Moq-ing around existing instance

We love unit testing! Seriously, it makes sense if you consider how many times a simple test had saved us from having to revisit that long forgotten project we’ve already moved on from. Not fun and not good for business.

Moq: our tool of choice

To be able to only test the code we want, we need to isolate it. Of course, there’s heaps libraries for that already. Moq is just one of them. It allows of to create objects based on given interfaces and set up the expected behaviour in a way that we can abstract away all code we don’t currently test. Extremely powerful tool

Sometimes you just need a bit more of that

Suppose we’re testing a object that depends on internal state that’s tricky to abstract away. We’d however like to use Mock to replace one operation without changing the others:

public class A
    public string Test {get;set;}
    public virtual string ReturnTest() => Test;
//and some code below:
void Main()
    var config = new A() {
        Test = "TEST"
    } ;

    var mockedConfig = new Mock<A>(); // first we run a stock standard mock
    mockedConfig.CallBase = true; // we will enable CallBase just to point out that it makes no difference  
    var o = mockedConfig.Object;
    Console.WriteLine(o.ReturnTest()); // this will be null because Test has not been initialised from constructor
    mockedConfig.Setup(c => c.ReturnTest()).Returns("mocked"); // of course if you set up your mocks - you will get the value
    Console.WriteLine(o.ReturnTest()); // this will be "mocked" now, no surprises

The code above illustrates the problem quite nicely. You’ll know if this is your case when you see it.

General sentiment towards these problems

“It can’t be done, use something else”, they say. Some people on StackOverflow suggest to ditch Moq completely and go for it’s underlying technology Castle DynamicProxy. And it is a valid idea – create a proxy class around yours and intercept calls to the method under test. Easy!

Kinda easy

One advantage or Moq (which is by the way built on top of Castle DynamicProxy) is that it’s not just creating mock objects, but also tracks invocations and allows us to verify those later. Of course, we could opt to write the reuiqred bits ourselves, but why reinvent the wheel and introduce so much code that no one will maintain?

How about we mix and match?

We know that Moq internally leverages Castle DynamicProxy and it actually allows us to generate proxies for instances (they call it Class proxy with target). Therefore the question is – how do we get Moq to make one for us. It seems there’s no such option out of the box, and simply injecting the override didn’t quite go well as there’s not much inversion of control inside the library and most of the types and properties are marked as internal, making inheritance virtually impossible.

Castle Proxy is however much more user firendly and has quite a few methods exposed and available for overriding. So let us define a ProxyGenerator class that would take the method Moq calls and add required functionality to it (just compare CreateClassProxyWithTarget and CreateClassProxy implementations – they are almost identical!)


class MyProxyGenerator : ProxyGenerator
    object _target;

    public MyProxyGenerator(object target) {
        _target = target; // this is the missing piece, we'll have to pass it on to Castle proxy
    // this method is 90% taken from the library source. I only had to tweak two lines (see below)
    public override object CreateClassProxy(Type classToProxy, Type[] additionalInterfacesToProxy, ProxyGenerationOptions options, object[] constructorArguments, params IInterceptor[] interceptors)
        if (classToProxy == null)
            throw new ArgumentNullException("classToProxy");
        if (options == null)
            throw new ArgumentNullException("options");
        if (!classToProxy.GetTypeInfo().IsClass)
            throw new ArgumentException("'classToProxy' must be a class", "classToProxy");
        CheckNotGenericTypeDefinition(classToProxy, "classToProxy");
        CheckNotGenericTypeDefinitions(additionalInterfacesToProxy, "additionalInterfacesToProxy");
        Type proxyType = CreateClassProxyTypeWithTarget(classToProxy, additionalInterfacesToProxy, options); // these really are the two lines that matter
        List<object> list =  BuildArgumentListForClassProxyWithTarget(_target, options, interceptors);       // these really are the two lines that matter
        if (constructorArguments != null && constructorArguments.Length != 0)
        return CreateClassProxyInstance(proxyType, list, classToProxy, constructorArguments);

if all of the above was relativaly straightforward, actually feeding it into Moq is going to be somewhat of a hack. As I mentioned, most of the structures are marked internal so we’ll have to use reflection to get through:


public class MyMock<T> : Mock<T>, IDisposable where T : class
    void PopulateFactoryReferences()
        // Moq tries ridiculously hard to protect their internal structures - pretty much every class that could be of interest to us is marked internal
        // All below code is basically serving one simple purpose = to swap a `ProxyGenerator` field on the `ProxyFactory.Instance` singleton
        // all types are internal so reflection it is
        // I will invite you to make this a bit cleaner by obtaining the `_generatorFieldInfo` value once and caching it for later
        var moqAssembly = Assembly.Load(nameof(Moq));
        var proxyFactoryType = moqAssembly.GetType("Moq.ProxyFactory");
        var castleProxyFactoryType = moqAssembly.GetType("Moq.CastleProxyFactory");     
        var proxyFactoryInstanceProperty = proxyFactoryType.GetProperty("Instance");
        _generatorFieldInfo = castleProxyFactoryType.GetField("generator", BindingFlags.NonPublic | BindingFlags.Instance);     
        _castleProxyFactoryInstance = proxyFactoryInstanceProperty.GetValue(null);
        _originalProxyFactory = _generatorFieldInfo.GetValue(_castleProxyFactoryInstance);//save default value to restore it later

    public MyMock(T targetInstance) {       
        // this is where we do the trick!
        _generatorFieldInfo.SetValue(_castleProxyFactoryInstance, new MyProxyGenerator(targetInstance));

    private FieldInfo _generatorFieldInfo;
    private object _castleProxyFactoryInstance;
    private object _originalProxyFactory;

    public void Dispose()
         // you will notice I opted to implement IDisposable here. 
         // My goal is to ensure I restore the original value on Moq's internal static class property in case you will want to mix up this class with stock standard implementation
         // there are probably other ways to ensure reference is restored reliably, but I'll leave that as another challenge for you to tackle
        _generatorFieldInfo.SetValue(_castleProxyFactoryInstance, _originalProxyFactory);

Then, given we’ve got the above working, the actual solution would look like so:

    var config = new A()
        Test = "TEST"
    using (var superMock = new MyMock<A>(config)) // now we can pass instances!
        superMock.CallBase = true; // you still need this, because as far as Moq is oncerned it passes control over to CastleDynamicProxy   
        var o1 = superMock.Object;
        Console.WriteLine(o1.ReturnTest()); // but this should return TEST

Hacking custom Start Action in Visual Studio 2017

As part of our technical debt collection commitment we do deal with weird situations where simply running a solution in Visual Studio might not be enough. A recent example to that was a web application split into two projects:

  • ASP.NET MVC – provided a shell for SPA as well as all backend integration – the entry point for application
  • React with Webpack – SPA mostly responsible for the UI and user intercations. Project was declared as Class Library with no code to run, while developers managed all javascript and Webpack off-band

Both projects are reasonably complex and I can see why the developers tried to split them. The best intention however makes running this tandem a bit awkward execrise. We run  npm run build  and expect it to drop files into correct places for MVC project that just relies on compiled javascript to be there. Using both technology stacks in one solution is nothing new. We’d have opted for this template back at project inception. Now was too late however.

Just make VS run both projects

Indeed, Microsoft has been generous enough to let us designate multiple projects as start up:

This however does not help our particular case as React project was created as Class Library and there’s no code to run. We need a way to kick that npm run build command line every time VS ‘builds’ the React project… How do we do it?

Okay, let’s use custom Start Action

Bingo! We can absolutely do this and bootstrap us a shell which then would run our npm command. Technically we can run npm directly, but I could never quite remember where to look for the executable.

There’s a slight issue with this approach though: it is not portable between developers’ machines. There are at least two reasons for that:

  • Input boxes on this dialog form do not support environment variables and/or relative paths.
  • Changes made in ths window go to .csproj.user file, that by default is .gitignore’d (here’s a good explanation why it should be)

So this does not work:

There might be a way however

  1. First and foremost, unload the solution (not just project). Project .user settings are loaded on solution start so we want it to be clean.
  2. Open up .user file in your favourite text editor, mine looks like this:

/c start /min npm run build

And change the path to whatever your requirements are:

/c start /min npm run build

We could potentially stop here, but the file is still user-specific and is not going into source control.

Merging all the way up to project file

As it turns out, we can just cut the elements we’re after (StartAction, StartProgram and StartArguments) and paste them into respective .csproj section (look out for the same Condition on PropertyGroup, that should be it)


/c start /min npm run build

Open the solution again and check if everything works as intended.

Another take on mocking away HttpContext.Current

Static classes and methods are a pain to unit test

Back in days of WebForms, HttpContext was the one object to rule them all. It’s got session state, request and response, cache, errors, server variables and so much more for developers to inspect and play with. HttpContext.Current was by far the easiest way to tap into all of this. But guess what? Static member invocation does not make mocking it out easy.

MVC controllers are much more unit-test friendly

Although technically HttpContext hasn’t gone anywhere with the coming of MVC, it’s been neatly wrapped into a HttpContextWrapper and exposed as controller instance .Context property. Just mock it out and everything will be fine.

End of story? Well may be

If you wanted to completely abstract from all HTTP specifics and happen to not need and utility methods that come with it – you’re sweet.
If, however, for some reason you feel like relying on some utility methods to reduce amount of non-productive mocking, try this trick:

public class MockHttpContext: IDisposable {
  public HttpContext Current {
  private AppDomain CurrentDomain {

  public MockHttpContext(string url, string query = null, IPrincipal principal = null) {
    CurrentDomain = Thread.GetDomain();
    var path = CurrentDomain.BaseDirectory;
    var virtualDir = "/";

    CurrentDomain.SetData(".appDomain", "*");
    CurrentDomain.SetData(".appPath", path);
    CurrentDomain.SetData(".appId", "testId");
    CurrentDomain.SetData(".appVPath", virtualDir);
    CurrentDomain.SetData(".hostingVirtualPath", virtualDir);
    CurrentDomain.SetData(".hostingInstallDir", HttpRuntime.AspInstallDirectory);
    CurrentDomain.SetData(".domainId", CurrentDomain.Id);

    // User is logged out by default
    principal = principal ?? new GenericPrincipal(
      new GenericIdentity(string.Empty),
      new string[0]

    Current = new HttpContext(
      new HttpRequest("", url, query),
      new HttpResponse(new StringWriter())
    ) {
      User = principal
    HttpContext.Current = Current;

  public void Dispose() {
    //clean up
    HttpContext.Current = null;

First it looks very similar to this implementation taken from SO (well, this is where it’s been taken off to begin with). But then what’s up with all these CurrentDomain.SetData calls? This allows us to mock paths and transition between relative and absolute urls as if we were hosted somewhere.
Consider the code:

public static string ToAbsoluteUrl(this string relativeUrl) {
  if (string.IsNullOrEmpty(relativeUrl)) return relativeUrl;
  if (HttpContext.Current == null) return relativeUrl;

  if (relativeUrl.StartsWith("/")) relativeUrl = relativeUrl.Insert(0, "~");
  if (!relativeUrl.StartsWith("~/")) relativeUrl = relativeUrl.Insert(0, "~/");

  var url = HttpContext.Current.Request.Url;
  var port = !url.IsDefaultPort ? ":" + url.Port : string.Empty;

  return $ "{url.Scheme}://{url.Host}{port}{VirtualPathUtility.ToAbsolute(relativeUrl)}"; // and this is where the magic happens. Now static invocation of VirtualPathUtility does not fail with NullReferenceException anymore!

The code outside makes afew assumptions regarding the environment being mocked, but it should be a trivial task to introduce more parameters/settings and mock everything away.