. Each plugin implements a common interface that main application is aware of. At runtime the application scans a folder and loads all assemblies into separate AppDomains. Under certain circumstances users/developers would like to be able to debug plugins in Visual Studio.
Given how seldom we would opt for this technique, documenting our solution might be an exercise in vain. But myself being a huge fan of weird and wonderful – I couldn’t resist going through with this case study.

Inventorying moving parts

First of all we’d need a way to inject code into the assembly. Apparently we can not directly replace methods we loaded from disk – SwapMethodBody() needs a DynamicModule. So we opted to define a subclass wrapper. Next, we need to actually stop execution and offer developers to start debugging. Using Debugger.Launch() is the easiest way to achieve that. Finally, we’d look at different ways to load assemblies into separate AppDomains to maintain existing convention.

Injecting Debugger.Launch()

The main attraction here – and Reflection.Emit is a perfect candidate for the job. Theory is fairly simple: we create a new dynamic assembly, module, type and a method. Then we generate code inside of the method and return wrapper instance:

public static object CreateWrapper(Type ServiceType, MethodInfo baseMethod)
    var asmBuilder = AppDomain.CurrentDomain.DefineDynamicAssembly(new AssemblyName($"newAssembly_{Guid.NewGuid()}"), AssemblyBuilderAccess.Run);
    var module = asmBuilder.DefineDynamicModule($"DynamicAssembly_{Guid.NewGuid()}");
    var typeBuilder = module.DefineType($"DynamicType_{Guid.NewGuid()}", TypeAttributes.Public, ServiceType);
    var methodBuilder = typeBuilder.DefineMethod("Run", MethodAttributes.Public | MethodAttributes.NewSlot);

    var ilGenerator = methodBuilder.GetILGenerator();

    ilGenerator.EmitCall(OpCodes.Call, typeof(Debugger).GetMethod("Launch", BindingFlags.Static | BindingFlags.Public), null);

    ilGenerator.EmitCall(OpCodes.Call, baseMethod, null);

     * the generated method would be roughly equivalent to:
     * new void Run()
     * {
     *   Debugger.Launch();
     *   base.Run();
     * }

    var wrapperType = typeBuilder.CreateType();
    return Activator.CreateInstance(wrapperType);

Triggering the method

After we’ve generated a wrapper – we should be in position to invoke the desired method. In this example I’m using all-reflection approach:

public void Run()
    var wrappedInstance = DebuggerWrapperGenerator.CreateWrapper(ServiceType, ServiceType.GetMethod("Run"));
    wrappedInstance.GetType().GetMethod("Run")?.Invoke(wrappedInstance, null);
 // nothing special here

The task becomes even easier if we know the interface to cast to.

Adding AppDomain into the mix

The above parts don’t depend much on where the code will run. However, trying to satisfy the layout requirement, we experimented with a few different configurations. In the end it appears that I’m able to confidently place the code in correct AppDomain by either leveraging .DoCallBack() or making sure that Launcher helper is created with .CreateInstanceAndUnwrap():

static void Main(string[] args)
    var appDomain = AppDomain.CreateDomain("AppDomainInMain", AppDomain.CurrentDomain.Evidence,
        new AppDomainSetup { ApplicationBase = AppDomain.CurrentDomain.SetupInformation.ApplicationBase });

    appDomain.DoCallBack(() =>
        var launcher = new Launcher(PathToDll);
static void Main(string[] args)
public class Launcher : MarshalByRefObject
    private Type ServiceType { get; }

    public Launcher(string pathToDll)
        var assembly = Assembly.LoadFrom(pathToDll);
        ServiceType = assembly.GetTypes().SingleOrDefault(t => t.Name == "Class1");

    public void Run()
        var wrappedInstance = DebuggerWrapperGenerator.CreateWrapper(ServiceType, ServiceType.GetMethod("Run"));
        wrappedInstance.GetType().GetMethod("Run")?.Invoke(wrappedInstance, null);

    public static void RunInNewAppDomain(string pathToDll)
        var appDomain = AppDomain.CreateDomain("AppDomainInLauncher", AppDomain.CurrentDomain.Evidence, AppDomain.CurrentDomain.SetupInformation);

        var launcher = appDomain.CreateInstanceAndUnwrap(typeof(Launcher).Assembly.FullName, typeof(Launcher).FullName, false, BindingFlags.Public|BindingFlags.Instance,
            null, new object[] { pathToDll }, CultureInfo.CurrentCulture, null);
        (launcher as Launcher)?.Run();

Testing it

In the end we’ve got the following prompt:

after letting it run through, we’d get something looking like this:

As usual, full code for this example sits in my GitHub if you want to take it for a spin.

Parsing OData queries

OData (Open Data Protocol) is an ISO approved standard that defines a set of best practices for building and consuming RESTful APIs. It allows us write business logic and not worry too much about request and response headers, status codes, HTTP methods, and other variables.

We won’t go into too much detail on how to write OData queries and how to use it – there’s plenty resources out there. We’ll rather have a look at a bit esoteric scenario where we consider defining our own parser and then walking the AST to get desired values.

Problem statement

Suppose we’ve got a filter string that we received from the client:

"?$filter =((Name eq 'John' or Name eq 'Peter') and (Department eq 'Professional Services'))"

And we’d like to apply custom validation to the filter. Ideally we’d like to get a structured list of properties and values so we can run our checks:

Filter 1:
    Key: Name
    Operator: eq
    Value: John
Operator: or

Filter 2:
    Key: Name
    Operator: eq
    Value: Peter

Operator: and

Filter 3:
    Key: Department
    Operator: eq
    Value: Professional Services

Some options are:

  • ODataUriParser – but it seems to have some issues with .net Core support just yet
  • Regular Expression – not very flexible
  • ODataQueryOptions – produces raw text but cannot broken down any further

What else?

One other way to approach this would be parsing. And there are plenty tools to do that (see flex or bison for example). In .net world, however, Irony might be a viable option: it’s available in .net standard 2.0 which we had no issues plugging into a .net core 3.1 console test project.


To start off, we normally need to define a grammar. But luckily, Microsoft have been kind enough to supply us with EBNF reference so all we have to do is to adapt it to Irony. I ended up implementing a subset of the grammar above that seems to cater for example statement (and a bit above and beyond, feel free to cut it down).

using Irony.Parsing;

namespace irony_playground
    [Language("OData", "1.0", "OData Filter")]
    public class OData: Grammar
        public OData()
            // first we define some terms
            var identifier = new RegexBasedTerminal("identifier", "[a-zA-Z_][a-zA-Z_0-9]*");
            var string_literal = new StringLiteral("string_literal", "'");
            var integer_literal = new NumberLiteral("integer_literal", NumberOptions.IntOnly);
            var float_literal = new NumberLiteral("float_literal", NumberOptions.AllowSign|NumberOptions.AllowSign) 
                                        | new RegexBasedTerminal("float_literal", "(NaN)|-?(INF)");
            var boolean_literal = new RegexBasedTerminal("boolean_literal", "(true)|(false)");

            var filter_expression = new NonTerminal("filter_expression");
            var boolean_expression = new NonTerminal("boolean_expression");
            var collection_filter_expression = new NonTerminal("collection_filter_expression");
            var logical_expression = new NonTerminal("logical_expression");
            var comparison_expression = new NonTerminal("comparison_expression");
            var variable = new NonTerminal("variable");
            var field_path = new NonTerminal("field_path");
            var lambda_expression = new NonTerminal("lambda_expression");
            var comparison_operator = new NonTerminal("comparison_operator");
            var constant = new NonTerminal("constant");

            Root = filter_expression; // this is where our entry point will be. 

            // and from here on we expand on all terms and their relationships
            filter_expression.Rule = boolean_expression;

            boolean_expression.Rule = collection_filter_expression
                                      | logical_expression
                                      | comparison_expression
                                      | boolean_literal
                                      | "(" + boolean_expression + ")"
                                      | variable;
            variable.Rule = identifier | field_path;

            field_path.Rule = MakeStarRule(field_path, ToTerm("/"), identifier);

            collection_filter_expression.Rule =
                field_path + "/all(" + lambda_expression + ")"
                | field_path + "/any(" + lambda_expression + ")"
                | field_path + "/any()";

            lambda_expression.Rule = identifier + ":" + boolean_expression;

            logical_expression.Rule =
                boolean_expression + (ToTerm("and", "and") | ToTerm("or", "or")) + boolean_expression
                | ToTerm("not", "not") + boolean_expression;

            comparison_expression.Rule =
                variable + comparison_operator + constant |
                constant + comparison_operator + variable;

            constant.Rule =
                | integer_literal
                | float_literal
                | boolean_literal
                | ToTerm("null");

            comparison_operator.Rule = ToTerm("gt") | "lt" | "ge" | "le" | "eq" | "ne";

            RegisterBracePair("(", ")");

NB: Irony comes with Grammar Explorer tool that allows us to load grammar dlls and debug them with free text input.

enter image description here

after we’re happy with the grammar, we need to reference it from our project and parse the input string:

class Program
    static void Main(string[] args)
        var g = new OData();
        var l = new LanguageData(g);
        var r = new Parser(l);
        var p = r.Parse("((Name eq 'John' or Name eq 'Grace Paul') and (Department eq 'Finance and Accounting'))"); // here's your tree
        // this is where you walk it and extract whatever data you desire 

Then, all we’ve got to do is walk the resulting tree and apply any custom logic based on syntax node type. One example how to do that can be found in this StackOverflow answer.

Entity Framework Core 3.1 – Peeking Into Generated SQL

Writing LINQ that produces optimal SQL can be even harder as developers often don’t have visibility into the process. It becomes even more confusing when the application is designed to run against different databases.

We often find ourselves questioning whether this particular query will fall in line with our expectations. And until not so long ago our tool of choice was a SQL Profiler, that ships with SQL Server. It’s plenty powerful but has one flaw – it pretty much requires the SQL Server installation. This might be a deal breaker for some clients using other DBs, like Postgres or MySQL (which are all supported by the way).

EF to the resque

Instead of firing off the profiler and fishing out the batches, we could have Entity Framework itself pass us the result. After all, it needs to build SQL before sending it off the the database, so all we have to do it to ask nicely. Stack Overflow is quite helpful here:

public static class IQueryableExtensions // this is the EF Core 3.1 version.
        public static string ToSql<TEntity>(this IQueryable<TEntity> query) where TEntity : class
            var enumerator = query.Provider.Execute<IEnumerable<TEntity>>(query.Expression).GetEnumerator();
            var relationalCommandCache = enumerator.Private("_relationalCommandCache");
            var selectExpression = relationalCommandCache.Private<SelectExpression>("_selectExpression");
            var factory = relationalCommandCache.Private<IQuerySqlGeneratorFactory>("_querySqlGeneratorFactory");

            var sqlGenerator = factory.Create();
            var command = sqlGenerator.GetCommand(selectExpression);

            string sql = command.CommandText;
            return sql;

        private static object Private(this object obj, string privateField) => obj?.GetType().GetField(privateField, BindingFlags.Instance | BindingFlags.NonPublic)?.GetValue(obj);
        private static T Private<T>(this object obj, string privateField) => (T)obj?.GetType().GetField(privateField, BindingFlags.Instance | BindingFlags.NonPublic)?.GetValue(obj);

The usage is simple

Suppose we’ve got the following inputs: One simple table, that we’d like to group by one field and total by another. Database Context is also pretty much boilerplate. One thing to note here is a couple of database providers we are going to try the query against.

public class SomeTable
    public int Id { get; set; }
    public int Foobar { get; set; }
    public int Quantity { get; set; }

class MyDbContext : DbContext
    public DbSet<SomeTable> SomeTables { get; set; }
    public static readonly LoggerFactory DbCommandConsoleLoggerFactory
        = new LoggerFactory(new[] {
        new ConsoleLoggerProvider ((category, level) =>
            category == DbLoggerCategory.Database.Command.Name &&
            level == LogLevel.Trace, true)
    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
        //run with SQL Server provider to get T-SQL
        optionsBuilder.UseNpgsql("Server=localhost;Port=5432;Database=test;User Id=;Password=;")
        //alternatively use other supported provider

The test bench would look something like so

class Program
    static void Main(string[] args)

        var context = new MyDbContext();
        var someTableData = context.SomeTables
                .GroupBy(x => x.Foobar)
                .Select(x => new { Foobar = x.Key, Quantity = x.Sum(y => y.Quantity) })
                .OrderByDescending(x => x.Quantity)
                .Take(10) // we've built our query as per normal
                .ToSql(); // this is the magic

And depending on our choice of provider the output would show ef core generated sql for SQL Server and Postgres

        -- MSSQL
        SELECT TOP(@__p_0) [s].[Foobar], SUM([s].[Quantity]) AS [Quantity]
        FROM [SomeTables] AS [s]
        GROUP BY [s].[Foobar]
        ORDER BY SUM([s].[Quantity]) DESC

        -- PG SQL
         SELECT s."Foobar", SUM(s."Quantity")::INT AS "Quantity"
        FROM "SomeTables" AS s
        GROUP BY s."Foobar"
        ORDER BY SUM(s."Quantity")::INT DESC
        LIMIT @__p_0

Getting started with Roslyn code analysis

It was going to happen eventually – our research on C# dynamic features eventually ended up with an attempt to parse bits of source code. There are quite a few solutions on the market, with NRefactory being our preferred tool over the years. There are however a few limitations: it does not support .NET core and C# 6.

It is a big deal

It might seem, that support for newer language spec is not critical. But in fact, it gets problematic very quickly even in more established projects. Luckily for us, Microsoft has chosen to open source Roslyn – the very engine that powers their compiler services. Their official documentation covers the platform pretty well and goes in great detail of writing Visual Studio code analysers. We however often have to deal with writing MSBuild tasks that load the whole solution and run analysis on class hierarchies (for example, to detect whether a single `SQL SELECT` statement is being called inside a foreach loop – we would fail the build and suggest to replace it with bulk select)


Roslyn is available via NuGet as a number of Microsoft.CodeAnalysis.* packages. We normally include these four:

Install-Package Microsoft.CodeAnalysis.Workspaces.MSBuild
Install-Package Microsoft.CodeAnalysis
Install-Package Microsoft.CodeAnalysis.CSharp
Install-Package Microsoft.Build # these classes are needed to support MSBuild workspace when it starts to load solution
Install-Package Microsoft.Build.Utilities.Core # these classes are needed to support MSBuild workspace when it starts to load solution
Install-Package Microsoft.Build.Locator # this is a helper to locate correct MSBuild toolchain (in case the machine has more than one installed)

Sometimes the environment gets confused as to what version MSBuild to use, and this is why starting a project with something like this is pretty much a must since VS2015:

// put this somewhere early in the program
if (!MSBuildLocator.IsRegistered) //MSBuildLocator.RegisterDefaults(); // ensures correct version is loaded up
    var vs2022 = MSBuildLocator.QueryVisualStudioInstances().Where(x => x.Name == "Visual Studio Community 2022").First(); // find the correct VS setup. There are namy ways to organise logic here, we'll just assume we want VS2022
    MSBuildLocator.RegisterInstance(vs2022); // register the selected instance
    var _ = typeof(Microsoft.CodeAnalysis.CSharp.Formatting.CSharpFormattingOptions); // this ensures library is referenced so the compiler would not try to optimise it away (if dynamically loading assemblies or doing other voodoo that can throw the compiler off) - probably less important than the above but we prefer to follow cargo cult here and leave it be

After initial steps, simplistic solution traversal would look something along these lines:

async Task AnalyseSolution()
	using (var w = MSBuildWorkspace.Create())
		var solution = await w.OpenSolutionAsync(@"MySolution.sln");		
		foreach (var project in solution.Projects)
			var docs = project.Documents; // allows for file-level document filtering
			var compilation = await project.GetCompilationAsync(); // allows for assembly-level analysis as well as SemanticModel 
			foreach (var doc in docs)
				var walker = new CSharpSyntaxWalker(); // CSharpSyntaxWalker is an abstract class - we will need to define our own implementation for this to actually work
				walker.Visit(await doc.GetSyntaxRootAsync()); // traverse the syntax tree

Syntax Tree Visitor

As with pretty much every single mainstream syntax analyser, the easiest way to traverse syntax trees is by using a Visitor Pattern. It allows to decouple tree nodes and processing logic. Which will allow room for expansion on either side (easy to add new logic, easy to add new tree node types). Roslyn has stub CSharpSyntaxWalker that allows us to only override required nodes for processing. It then takes care of everything else.

With basics out of the way, let’s look into classes that make up our platform here. Top of the hierarchy is MSBuild Workspace followed by Solution, Project and Document. Roslyn makes a distinction between parsing code and compiling it. Meaning some analytics will only be available in Compilation class that is available for project as well as for individual documents down the track.

Traversing the tree

Just loading the solution is kind of pointless though. We’d need to come up with processing logic – and the best place to do it would be a CSharpSyntaxWalker subclass. Suppose, we’d like to determine whether class constructor contains if statements that are driven by parameters. This might mean we’ve got overly complex classes and could benefit from refactoring these out:

public class ConstructorSyntaxWalker : CSharpSyntaxWalker
    public List<IParameterSymbol> Parameters { get; set; }
    public int IfConditions { get; set; }
    bool processingConstructor = false;

    SemanticModel sm;

    public ConstructorSyntaxWalker(SemanticModel sm)
    { = sm;
        Parameters = new List<IParameterSymbol>();

    public override void VisitConstructorDeclaration(ConstructorDeclarationSyntax node)
        processingConstructor = true;
        processingConstructor = false;

    public override void VisitIfStatement(IfStatementSyntax node)
        if (!processingConstructor) return; // we only want to keep traversing if we know we're inside constructor body
        Parameters.AddRange(sm.AnalyzeDataFlow(node).DataFlowsIn.Cast<IParameterSymbol>()); // .AnalyzeDataFlow() is one of the most commonly used parts of the platform: it requires a compilation to work off and allows tracking dependencies. We could then check if these parameters are supplied to constructor and make a call whether this is allowed 
        IfConditions++; // just count for now, nothing fancy

Then, somewhere in our solution (or any other solution, really!) We have a class definition like so:

public class TestClass
    public TestClass(int a, string o) 
        if (a == 1) DoThis() else DoSomethingElse();
        if (o == "a") Foo() else Bar();

If we wanted to throw an exception and halt the build we could invoke out SyntaxWalker:

public static async Task Main()
    await AnalyseSolution();
async static Task AnalyseSolution()

    using (var w = MSBuildWorkspace.Create())
        var solution = await w.OpenSolutionAsync(@"..\..\..\TestRoslyn.sln"); // let's analyse our own solution. But can be any file on disk
        foreach (var project in solution.Projects)
            var docs = project.Documents; // allows for file-level document filtering
            var compilation = await project.GetCompilationAsync(); // allows for assembly-level analysis as well as SemanticModel 
            foreach (var doc in docs)
                var walker = new ConstructorSyntaxWalker(await doc.GetSemanticModelAsync());
                walker.Visit(await doc.GetSyntaxRootAsync()); // traverse the syntax tree
                if (walker.IfConditions > 0 && walker.Parameters.Any()) throw new Exception("We do not allow branching in constructors.");

And there we have it. This is a very simplistic example, but possibilities are endless!

Entity Framework Core 3 – Custom Functions (Using IMethodCallTranslator)

Every now and then Stack Overflow provides fantastic opportunities to learn something new. One user asked whether SQL Server’s DECRYPTBYPASSPHRASE can be implemented with Entity Framework Core 2.2 so they can fetch encrypted strings in SQL.

Continue reading “Entity Framework Core 3 – Custom Functions (Using IMethodCallTranslator)”

EF Core 3: Getting model metadata from dynamically loaded assembly with IL Emit

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.HideBySig
                | MethodAttributes.NewSlot
                | MethodAttributes.Virtual,
                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)
                .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)))
            // emits the equivalent of optionsBuilder.UseInMemoryDatabase("test");
            var gen = onConfiguringMethod.GetILGenerator();
            gen.Emit(OpCodes.Ldstr, Guid.NewGuid().ToString());
            gen.Emit(OpCodes.Call, useInMemoryDatabaseMethodSignature);

            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

This is runnable

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

Custom Routing in .NET WebAPI

We all need to do weird things sometimes. One assignment we’ve got was to implement an API that would totally obfuscate all parameters in a Base64 encoded string. This will clearly go against stock standard routing and action mapping that ASP.NET WebAPI comes with out of the box. But that got us thinking about ways we can achieve it nonetheless.

By default

Normally, the router will:

  1. get the request URI,
  2. match it against given templates (those "{controller}/{action}" things), and
  3. invoke an {action} on {controller} with whatever parameters happen to be passed along

Then we realise

We’re constrained to full .net framework on the project and fancy .net core middleware are not a thing yet. Luckily for us custom Message Handler is a thing so theoretically we could bootstrap ourselves through that and override IHttpControllerSelector (and potentially IHttpActionSelector).


Writing code directly in global.asax is an option, but as it calls through to WebApiConfig.Register() by default:


it’s probably a better place for things to do with WebAPI.


    public static class WebApiConfig
        public static void Register(HttpConfiguration config)
            // Web API configuration and services
            // Web API routes
            config.MessageHandlers.Add(new TestHandler()); // if you define a handler here it will kick in for ALL requests coming into your WebAPI (this does not affect MVC pages though)
            config.Services.Replace(typeof(IHttpControllerSelector), new MyControllerSelector(config)); // you likely will want to override some more services to ensure your logic is supported, this is one example

            // your default routes
            config.Routes.MapHttpRoute(name: "DefaultApi", routeTemplate: "api/{controller}/{id}", defaults: new {id = RouteParameter.Optional});

            //a non-overlapping endpoint to distinguish between requests. you can limit your handler to only kick in to this pipeline
            config.Routes.MapHttpRoute(name: "Base64Api", routeTemplate: "apibase64/{query}", defaults: null, constraints: null
                //, handler: new TestHandler() { InnerHandler = new HttpControllerDispatcher(config) } // here's another option to define a handler

and then define our handler:


    public class TestHandler : DelegatingHandler
        protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
            //suppose we've got a URL like so: http://localhost:60290/api/VmFsdWVzCg==
            var b64Encoded = request.RequestUri.AbsolutePath.Remove(0, "/apibase64/".Length);
            byte[] data = Convert.FromBase64String(b64Encoded);
            string decodedString = Encoding.UTF8.GetString(data); // this will decode to values
            request.Headers.Add("controllerToCall", decodedString); // let us say this is the controller we want to invoke
            HttpResponseMessage resp = await base.SendAsync(request, cancellationToken);
            return resp;

Depending on what exactly we want handler to do, we might also have to supply a custom ControllerSelector implementation:


// add this line in your Register method
config.Services.Replace(typeof(IHttpControllerSelector), new MyControllerSelector(config));


    public class MyControllerSelector : DefaultHttpControllerSelector
        public MyControllerSelector(HttpConfiguration configuration) : base(configuration)

        public override string GetControllerName(HttpRequestMessage request)
            //this is pretty minimal implementation that examines a header set from TestHandler and returns correct value
            if (request.Headers.TryGetValues("controllerToCall", out var candidates))
                return candidates.First();
                return base.GetControllerName(request);

Applying this in real life?

Pretty neat theory. We however couldn’t quite figure out a way to take it to our customers that wouldn’t raise a few questions on whether we’re doing something shady there.

Making Swagger to get the authorization token from URL query string

Swagger is extremely useful when developing and debugging Web APIs. Some dev environments however got a bit of security added on top which can get a bit too painful to work around.

Enter API key

It doesn’t need to be tedious! We’ll be looking at overriding Swagger-UI’s index page so we can plug a custom handler into onComplete callback. Solution is extremely simple:

  1. Grab latest index.html from Swashbuckle’s source repo (ideally, get the matching version)
  2. Tweak configObject to add an OnComplete callback handler so it will call preauthorizeApiKey when the UI is ready
  3. Override IndexStream in UserSwaggerUI extension method to serve the custom html

I ended up having the following setup (some bits are omitted for brevity):


<!-- your standard HTML here, nothing special -->
    // some boilerplate initialisation
    // Begin Swagger UI call region
    configObject.onComplete = () => {

        // get the authorization portion of the query string
        var urlParams = new URLSearchParams(;
        if (urlParams.has('authorization')) {
            var apikey = urlParams.get('authorization');

            // this is the important bit, see documentation
            ui.preauthorizeApiKey('api key', apikey );// key name must match the one you defined in AddSecurityDefinition method in Startup.cs
    const ui = SwaggerUIBundle(configObject);
    window.ui = ui        


    public void ConfigureServices(IServiceCollection services)
        services.AddSwaggerGen(c => {
            c.SwaggerDoc("v1", new Info { Title = "You api title", Version = "v1" });
            c.AddSecurityDefinition("api key", new ApiKeyScheme() // key name must match the one you supply to preauthorizeApiKey call in JS
                Description = "Authorization query string expects API key",
                In = "query",
                Name = "authorization",
                Type = "apiKey"

            var requirements = new Dictionary<string, IEnumerable<string>> {
                { "api key", new List<string>().AsEnumerable() }

    // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
    public void Configure(IApplicationBuilder app, IHostingEnvironment env)
        app.UseSwaggerUI(c =>
            c.IndexStream = () => File.OpenRead("wwwroot/swashbuckle.html"); // this is the important bit. see documentation
            c.SwaggerEndpoint("/swagger/v1/swagger.json", "My API V1"); // very standard Swashbuckle init

After having finished all that, calling the standard swagger URL with ?authorization=1234567890 should automatically authorize the page.

Integration testing aide for MVC core routing

Sometimes unit tests just don’t cut it. This is where integration tests come in. This however brings a whole new set of issues with finding the beast way to isolate the aspects under test and mock everything else away.

Problem statement

Suppose, we’ve got an api and a test that needs to make an http call to our api endpoint, like so:

  public class TestController : ControllerBase {

    public IActionResult OkTest() {
      return Ok(true);
public class TestControllerTests {

    private readonly HttpClient _client;

    public TestControllerTests() {
      _client = TestSetup.GetTestClient();

    public async Task OkTest() {
      var path = GetPathHere(nameof(OkTest)); // should return "/api/test/oktest".
      var response = await _client.GetAsync(path);


Knowing that ASP.NET Core comes with such a lightweight package now, and exposes so many extensibility points, one approach we found efficient was to build up the whole Host and query its properties:

private string GetPathHere(string actionName)
        var host = Program.CreateWebHostBuilder(new string[] { }).Build();
        IActionDescriptorCollectionProvider provider = (host.Services as ServiceProvider).GetService<IActionDescriptorCollectionProvider>();
        return provider.ActionDescriptors.Items.First(i => (i as ControllerActionDescriptor)?.ActionName == actionName).AttributeRouteInfo.Template;

    public void OkTestShouldBeFine()
        var path = GetPathHere(nameof(ValuesController.OkTest)); // "api/test/oktest"


This is pretty basic case we’ve been dealing with, and the code makes quite a few assumptions. This approach however seems to hold up pretty well and surely will be our starting point next time round we test MVC actions!