Task From A Code Test

Earlier this week I took a coding test and the first task in the test was interesting and alittle fun to complete. At the start you are given a text file with integers (one per line) that are used as an input into another process. This process has been failing because of problems with the input file. You are tasked with writing a program to validate the input files and output any errors to a separate file for use in troubleshooting. The task is to validate the file according to a set of rules.

The rules are as follows:
1.Verify that all integers in the file fall between 1 and N where N is the total number of lines in the text file.
2.Verify that all integers between 1 and N exist in the file (So if the file contains 3 lines, they should be 1, 2, and 3 in any order).
3.Output any errors to another file.

Rather than use a bunch of loops directly to iterate through the lines, I opted for a LINQ approach. Having done LINQ for so long, that approach usually looks better to me than writing the loops directly. The code is on GitHub and is available by clicking here.

Rebooting & Blogging

I had originally created this blog years ago and tried to get into blogging. I got a few posts up but then kind of lost interest when life started happening. So when I was recently and unexpectedly given an opportunity to “reboot” my career, I remembered some .NET Rocks! podcasts from the last year that addressed taking your carreer to the next level (Working on your Career with John Sonmez and Managing Your Career with John Sonmez). I decided to go check out his website SimpleProgrammer.com to see what I could find that might help me. One of the things I found was his online 3 week blogging course. I wasn’t terribly motivated to blog, but I know blogging would help me and I decided to give it a shot. There are no silver bullets with this, but the course gives you a path forward to building or re-building your blog as the case may be. It turns out that is exactly what I needed to get going.

If you are looking to start a blog or just find something to help jumpstart your blog, I would recommend checking out John Sonmez’s Blog Course.

Registering Types With the SimpleIoC Container

I’ve been kicking the tires on the Command Query Seperation principle/pattern lately. I’ve been using the CQS-Sample as a starting point. So far, the approach to breaking things down into Commands and Queries is pretty slick and much easier to unit test. The interfaces are used for dependency injection with an IoC container like SimpleIoC (which comes with MVVMLight). If you have done any work with IoC containers, you’ll know that you need to register your types with the container either explicitly or through some kind of convention. With a few classes it’s easy to just bang out the few lines of code you need to do it. In this case, the number of commands and queries is growing and having to explicitly register each one is becoming monotonous. So how to I register my queries and commands without having to write any code to explicitly register my types?

Assumptions:

  1. We are working with MVVMLight and SimpleIoC.
  2. For any query or command processor interface there is exactly 1 implementation of it.
  3. All queries are marked with an IQuery interface.
  4. All commands are marked with an ICommand interface (not to be confused with the System.Windows.Input.ICommand).
  5. All command processors implement ICommandProcessor<TCommand> where TCommand is a class that inherits from ICommand.

Step 1: Get an instance of MethodInfo for SimpleIoC’s Register<TInterface, TClass>() method.

Normally, getting the MethodInfo for a method you’d like to invoke is more straight forward. This method has 8 overloads so some LINQ was needed to get the correct one.

var registerMethodInfo = SimpleIoc.Default.GetType().GetMethods()
.Where(m => m.Name == "Register")
.Select(m => new
{
Method = m,
Params = m.GetParameters(),
Args = m.GetGenericArguments()
})
.Where(x => x.Params.Length == 0)
.Where(x => x.Args.Length == 2)
.Select(x => x.Method)
.First();

Step 2: Get a list of all types in the current AppDomain.

This step is pretty straightforward. Notice the .ToList() call at the end of method chain. This is to make sure that when we use types in the following steps, we won’t be enumerating the IEnumerable of types more than once.

var types = AppDomain.CurrentDomain.GetAssemblies().SelectMany(a => a.GetTypes()).ToList();

Step 3: Get all query interfaces that inherit from IQuery.

var queryInterfaces = from t in types
where t != typeof (IQuery)
where t.IsInterface
where typeof (IQuery).IsAssignableFrom(t)
select t;

Step 4: Create a query implementation class to query interface mapping

This step creates the query interface to query type mapping. The result is projected into an anonymous type that will be easy to use in the next step.

var queryMappings = from queryInterface in queryInterfaces
from t in types
where t.GetInterface(queryInterface.Name) == queryInterface
select new {queryInterface, Query = t};

Step 5: Register the query types

This is the last bit of reflection needed to register the queries. This code iterates of the query mappings from the previous step, gets a generic MethodInfo using the interface and implementation, and then invokes it on the current SimpleIoC instance.

foreach (var queryMapping in queryMappings)
{
var genericRegisterMethodInfo = registerMethodInfo.MakeGenericMethod(queryMapping.queryInterface, queryMapping.Query);
genericRegisterMethodInfo.Invoke(SimpleIoc.Default, null);
}

Step 6: Get a list of classes that inherit from ICommand.

var commands = from t in types
where t != typeof (ICommand)
where typeof (ICommand).IsAssignableFrom(t)
select t;

Step 7: Make the ICommandProcessor<T> interfaces.

This is an interesting step in the process. The command processors do not have a common non-generic interface, so we’ll have to make the generic type of ICommandProcessor<TCommand>.

 

var commandProcessorInterfaces = from command in commands
select new {CommandProcessorInterface = typeof (ICommandProcessor<>).MakeGenericType(command)};

Step 8: Map ICommandProcessor<T> interfaces to ICommandProcessor<T> implementations

Now we’ll create the mapping between the ICommandProcessor<TCommand>s and their implementations.

var commandProcessorMappings = from commandProcessorInterface in commandProcessorInterfaces
from t in types
where t.GetInterface(commandProcessorInterface.CommandProcessorInterface.Name) == commandProcessorInterface.CommandProcessorInterface
select new {commandProcessorInterface.CommandProcessorInterface, CommandProcessor = t};

Step 9: Register the command processor types

Same thing as step 5. Just iterating over the command mappings to make the generic MethodInfos and then invoking them on the current SimpleIoC instance.

foreach (var commandProcessorMapping in commandProcessorMappings)
{
var genericRegisterMethodInfo = registerMethodInfo.MakeGenericMethod(commandProcessorMapping.CommandProcessorInterface, commandProcessorMapping.CommandProcessor);
genericRegisterMethodInfo.Invoke(SimpleIoc.Default, null);
}

And that’s how I register all my command processors and queries through reflection. LINQ and Reflection FTW!

Are there better approaches? Sure. Are there better IoC containers out there? You betcha. But, this approach is Good Enough™ for now.

How to Detect Concurrent Events Using LINQ and Reactive Extensions

Here’s the scenario…

I have 3 referees around a powerlifting platform. Each has a set of 4 switches in their hand that is used to indicate if they thought a lift was good or not. One switch indicates the lift was good and the other 3 indicate the lift was no good and the general reason why. A referee’s light box displays the result by turning lights on and off. The individual lights should only turn on when all 3 referees have pushed their buttons. How do I determine when all 3 referees have pushed their buttons?

To get started, we need to create a simple event to indicate a referees decision.
public class RefereeDecisionEvent
{
public DateTimeOffset Timestamp { get; set; }
public DecisionCode Decision { get; set; }
}

The Decision property will hold a referee’s decision represented as a DecisionCode enumeration.

public enum DecisionCode
{
White = 0,
Red = 1,
Blue = 2,
Yellow = 4
}

White indicates the referee decided it was a good lift. Red, Blue, and Yellow represent potential reasons why the referee decided the lift was no good.

We’ll also need 3 event sources. Each event source represents a different referee; left, center, right. The actual implementation of the sources is outside the scope of this blog post. That said, I’ll just generate some events for each source at different intervals to give us some simple test data.

Step 1 is to join the left source’s events with the right source’s events. Since the left and right event sources create new events at different intervals, the events will most likely not occur at exactly the same time. We can fix this by increasing the duration of both sequences of events by 3 seconds. Now, we are joining the events from the left and the right sources when they occur within 3 seconds of each other.

var step1 = leftSource.Join(rightSource,
l => Observable.Interval(TimeSpan.FromSeconds(3)),
r => Observable.Interval(TimeSpan.FromSeconds(3)),
(l, r) => new {Left = l, Right = r});

Step 2 is to join the center source’s events with the now joined left and right side events. We’ll extend the durations of the events 3 seconds so that we can get a combined event that shows the left, center, and right events that all occurred withing 3 seconds of each other.

var step2 = centerSource.Join(step1,
l => Observable.Interval(TimeSpan.FromSeconds(3)),
r => Observable.Interval(TimeSpan.FromSeconds(3)),
(l, r) => new {r.Left, Center = l, r.Right});

In Step 3, we’ll project the results of step 2 into an event that is better laid out for our purposes. That Observable can then be subscribed to by an Observer that handles turning the lights on and off.

var step3 = from e in step2
select new
{
e.Left,
e.Center,
e.Right,
Status = new[] {e.Left.Decision, e.Center.Decision, e.Right.Decision}.Count(x => x == DecisionCode.White) >= 2 ? "Good" : "Bad"
};

And that’s how it’s done.