c# - Serialising predicates over wcf -

i have wcf service exposes bunch of methods return business objects. under hood has nice repository layer uses interface methods this:

ienumerable<user> getusers(func<user, bool> predicate); 

so if want users within given role can do:

var plebs = getusers(u => u.roles.contains(plebrole)); 

now want expose any-filter-can-be-satisfied thinking on wcf interface. wcf api needs accessible non .net clients want use (relatively) simple types.

i have filter object holds property name , and value:

[datacontract] public class filter {     [datamember] public string property { get; set; }     [datamember] public string value { get; set; } } 

so can expose wcf method this:

ienumerable<user> getusers(ienumerable<filter> filters); 

then can build predicates based on comes in, in clients filters. 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 user).forename == value },         { "isadult", x => (x user).isadult.tostring() == value },         ...       }     },     {       typeof (group), new dictionary<string, expression<func<t, bool>>>       {         { "name", x => (x group).name == value },         ...       }     },     ...   };   // resharper restore possiblenullreferenceexception } 

until started writing getknownpredicates method, code didn't stink. does. how fix it?

if want super-fancy use system.linq.expressions.expression class dynamically build predicate based on passed-in filter. know type you're going search, need create property expression using filter.property , constant expression filter.value. use them compose equal expression , you're near finish line.

getting used composing expressions can pain, debugger helpful , show code expression compose it, dive in , try out!