Sometimes it's concurrency, but most of the time it's that I want to know I own the array. I want to know that `_array[5]` will be the same, no matter when I access it (excluding intentional modifications by callee). For example, if I take `T[]` as a parameter to my constructor, I might think `_array[5]` will be the same, but nothing stops the caller from doing `Array.Sort(theParameter)` sometime later.

    // in one function of the caller's type
    _data = new int[] { 1, 2, 3, 4, 5, 6, 7 }; // could come from some data collection engine
    _dataTable = new(_data);
 
    // then, sometime later, in another function of the caller's type
    Array.Clear(_data); // reset for next iteration or something
    // data table now has an array of zeros if it didn't copy the data

This is just an example, and the problem is not only limited to arrays; Mutable classes can be mutated by the caller after the callee is given them. Basically, the issue is multiple mutable references. The only solution to the above problem is a defensive copy by the "data table" constructor. I would like the ability to say, "the caller cannot modify this reference anymore".

This is something Rust gets right, IMO. If I want the callee and the caller to share mutable ownership, RefCell<T> or Mutex<T> can be used, and the usage of such a type makes that clear. If I want thread safety, I can wrap the mutex in an Arc<T>. And, if I don't want shared ownership, a plain T can be used, and ownership passes into the callee. The issue is: in C#, without defensive measures, all objects are just T* with no ownership or thread-safety.