How to make sure that a list cannot be modified

Question | Sep 13, 2016 | hlone 

Suppose we have a List of integers that we want to pass to a method Reader:

class Example { 

  public void Run() {
     // Add more data to the list
        how to pass lst to Reader() here
        as read-only ?
  //.... more code
  void Reader( ICollection<int> coll ) {
     // some code 
  List<int> lst = new List<int>{ 1 };


While passing lst to Reader from Run, how can we enforce that the list is not modifiable inside method Reader?