c# list<int> how to insert a new value in between two values

List<int> initializers = new List <int>();

initializers.Add(1);
initializers.Add(3);

int index = initializers.IndexOf(3);
initializers.Insert(index, 2);

Gives you 1,2,3.

Use List<T>.Insert:

initializers.Insert(index, value);

You can just use List.Insert() instead of List.Add() to insert items at a specific position.