Question

i have following tables

T1    
    ====   ======
     ID     Desc
    ====   ======
     1       t1
     2       t2
     3       t3
     4       t4


T2
=====   =======  ========
 ID      T1ID     PT1ID
=====    ======  =========
 1        2         1
 2        3         2

In T2 both T1ID and PT1ID are foreign key for T1. In my output i want row with ID=4 as it does not have any matching row in both foreign key columns

Answer 1

This should do want you want:

var notInT2 = T1.Where(i => T2.All(e => e.T1ID != i.Id) &&
                            T2.All(e => e.PT1ID != i.Id));

To test it:

class RowT1 { public Int32 Id;}
class RowT2 { public Int32 T1ID; public Int32 PT1ID;    }

class Programm
{
    static void Main()
    {
        var T1 = new List<RowT1>(){new RowT1(){Id=1},
                                   new RowT1(){Id=2},
                                   new RowT1(){Id=3},
                                   new RowT1(){Id=4}};

        var T2 = new List<RowT2>(){new RowT2(){T1ID=2, PT1ID=1}, 
                                   new RowT2(){T1ID=3, PT1ID=2}};

        var notInT2 = T1.Where(i => T2.All(e => e.T1ID != i.Id) &&
                                    T2.All(e => e.PT1ID != i.Id));

        Console.ReadLine();
    }
}