How to use local variables in a lambda expression

Question

I have 2 list object of type of some class,

 class person
    {
        public string id { get; set; }
        public string name { get; set; }
    }

List<person> pr = new List<person>();
pr.Add(new person { id = "2", name = "rezoan" });
pr.Add(new person { id = "5", name = "marman" });
pr.Add(new person { id = "3", name = "prithibi" });

List<person> tem = new List<person>();
tem.Add(new person { id = "1", name = "rezoan" });
tem.Add(new person { id = "2", name = "marman" });
tem.Add(new person { id = "1", name = "reja" });
tem.Add(new person { id = "3", name = "prithibi" });
tem.Add(new person { id = "3", name = "prithibi" });

Now i have to get all the ids from "pr" ListObject that has no entry or odd number of entries in the "tem" ListObejct. using lamda.

To do this i have used,

HashSet<string> inconsistantIDs = new HashSet<string>(pr.Select(p => p.id).Where(p => tem.FindAll(t => t.id == p).Count == 0 || tem.FindAll(t => t.id == p).Count % 2 != 0));

and it works fine.

but you can see from the code i have used tem.FindAll(t => t.id == p).Count twice to comapre with ==0 and %2!=0.

Is there any way to use tem.FindAll(t => t.id == p).Count once and save it to a temporary variable and then compare this variable with ==0 and %2!=0.

More simply i just want to use it once for two condition here.

Answer 1

Use a statement lambda instead of an expression lambda

var inconsistantIDs = new HashSet<string>(
           pr.Select(p => p.id).Where(p => 
                  {
                    var count = tem.FindAll(t => t.id == p).Count;
                    return count == 0 || count % 2 != 0;
                  }
           ));

Answer 2

Perhaps simply:

var query = pr.Where(p => { int c = tem.Count(p2 => p.id == p2.id); return c == 0 || c % 2 != 0; });

returns two persons:

2   "rezoan"
5   "marman"