var fooIds = new List<int> { 2, 3, 4}
var foosToChange = Foo.Where(f => fooIds.Contains(f.Id));
// Update foosToChange and save here

var fooIds = new []
  {
    new { prop1 = "12345", prop2 = 2017111701 },
    new { prop1 = "hij", prop2 = 2018060101 }
  };

var foosToChange = Foo.Where(f => fooIds.Contains(???));

var foosToChange = Foo.Where(f => fooIds.Any(x => x.prop1 == f.Id1 && x.prop2 == f.Id2));

A[] Foo = new A[]{ new A{ Id1 = "12345", Id2 = "2017111701" }, new A { Id1 = "fakeid", Id2 = "2017111701" } };

        var res = Foo.Where(f => fooIds.Any(x => x.prop1 == f.Id1 && x.prop2 == f.Id2));

        // res will return first element