This week orders from the board of directors re-prioritised the current sprint to include a new feature. The feature would add a process which would trigger a chain of events once costs exceeded a predefined threshold.

The feature could of been completed by checking the predefined threshold once costs had been submitted. However a future piece of work will also require the checking of predefined values, we decided to write the feature in a more versatile way.

The system needed some sort of rules engine to check if there are any discounts available or if any internal costs have gone beyond a threshold. Sitting down with Macs Dickinson and Shahid Azim we started to spec out the rules engine. Initial thoughts were that it was going to be a complicated monster, having lots of different customisable rule types to be stored in a database. However with a couple of well known design patterns we were able to switch that to be a super simple rules engine. With two interfaces and an extension method we were able to create the rules engine. At the lowest level we have Conditions,

public interface ICondition
{
    public bool IsSatisfied()
}

public class IsGreaterThan : ICondition
{
    public IsGreaterThan(decimal threshold, decimal actual)
    {
        _threshold = threshold;
        _actual = actual;
    }
}

And a Rule has a list of conditions plus some setup methods, a validation method and a action method.

public interface IRule<T>
{
    void ClearConditions();
    void Initialize(T obj);
    bool IsValid();
    T Apply(T obj);
}

public class MyRule<MyClass> : IRule<MyClass>
{
    public MyRule()
    {
        Conditions = new List<ICondition>();
    }

    public void ClearConditions()
    {
        Conditions.Clear();
    }

    public bool IsValid()
    {
        return Conditions.All(x => x.IsSatisfied());
    }

    public void Initialize(MyClass obj)
    {
        Conditions.Add(new IsGreaterThan(5, obj.SomeProperty))
    }

    public MyClass Apply(MyClass obj)
    {
        obj.SomeProperty = obj.SomeProperty * 0.9; // 10% reduction
    }
  	
     public IList<ICondition> Conditions { get; set; }
}

The main rule engine is an extension method which can be applied to any class.

public static class RulesEngine
{
    public static T ApplyRule<T>(this T obj, IRule<T> rule) where T : class
    {
        rule.ClearConditions();
        rule.Initialize(obj);
        if (rule.IsValid())
        {
            rule.Apply(obj);
        }
        return obj;
    }
}

Finally to implement a rule, extend the object you want to apply the rule on.

public class Program
{
    public void Main()
    {
        var myClass = new MyClass{ SomeProptey = 6.0m };
        myClass.ApplyRule(new MyRule());
    }
}

You may have noticed the use of the Strategy Pattern. With the rules being the strategy, encapsulating the different rule logic and the RulesEngine executing the common interface to call the different rule algorithms. Using an extension method means we can apply a rule to any class.

You can check this out and an example on github.