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9226

answers:

6

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

+1  A: 

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.

Dylan Beattie
This should work. Instead of dynamic where clause you reduced it to a single one. And that nice LINQ syntax you mentioned could be:uint roleMask = UserRoles.Aggregate(0, (combined, role) => combined | role);
Lucas
+1  A: 

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;    
}
Carlton Jenke
users.AddRange(usersInRole);
David B
Also, you need to ToList in GetUsersFromRole
David B
+1  A: 

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

alexmac
A: 
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.

Michael Damatov
This will return the users who match ALL the given roles. His method return users who match ANY of the given roles.
Lucas
+7  A: 

You can use the PredicateBuilder class.

ilitirit
+1  A: 

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 u in Users
             from role in UserRoles
             where (u.UserRolesBitmask & role) == role)
             select u;
Lucas