LINQ ThenBy in circle

Question

Why I have different results in gg?

1. Version:

   var kk = ff.OrderBy(el => el.path[0]);
   
   for (var i = 1; i <= 2; i++)
     {
         kk = kk
             .ThenBy(el => el.path.Length > i
             ? el.path[i]
             : 0);
     }
   
   var gg = kk.ToList();
   

2. Version:

   var kk = ff.OrderBy(el => el.path[0]);
   
    kk = kk
         .ThenBy(el => el.path.Length > 1
          ? el.path[1]
          : 0)
         .ThenBy(el => el.path.Length > 2
          ? el.path[2]
          : 0);
   
   var gg = kk.ToList();
   

I need result by Version 2, but i need in cycle

Answer 1

This is because any variable that is declared outside a lambda expression is not captured inside the lambda i.e. you don't create multiple versions of i in each of the lambdas.

For example,

List<Action> actions = new List<Action>();
for (int i = 0 ; i < 10 ; i++) {
    actions.Add(() => Console.WriteLine(i));
}
actions.ForEach(x => x.Invoke());

Prints 10 10 times, instead of 0 to 9 as you would expect. The same thing happens in your code. Your first code snippet is equivalent to this:

 kk = kk
  .ThenBy(el => el.path.Length > 1
   ? el.path[3]
   : 0)
  .ThenBy(el => el.path.Length > 2
   ? el.path[3]
   : 0);