Serialising predicates over wcf

I have a WCF service that exposes a bunch of methods that return business objects. Under its hood it has a nice repository layer that uses interface methods like this:

IEnumerable<User> GetUsers(Func<User, bool> predicate);

So if I want all users within a given role I can do:

var plebs = GetUsers(u => u.Roles.Contains(plebRole));

Now I want to expose this any-filter-can-be-satisfied thinking over the WCF interface. The WCF api needs to be accessible to non .Net clients so I want to use (relatively) simple types.

I have a Filter object that holds a property name and and value:

[DataContract] public class Filter {
    [DataMember] public string Property { get; set; }
    [DataMember] public string Value { get; set; }

So now I can expose a WCF method like this:

IEnumerable<User> GetUsers(IEnumerable<Filter> filters);

Then I can build predicates based on what comes in, in the clients filters. Now it gets messy:

private static Expression<Func<T, bool>> GetPredicate<T>(Filter filter)
  var knownPredicates = GetKnownPredicates<T>(filter.Value);
  var t = typeof(T);
  return knownPredicates.ContainsKey(t) && knownPredicates[t].ContainsKey(filter.Property)
           ? knownPredicates[t][filter.Property]
           : True<T>();

private static Dictionary<Type, Dictionary<string, Expression<Func<T, bool>>>> GetKnownPredicates<T>(string value)
  // ReSharper disable PossibleNullReferenceException
  return new Dictionary<Type, Dictionary<string, Expression<Func<T, bool>>>>
      typeof (User), new Dictionary<string, Expression<Func<T, bool>>>
        { "Forename", x => (x as User).Forename == value },
        { "IsAdult", x => (x as User).IsAdult.ToString() == value },
      typeof (Group), new Dictionary<string, Expression<Func<T, bool>>>
        { "Name", x => (x as Group).Name == value },
  // ReSharper restore PossibleNullReferenceException

Until I started writing the GetKnownPredicates method, the code didn't really stink. Now it does. How do I fix it?


If you want to be super-fancy you could use the System.Linq.Expressions.Expression class to dynamically build a predicate based on the passed-in filter. You know the type that you're going to search, so all you need to do is to create a property expression using Filter.Property and then a constant expression with Filter.Value. Use them to compose an equal expression and you're near the finish line.

Getting used to composing expressions can be a pain, but the debugger is really helpful and will show you the code for your expression as you compose it, so dive in and try it out!

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