How to implement a rule engine in C#


Recently there was an interesting question on StackOverflow about creating a rule engine in C#.

Say we have a collection of Users (with Name, Age, ...) and rule definitions like this:

static List<Rule> rules = new List<Rule> {
     new Rule ("Age""GreaterThan""20"),
     new Rule ( "Name""Equal""John"),
   new Rule ( "Tags""Contains""C#" )
};

and we want to be able to evaluate the rules:

// Returns true if User satisfies given rule (e.g. 'user.Age > 20')
bool Matches(User user, Rule rule)
{
    // how to implement this?
}
 
One  obvious solution is to use reflection.
Here we will show a solution which uses Expression trees to
compile the rules into fast executable delegates. We can then evaluate the rules as if they were normal boolean functions.

public static Func<T, bool> CompileRule<T>(Rule r)
{
    var paramUser = Expression.Parameter(typeof(User));
    Expression expr = BuildExpr<T>(r, paramUser);
    // build a lambda function User->bool and compile it
    return Expression.Lambda<Func<T, bool>>(expr, paramUser).Compile();
}
 
static Expression BuildExpr<T>(Rule r, ParameterExpression param)
{
    var left = MemberExpression.Property(param, r.MemberName);
    var tProp = typeof(T).GetProperty(r.MemberName).PropertyType;
    ExpressionType tBinary;
    // is the operator a known .NET operator?
    if (ExpressionType.TryParse(r.Operator, out tBinary))    {
        var right = Expression.Constant(Convert.ChangeType(r.TargetValue, tProp));
        // use a binary operation, e.g. 'Equal' -> 'u.Age == 15'
        return Expression.MakeBinary(tBinary, left, right);
    } else {
        var method = tProp.GetMethod(r.Operator);
        var tParam = method.GetParameters()[0].ParameterType;
        var right = Expression.Constant(Convert.ChangeType(r.TargetValue, tParam));
        // use a method call, e.g. 'Contains' -> 'u.Tags.Contains(some_tag)'
        return Expression.Call(left, method, right);
    }
}
 
Now we can implement the rule validation by compling the rules and then simply invoking them:

var rule = new Rule ("Age""GreaterThan""20");
Func<Userbool> compiledRule = CompileRule<User>(rule);
 
// true if someUser.Age > 20
bool isMatch = compiledRule(someUser);

Of course we compile all the rules just once and then use the compiled delegates:

// Compile all the rules once.
var compiledRules = rules.Select(r => CompileRule<User>(r)).ToList();
 
// Returns true if user satisfies all rules.
public bool MatchesAllRules(User user)
{
    return compiledRules.All(rule => rule(user));
}
 
Note that the “compiler” is so simple because we are using 'GreaterThan' in the rule definition, and 'GreaterThan' is a known .NET name for the operator, so the string can be directly parsed. The same goes for 'Contains' – it is a method of List. If we need custom names we can build a very simple dictionary that just translates all operators before compiling the rules:

Dictionary<stringstring> nameMap = new Dictionary<stringstring> {
"greater_than""GreaterThan" },
"hasAtLeastOne""Contains" }
};

That’s it. As an improvement we could add error messages to the rule definitions and print the error messages for the unsatisfied rules.

If you found this useful, feel free to rate the answer on StackOverflow.

Posted by Martin Konicek on 1:10 PM 64 comments