How do you add dynamic 'where' clauses to a linq query?

Question

I've got a User table with a bitmask that contains the user's roles. The linq query below returns all the users whose roles include 1, 4 or 16.

var users = from u in dc.Users
            where ((u.UserRolesBitmask & 1) == 1)
               || ((u.UserRolesBitmask & 4) == 4)
               || ((u.UserRolesBitmask & 16) == 16)
            select u;

I'd like to rewrite this into the method below to returns all the users from the given roles so I can reuse it:

private List<User> GetUsersFromRoles(uint[] UserRoles) {}

Any pointers on how to dynamically build my query? Thanks

Answer 1

You can use the PredicateBuilder class.

PredicateBuilder has been released in the LINQKit NuGet package

LINQKit is a free set of extensions for LINQ to SQL and Entity Framework power users.

Answer 2

Assuming your UserRoles values are themselves bitmasks, would something like this work?

private List<User> GetUsersFromRoles(uint[] UserRoles) {
    uint roleMask = 0;
    for (var i = 0; i < UserRoles.Length;i++) roleMask= roleMask| UserRoles[i];
    // roleMasknow contains the OR'ed bitfields of the roles we're looking for

    return (from u in dc.Users where (u.UserRolesBitmask & roleMask) > 0) select u);
}

There's probably a nice LINQ syntax that'll work in place of the loops, but the concept should be the same.

Answer 3

There are a couple of ways you can do this:

LINQ Dynamic query libraries: http://weblogs.asp.net/scottgu/archive/2008/01/07/dynamic-linq-part-1-using-the-linq-dynamic-query-library.aspx

Expression Trees & Lamda expressions: http://msdn.microsoft.com/en-us/library/bb882637.aspx

Answer 4

Here's one way of adding a variable number of where clauses to your LINQ query. Note that I haven't touched your bitmask logic, I just focused on the multiple wheres.

// C#
private List<User> GetUsersFromRoles(uint[] UserRoles)
{
   var users = dc.Users;

   foreach (uint role in UserRoles)
   {
      users = users.Where(u => (u.UserRolesBitmask & role) == role);
   }

   return users.ToList();
}

---

EDIT: Actually, this will AND the where clauses and you wanted to OR them. The following approach (a inner join) works in LINQ to Objects but can not be translated to SQL with LINQ to SQL:

var result = from user in Users
             from role in UserRoles
             where (user.UserRolesBitmask & role) == role
             select user;

Answer 5

How's this? It is not dynamic linq, but accomplishes the goal.

private List<User> GetUsersFromRoles(uint[] userRoles) 
{
    List<User> users = new List<User>();

    foreach(uint userRole in UserRoles)
    {
        List<User> usersInRole = GetUsersFromRole(userRole);
        foreach(User user in usersInRole )
        {
            users.Add(user);
        }
    }
    return users;
}    

private List<User> GetUsersFromRole(uint userRole) 
{
    var users = from u in dc.Users
            where ((u.UserRolesBitmask & UserRole) == UserRole)
            select u;

    return users;    
}

Answer 6

private List<User> GetUsersFromRoles(uint UserRoles) {
  return from u in dc.Users            
         where (u.UserRolesBitmask & UserRoles) != 0
         select u;
}

UserRoles parameter should be provided, however, as a bit mask, instead of array.