using System;

using System.Collections.Generic;

​

public class Program {

 public static void Main() {

  var l1 = new List < A > () {

   new A {

    B = "a"

   }, new A {

    B = "b"

   }

  };

  var l2 = new List < A > () {

   new A {

    B = "a"

   }, new A {

    B = "c"

   }

  };

​

  var c = new List < List < A >> () {

   l1,

   l2

  };

​

  var cp = c.FindCommon();

​

  foreach(var ci in cp) {

   Console.WriteLine(ci.B);

  }

​

  Console.WriteLine("nothing common");

 }

​

​

}

​

public class A {

 public string B;

}

​

public static class Ext {

 public static List < T > FindCommon < T > (this IEnumerable < List < T >> lists) {

  var map = new Dictionary < T,

   int > ();

  var listCount = 0; // number of lists

​

  foreach(var list in lists) {

   listCount++;

   foreach(var item in list) {

    // Item encountered, increment count

    int currCount;

    if (!map.TryGetValue(item, out currCount)) {

     currCount = 0;

    }

​

    currCount++;

    map[item] = currCount;

   }

  }

​

  var result = new List < T > ();

  foreach(var kvp in map) {

   // Items whose occurrence count is equal to the number of lists are common to all the lists

   if (kvp.Value == listCount) {

    result.Add(kvp.Key);

   }

  }

​

  return result;

 }

}