Question

This is a simplified version of the original problem.

I have a class called Person:

public class Person {
  public string Name { get; set; }
  public int Age { get; set; }
  public int Weight { get; set; }
  public DateTime FavouriteDay { get; set; }
}

…and lets say an instance:

var bob = new Person {
  Name = "Bob",
  Age = 30,
  Weight = 213,
  FavouriteDay = '1/1/2000'
}

I would like to write the following as a string in my favourite text editor….

(Person.Age > 3 AND Person.Weight > 50) OR Person.Age < 3

I would like to take this string and my object instance and evaluate a TRUE or FALSE – i.e. evaluating a Func<Person, bool> on the object instance.

Here are my current thoughts:

1. Implement a basic grammar in ANTLR to support basic Comparison and Logical Operators. I am thinking of copying the Visual Basic precedence and some of the featureset here: http://msdn.microsoft.com/en-us/library/fw84t893(VS.80).aspx
2. Have ANTLR create a suitable AST from a provided string.
3. Walk the AST and use the Predicate Builder framework to dynamically create the Func<Person, bool>
4. Evaluate the predicate against an instance of Person as required

My question is have I totally overbaked this? any alternatives?

---

EDIT: Chosen Solution

I decided to use the Dynamic Linq Library, specifically the Dynamic Query class provided in the LINQSamples.

Code below:

using System;
using System.Linq.Expressions;
using System.Linq.Dynamic;

namespace ExpressionParser
{
  class Program
  {
    public class Person
    {
      public string Name { get; set; }
      public int Age { get; set; }
      public int Weight { get; set; }
      public DateTime FavouriteDay { get; set; }
    }

    static void Main()
    {
      const string exp = @"(Person.Age > 3 AND Person.Weight > 50) OR Person.Age < 3";
      var p = Expression.Parameter(typeof(Person), "Person");
      var e = System.Linq.Dynamic.DynamicExpression.ParseLambda(new[] { p }, null, exp);
      var bob = new Person
      {
        Name = "Bob",
        Age = 30,
        Weight = 213,
        FavouriteDay = new DateTime(2000,1,1)
      };

      var result = e.Compile().DynamicInvoke(bob);
      Console.WriteLine(result);
      Console.ReadKey();
    }
  }
}

Result is of type System.Boolean, and in this instance is TRUE.

Many thanks to Marc Gravell.

Include System.Linq.Dynamic nuget package, documentation here

Answer 1

Would the dynamic linq library help here? In particular, I’m thinking as a Where clause. If necessary, put it inside a list/array just to call .Where(string) on it! i.e.

var people = new List<Person> { person };
int match = people.Where(filter).Any();

If not, writing a parser (using Expression under the hood) isn’t hugely taxing – I wrote one similar (although I don’t think I have the source) in my train commute just before xmas…