Category: C#

Finding refactoring candidates using reflection

If some of your types are always used together, that is probably a sign that you are missing an abstraction that bundles them. For example, if I always see the types Rectangle and Color together, it’s probably a good idea to create a ColoredRectangle class that combines the two. However, these patterns tend to emerge over time, so it’s hard to actually find them manually.

Reflection can help find these relationships between types. For example, you can look at all the function/method parameter lists in your code and mark all types appearing there as ‘being used together’. Then count how often these tuples appear, and you might have a good candidate for refactoring.

Here’s how to do that in C#. First pick a few assemblies you want to analyze. One way to get them is using Assembly.GetAssembly(typeof(SomeTypeFromYourAssembly)). Then get all the methods from all the types:

IEnumerable<MethodInfo> GetParameterTypesOfAllMethods(IEnumerable<Assembly> assemblies)
{
  var flags = BindingFlags.Instance | BindingFlags.Static | BindingFlags.Public
    | BindingFlags.NonPublic | BindingFlags.DeclaredOnly;
  foreach (var assembly in assemblies)
  {
    foreach (var type in assembly.GetTypes())
    {
      foreach (var method in type.GetMethods(flags))
      {
        yield return method;
      }
    }
  }
}

The flags are important: the default will not include NonPublic and DeclaredOnly. Without those, the code will not report private methods but give you methods from base classes that we do not want here.

Now this is where things become a little more muddy, and specific to your application. I am skipping generated methods with “IsSpecialName”, and then I’m only looking at non-generic class parameters:

foreach (var method in GetParameterTypesOfAllMethods(assemblies))
{
  if (method.IsSpecialName)
    continue;

  var parameterList = method.GetParameters();

  var candidates = parameterList
      .Select(x => x.ParameterType)
      .Where(x => !x.IsGenericParameter)
      .Where(x => x.IsClass);

  /* more processing here */
}

Then I convert the types to a string using ToString() to get a nice identifier that includes filled generic parameters. I sort and join the type ids to get a key for my tuple and count the number of appearances in a Dictionary<string, int>:

var candidateNames = candidates
    .Select(x => x.ToString())
    .OrderBy(x => x)
    .ToList();

if (candidateNames.Count <= 1)
  continue;

if (candidateNames.Any(string.IsNullOrWhiteSpace))
  continue;

var key = string.Join(",", candidateNames);

if (!lookup.ContainsKey(key))
{
  lookup.Add(key, 1);
}
else
{
  lookup[key]++;
}

Once that is done, you can sort the resulting lookup, print out all the tuples, and see if there are any good candidates.

There’s much room for improvement with a method like this. For example, skipping non-class types is a pretty arbitrary choice. And you will not find new tuples built from built-in types this way. However, because those types offer very little semantic by themselves, it can be hard to correlate multiple occurrences simply by their types.

Arrow Anti-Pattern

When you write code, it can happen that you nest some ifs or loops inside each other. Here is an example:

Because of the shape of the indentation, this code smell is called an anti-arrow pattern. The deepest indentation depth is the tip of the arrow. In my opinion, such a style is detrimental to readability and comprehension.

In the following, I would like to present a simple way of resolving such arrow anti-patterns.

Extract Method

First we extract the arrow pattern as a new method. This allows us to use return values instead of variable assignments and makes the code clearer.

public string PrintElephantMessage(Animal animal)
{
    Console.WriteLine(IsAElephant(animal));
}
public string IsAElephant(Animal animal)
{
    if (animal.IsMammal())
    {
        if (animal.IsGrey())
        {
            if (animal.IsBig())
            {
                if (animal.LivesOnLand())
                {
                    return "It is an elephant";
                }
                else
                {
                    return "It is not an elephant. Elephants live on land";
                }
            }
            else
            {
                return "It is not an elephant. Elephants are big";
            }
        }
        else
        {
            return "It is not an elephant. Elephants are grey";
        }
    }
    else
    {
        return "It is not an elephant. Elephants are mammals";
    }
}

Turn over ifs

A quick way to eliminate the arrow anti-pattern is to invert the if conditions. This will make the code look like this:

public string IsAElephant(Animal animal)
{
    if (!animal.IsMammal())
    {
        return "It is not an elephant. Elephants are mammals";
    }
    if (!animal.IsGrey())
    {
        return "It is not an elephant. Elephants are grey";
    }
    if (!animal.IsBig())
    {
        return "It is not an elephant. Elephants are big";
    }
    if (!animal.LivesOnLand())
    {
        return "It is not an elephant. Elephants live on land";
    }
    return "It is an elephant";
}

Some IDEs like Visual Studio can help flip the Ifs.

Conclusion

Arrow anti-pattern are code smells and make your code less legible. Fortunately, you can refactor the code with a few simple steps.

Cancellation Token in C#

Perhaps you know this problem from your first steps with asynchronous functions. You want to call them, but a CancellationToken is requested. But where does it come from if you do not have one at hand? At first glance, a quick way is to use an empty CancellationToken. But is that really the solution?

In this blog article, I explain what CancellationTokens are for and what the CancellationTokenSource is for.

Let us start with a thought experiment: You have an Application with different Worker threads. The Application is closed. How can you make the whole Application with all the Worker threads close?
One solution would be a global bool variable “closed”, which is set in this case. All Workers have a loop built in, which regularly checks this bool and terminates if it is set. Basically, the Application and all Workers must cooperate and react to the signal.

class Application
{
    public static bool closed = false;
    Application()
    {
        var worker= new Worker();
        worker.Run();

        Thread.Sleep(2500);
        closed = true;

        // Wait for termination of Tasks
        Thread.Sleep(2500);
    }
}
class Worker
{
    public async void Run()
    {
        bool moreToDo = true;
        while (moreToDo)
        {
            await DoSomething();
            if (Application.closed)
            {
                return;
            }
        }
    }
}

This is what the CancellationToken can be used for. The token is passed to the involved Workers and functions. Throught it, the status of whether or not it was cancelled can be queried. The token is provided via a CancellationTokenSource, which owns and manages the token.

class Application
{
    Application()
    {
        var source = new CancellationTokenSource();
        var token = source.Token;
        var worker = new Worker();
        worker.Run(token);

        Thread.Sleep(2500);
        source.Cancel();

        // Wait for cancellation of Tasks and dispose source
        Thread.Sleep(2500);
        source.Dispose();
    }
}
class Worker
{
    public async void Run(CancellationToken token)
    {
        bool moreToDo = true;
        while (moreToDo)
        {
            await DoSomething(token);
            if (token.IsCancellationRequested)
            {
                return;
            }
        }
    }   
}

The call to source.Cancel() sets the token to canceled. CancellationTokens and sources offer significantly more built-in possibilities, such as callbacks, WaitHandle, CancelAfter and many more.

So using an empty CancellationToken is not a good solution. This means that cancellations cannot be passed on and a task could continue to run in the background even though the Application should have been terminated a long time ago.

I hope I have been able to bring the topic a little closer to you. If you have already encountered such problems and found solutions, please leave a comment.

Unit-Testing Deep-Equality in C#

In the suite of redux-style applications we are building in C#, we are making extensive use of value-types, which implies that a value compares as equal exactly if all of its contents are equal also known as “deep equality”, as opposed to “reference equality” or “shallow equality”. Both of those imply deep equality, but the other way around is not true. The same object is of course equal to itself, not matter how deep you look. And an object that references the same data as another object also has equal content. But a simple object that contains different lists with equal content will be unequal under shallow comparison, but equal under deep comparison.

Though init-only records already provide a per-member comparison as Equals be default, this fails for collection types such as ImmutableList<> that, against all intuition but in accordance to , only provide reference-equality. For us, this means that we have to override Equals for any value type that contains a collection. And this is were the trouble starts. Once Equals is overridden, it’s extremely easy to forget to also adapt Equals when adding a new property. Since our redux-style machinery relies on a proper “unequal”, this would manifest in the application as a sporadically missing UI update.

So we devised a testing strategy for those types, using a little bit of reflection:

  1. Create a sample instance of the value type with no member retaining its default value
  2. Test, by going over all properties and comparing to the same property in a default instance, if indeed all members in the sample are non-default
  3. For each property, run Equals the sample instance to a modified sample instance with that property set to the value from a default instance.

If step 2 fails, it means there’s a member that’s still at its default value in the sample instance, e.g. the test wasn’t updated after a new property was added. If step 3 fails, the sample was updated, but the new property is not considered in Equals – and it can even tell which property is missing.

The same problems of course arise with GetHashCode, but are usually less severe. Forgetting to add a property just makes collisions more likely. It can be tested much in the same way, but can potentially lead to false positives: collisions can occur even if all properties are correctly considered in the function. In that case, however, the sample can usually be altered to remove the collision – and it is really unlikely. In fact, we never had a false positive.

When laziness broke my code

I was just integrating a new task-graph system for a C# machine control system when my tests started to go red. Note that the tasks I refer to are not the same as the C# Task implementation, but the broader concept. Task-graphs are well known to be DAGs, because otherwise the tasks cannot be finished. The general algorithm to execute a task-graph like this is called topological sorting, and it goes like this:

  1. Find the number of dependencies (incoming edges) for each task
  2. Find the tasks that have zero dependencies and start them
  3. For any finished tasks, decrement the follow-up tasks dependency count by one and start them if they reach zero.

The graph that was failed looked like the one below. Task A was immediately followed by a task B that was followed by a few more tasks.

I quickly figured out that the reason that the tests were failing was that node B was executed twice. Looking at the call-stack for both executions, I could see that the first time B was executed was when A was completed. This is correct as per step 3 in the algorithm. However, the second time it was started was directly from the initial Run method that does the work from step 2: Starting the initial tasks that are not being started recursively. I was definitely not calling Run twice, so how did that happen?

public void Run()
{
    var ready = tasks
        .Where(x => x.DependencyCount == 0);

    StartGroup(ready);
}

Can you see it? It is important to note that many of the tasks in this graph are asynchronous. Their completion is triggered by an IObserver, a C# Task completing or some other event. When the event is processed, StartGroup is used to start all tasks that have no more dependencies. However, A was no such task, it was synchronous, so the StartGroup({B}) call happened while Run was still on the stack.

Now what happened was that when A (instantly!) completed, it set the DependencyCount of B to 0. Since ready in the code snippet is lazily evaluated from within StartGroup, the ‘contents’ actually change while StartGroup is running.

The fix was adding a .ToList after the .Where, a unit test that checked that this specifically would not happen again, and a mental note that lazy evaluation can be deceiving.

Composition of C# iterator methods

Iterator methods in C# or one of my favorite features of that language. I do not use it all that often, but it is nice to know it is there. If you are not sure what they are, here’s a little example:

public IEnumerable<int> Iota(int from, int count)
{
  for (int offset = 0; offset < count; ++offset)
    yield return from + offset;
}

They allow you to lazily generate any sequence directly in code. For example, I like to use them when generating a list of errors on a complex input (think compiler errors). The presence of the yield contextual keyword transforms the function body into a state machine, allowing you to pause it until you need the next value.

However, this makes it a little more difficult to compose such iterator methods, and in reverse, refactor a complex iterator method into several smaller ones. It was not obvious to me right away how to do it at all in a ‘this always works’ manner, so I am sharing how I do it here. Consider this slightly more complex iterator method:

public IEnumerable<int> IotaAndBack(int from, int count)
{
  for (int offset = 0; offset < count; ++offset)
    yield return from + offset;

  for (int offset = 0; offset < count; ++offset)
    yield return from + count - offset - 1;
}

Now we want to extract both loops into their own functions. My one-size-fits-all solution is this:

public IEnumerable<int> AndBack(int from, int count)
{
  for (int offset = 0; offset < count; ++offset)
    yield return from + count - offset - 1;
}

public IEnumerable<int> IotaAndBack(int from, int count)
{
  foreach (var x in Iota(from, count))
     yield return x;

  foreach (var x in AndBack(from, count))
     yield return x;
}

As you can see, a little ‘foreach harness’ is needed to compose the parts into the outer function. Of course, in a simple case like this, the LINQ version Iota(from, count).Concat(AndBack(from, count)) also works. But that only works when the outer function is sufficiently simple.

WPF Redux Sample Application

A while ago, I wrote about how we are using the redux architexture in our C# applications. I have just pushed an example showing ReduxSimple with WPF and our extensions in a .NET 5 application to our github account. The example itself is just a counter with an increment and a decrement button, but it already shows the whole redux cycle.

The store setup in App.xaml.cs shows how the ReducerBuilder can be used to build a State reducer from the Reducer class via reflection.

I also added a small prime-number factorization to show how to use ‘expensive’ functions in the view part of the application using our SelectorGraph. This makes it possible to properly derive view data from the state, only updating them once when one of their inputs changes. In the example, that is the counter. So the number will only be factorized when the counter changes, while all other future state changes do update the selector.

The example does not use the UIDuplexBinder yet. It allows read/write binding of WPF controls to an IObservable and an action-creator, and is hopefully pretty straight-forward to use. Please enjoy!

WPF: Recipe for customizable User Controls with flexible Interactivity

The most striking feature of WPF is its peculiar understanding of flexibility. Which means, that usually you are free to do anything, anywhere, but it instantly hands back to you the responsibility to pay super-close attention of where you actually are.

As projects with grow, their user interfaces usually grow, and over time there usually appears the need to re-use any given component of that user interface.

This not only is the working of the DRY principle at the code level, Consistency is also one of the Nielsen-Norman Usability Heuristics, i.e. a good plan as to not confuse your users with needless irritations. This establishes trust. Good stuff.

Now say that you have a re-usable custom button that should

  1. Look a certain way at a given place,
  2. Show custom interactivity (handling of mouse events)
  3. Be fully integrated in the XAML workflow, especially accepting Bindings from outside, as inside an ItemsControl or other list-type Control.

As usual, this was a multi-layered problem. It took me a while to find my optimum-for-now solution, but I think I managed, so let me try to break it down a bit. Consider the basic structure:

<ItemsControl ItemsSource="{Binding ListOfChildren}">
	<ItemsControl.ItemTemplate>
		<DataTemplate>
			<Button Style="{StaticResource FancyButton}"
				Command="{Binding SomeAwesomeCommand}"
				Content="{Binding Title}"
				/>
		</DataTemplate>
	</ItemsControl.ItemTemplate>
</ItemsControl>
Quick Note about the Style

We see the Styling of FancyButton (defined in some ResourceDictionary, merged together with a lot of stuff in the App.xaml Applications.Resources), and I want to define the styling here in order to modify it in some other places i.e. this could be defined in said ResourceDictionary like

<Style TargetType="{x:Type Button}" x:Key="FancyButton"> ... </Style>

<Style TargetType="{x:Type Button}" BasedOn="{StaticResource FancyButton}" x:Key="SmallFancyButton"> ... </Style>
<Style TargetType="{x:Type Button}" BasedOn="{StaticResource FancyButton}" x:Key="FancyAlertButton"> ... </Style>
... as you wish ...
Quick Note about the Command

We also see SomeAwesomeCommand, defined in the view model of what ListOfChildren actually consists of. So, SomeAwesomeCommand is a Property of a custom ICommand-implementing class, but there’s a catch:

Commands on a Button work on the Click event. There’s no native way to assign that to different events like e.g. DragOver, so this sounds like our new User Control would need quite some Code Behind in order to wire up any Non-Click-Event with that Command. Thankfully, there is a surprisingly simple solution, called Interaction.Triggers. Apply it as

  1. installing System.Windows.Interactivity from NuGet
  2. adding the to your XAML namespaces: xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
  3. Adding the trigger inside:
<Button ...>
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="DragOver">
            <i:InvokeCommandAction Command="WhateverYouFeelLike"/>
        </i:EventTrigger>
    </i:Interaction.Triggers>
</Button>

But that only as a side note, remember that the point of having our separate User Control is still valid; considering that you would want to have some extra interactivity in your own use cases.

Now: Extracting the functionality to our own User Control

I chose not to derive some class from the Button class itself because it would couple me closer to the internal workings of Button; i.e. an application of Composition over Inheritance. So the first step looks easy: Right click in the VS Solution Explorer -> Add -> User Control (WPF) -> Create under some name (say, MightyButton) -> Move the <Button.../> there -> include the XAML namespace and place the MightyButton in our old code:

// old place
<Window ...
	xmlns:ui="clr-namespace:WhereYourMightyButtonLives
	>
	...
	<ItemsControl ItemsSource="{Binding ListOfChildren}">
		<ItemsControl.ItemTemplate>
			<DataTemplate>
				<ui:MightyButton Command="{Binding SomeAwesomeCommand}"
						 Content="{Binding Title}"
						 />
			</DataTemplate>
		</ItemsControl.ItemTemplate>
	</ItemsControl>
	...
</Window>

// MightyButton.xaml
<UserControl ...>
	<Button Style="{StaticResource FancyButton}"/>
</UserControl>

But now it get’s tricky. This could compile, but still not work because of several Binding mismatches.

I’ve written a lot already, so let me just define the main problems. I want my call to look like

<ui:DropTargetButton Style="{StaticResource FancyButton}"
		     Command="{Binding OnBranchFolderSelect}"
		     ...
		     />

But again, these are two parts. Let me clarify.

Side Quest: I want the Style to be applied from outside.

Remember the idea of having SmallFancyButton, FancyAlertButton or whatsoever? The problem is, that I can’t just pass it to <ui:MightyButton.../> as intended (see last code block), because FancyButton has its definition of TargetType="{x:Type Button}". Not TargetType="{x:Type ui:MightyButton}".

Surely I could change that. But I will regret this when I change my component again; I would always have to adjust the FancyButton definition every time (at several places) even though it always describes a Button.

So let’s keep the Style TargetType to be Button, and just treat the Style as something to be passed to the inner-lying Button.

Main Quest: Passing through Properties from the ListOfChildren members

Remember that any WPF Control inherits a lot of Properties (like Style, Margin, Height, …) from its ancestors like FrameworkElement, and you can always extend that with custom Dependency Properties. Know that Command actually is not one of these inherited Properties – it only exists for several UI Elements like the Button, but not in a general sense, so we can easily extend this.

Go to the Code Behind, and at some suitable place make a new Dependency Property. There is a Visual Studio shorthand of writing “propdp” and pressing Tab twice. Then adjust it to read like

public ICommand Command
        {
            get { return (ICommand)GetValue(CommandProperty); }
            set { SetValue(CommandProperty, value); }
        }

        public static readonly DependencyProperty CommandProperty =
            DependencyProperty.Register("Command", typeof(ICommand), typeof(DropTargetButton), new PropertyMetadata(null));

With Style, we have one of these inherited Properties. Nevertheless, I want my Property to be called Style, which is quite straightforward by just employing the new keyword (i.e. we really want to shadow the inherited property, which is tolerable because we already know our FancyButton Style to its full extent.)

public new Style Style
        {
            get { return (Style)GetValue(StyleProperty); }
            set { SetValue(StyleProperty, value); }
        }

        public static readonly new DependencyProperty StyleProperty =
            DependencyProperty.Register("Style", typeof(Style), typeof(DropTargetButton), new PropertyMetadata(null));

And then we’re nearly there, we just have to make the Button inside know where to take these Properties. In an easy setting, this could be accomplished by making the UserControl constructor set DataContext = this; but STOP!

If you do that, you lose easy access to the outer ItemsControl elements. Sure you could work around – remember the WPF philosophy of allowing you many ways – but more practicable imo is to have an ElementName. Let’s be boring and take “Root”.

<UserControl x:Class="ComplianceManagementTool.UI.DropTargetButton"
	     ...
             xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"
             xmlns:local="clr-namespace:ComplianceManagementTool.UI"
             x:Name="Root"
             >
    <Button Style="{Binding Style, ElementName=Root}"
            AllowDrop="True"
            Command="{Binding Command, ElementName=Root}"
            Content="{Binding Text, ElementName=Root}"
            >
        <i:Interaction.Triggers>
            <i:EventTrigger EventName="DragOver">
                <i:InvokeCommandAction Command="{Binding Command, ElementName=Root}"/>
            </i:EventTrigger>
        </i:Interaction.Triggers>
    </Button>
</UserControl>

As some homework, I’ve left you the Content property to add as a Dependency Property, as well. You could go ahead and add as many DPs to your User Control, and inside that Control (which is quite maiden-like still, if we ignore all that DP boilerplate code) you could have as many complex interactivity as you would require, without losing the flexibility of passing the corresponding Commands from the outside.

Of course, this is just one way of about seventeen plusminus thirtythree, add one or two, which is about the usual number of WPF ways of doing things. Nevertheless, this solution now lives in our blog, and maybe it is of some help to you. Or Future-Me.

Using a C++ service from C# with delegates and PInvoke

Imagine you want to use a C++ service from contained in a .dll file from a C# host application. I was using a C++ service performing some hardware orchestration from a C# WPF application for the UI. This service pushes back events to the UI in undetermined intervals. Let’s write a small C++ service like that real quick:

#include <thread>
#include <string>

using StringAction = void(__stdcall*)(char const*);

void Report(StringAction onMessage)
{
  for (int i = 0; i < 10; ++i)
  {
    onMessage(std::to_string(i).c_str());
    std::this_thread::sleep_for(std::chrono::seconds(1));
  }
}

static std::thread thread;

extern "C"
{
  __declspec(dllexport) void __stdcall Start(StringAction onMessage)
  {
    thread = std::thread([onMessage] {Report(onMessage);});
  }

  __declspec(dllexport) void __stdcall Join()
  {
    thread.join();
  }
}

Compile & link this as a .dll that we’ll call Library.dll for now. Catchy, no?

Now we write a small helper class in C# to access our nice service:

class LibraryLoader
{
  public delegate void StringAction(string message);

  [DllImport("Library.dll", CallingConvention = CallingConvention.StdCall)]
  private static extern void Start(StringAction onMessage);

  [DllImport("Library.dll", CallingConvention = CallingConvention.StdCall)]
  public static extern void Join();

  public static void StartWithAction(Action<string> action)
  {
    Start(x => action(x));
  }
}

Now we can use our service from C#:

LibraryLoader.StartWithAction(x => Console.WriteLine(x));
// Do other things while we wait for the service to do its thing...
LibraryLoader.Join();

If this does not work for you, make sure the C# application can find the C++ Library.dll, as VS does not help you with this. The easiest way to do this, is to copy the dll into the same folder as the C# application files. When you’re starting from VS 2019, that is likely something like bin\Debug\net5.0. You could also adapt the PATH environment variable to include the target directory of your Library.dll.

If you’re getting a BadImageFormatException, make sure the C# application is compiled for the same Platform target as the C++ application. By default, VS builds C++ for “x86”, while it builds C# projects for “Any CPU”. You can change this to x86 in the project settings under Build/Platform target.

Now if this is all you’re doing, the application will probably work fine and report its mysterious number sequence flawlessly. But if you do other things, e.g. something that triggers garbage collection, like this:

LibraryLoader.StartWithAction(x => Console.WriteLine(x));
Thread.Sleep(2000);
GC.Collect();
LibraryLoader.Join();

The application will crash with a very ominous ExecutionEngineException after 2 seconds. In a more realistic environment, e.g. my WPF application, this happened seemingly at random.

Now why is this? The Action<string> we registered to print to the console gets garbage collected, because there is nothing in the managed environment keeping it alive. It exists only as a dependency to the function pointer in C++ land. When C++ wants to message something, it calls into nirvana. Not good. So let’s just store it, to keep it alive:

static StringAction messageDelegate;
public static void StartWithAction(Action<string> action)
{
  messageDelegate = x => action(x);
  Start(messageDelegate);
}

Now the delegate is kept alive in the static variable, thereby matching the lifetime of the C++ equivalent, and the crash is gone. And there you have it, long-lasting callbacks from C++ to C#.

A very strange bug

A week ago, one of our junior programmers encountered a strange bug in his WPF application. This particular application has a main window with pages, i.e. views, that can be switched between, e.g. via the main menu. The first page, however, is the login page. And while on the login page, the main menu should be disabled, so users cannot go where they are not authorized to go.

And this worked fine. A simple boolean in the main window’s view-model was used to disable the menu when in on login page, and enable it otherwise. We have a couple of applications that behave this way, and there were enough examples to get this to work.

Now the programmer introduced a new feature: when the application is started for the first time, there should be a configuration page right after the login page. During the configuration, the main menu should still be disabled. When the user hits the save button on the configuration page, the configuration should be stored and they should get to the dashboard with an enabled main menu.

New Feature, new Bug

Of course, this required changing the condition for when the main menu is disabled: When on either of the two pages, keep it disabled. But now the very strange bug appeared. When going to the dashboard from the configuration page, the main menu was correctly enabled, but all of its menu entries were still disabled. And this only happened when opening the main menu for the first time. When closing and opening it again, all menu entries were correctly enabled.

Now a lot of hands-on debugging ensued. The junior developer used all of the tools at his disposal: web searching, debug output, consulting other senior developers. The leads were plenty, too. Could it be a broken INotifyPropertyChanged implementation? Was ICommand.CanExecute not returning the correct value? Can we attach our own CanExecute handlers to the associated CommandBindings to at least get around the issue? Do we manually have to trigger a refresh of the enabled state?

Nothing worked, and no new information was gained. Even after fiddling around with the problem for a few days, there was no solution, no new insight to be found, not even a workaround. All our code seemed to be working alright.

From good to bad

One of my debugging mantras, that always helped me with the nastiest of bugs, is:

Work from a good, bug-free scenario to the bad, buggy scenario. Use small increments and bisection to find the step that breaks it.

In this situation, we were lucky. We had a good, working scenario in the same application. Starting the application without the “first time configuration” was working nicely. So what was the difference? From the login page the user also hit a button to change to the dashboard page.

The only difference was: the configuration was not stored in between. So we commented that out. Finally! Progress! We could not believe it. Commenting out the “store the configuration” code made our menu items work. Time to dig deeper: The store-the-configuration code was using a helper dialog called TaskDialog that awaits a given Task while showing an “in progress” animation. Our industrious junior developer thought that might be a good idea for storing the configuration data using File.WriteAllTextAsync. Further bisection revealed that it was not actually the “save” Task that was causing the problem, but our TaskDialog: Removing the await from the TaskDialog, our MainWindow‘s main menu was still broken.

This was surprising since the TaskDialog had been in-production, seemingly working alright for quite some time. Yet all our clues hinted at it being the culprit. In its implementation, it runs the given Task directly in its async “Loaded” event handler. Once it is done, it sets the DialogResult to true.

So we hypothesized that it is probably not a good idea to close the dialog while it is currently in the process of opening. The configuration saving task was probably very fast and never yielding, so only that was showing the strange behavior, while all our previous use cases were “slow enough” and yielded at least once.

Hence we tried a small modification: We delayed the execution of our Task and the subsequent DialogResult = true; slightly to the next “event frame” using Application.Current.Dispatcher.InvokeAsync. And that did the trick! The main menu items were finally correctly enabled after leaving the configuration page.

And this is how we solved this very weird bug, where the trigger does not appear to relate to the symptom at all. There is probably still a bug causing this weird behavior somewhere in WPF, but at least we are not longer triggering it with our TaskDialog. Remember, start from the good case, iterate and bisect!