An array of List in c#

Since no context was given to this question and you are a relatively new user, I want to make sure that you are aware that you can have a list of lists. It's not the same as array of list and you asked specifically for that, but nevertheless:

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

you can initialize them through collection initializers like so:

List<List<int>> myList = new List<List<int>>(){{1,2,3},{4,5,6},{7,8,9}};

simple approach:

        List<int>[] a = new List<int>[100];
        for (int i = 0; i < a.Length; i++)
        {
            a[i] = new List<int>();
        }

or LINQ approach

        var b = Enumerable.Range(0,100).Select((i)=>new List<int>()).ToArray();

I can suggest that you both create and initialize your array at the same line using linq:

List<int>[] a = new List<int>[100].Select(item=>new List<int>()).ToArray();

If you want a more efficient code, you can also use:

List<int>[] a = Enumerable.Range(0, 100).Select(_ => new List<int>()).ToArray();

The reason is that it first creates an array of 100 null references to List, and then it uses LINQ to iterate over each element and replace the null reference with a new List instance. This means that the code is doing more work than necessary, as it is first creating an array of null references, and then iterating over it to create new List instances.

In contrast, the second code creates the List instances directly using Enumerable.Range and Select, without first creating an array of null references. This means that it is more efficient, as it avoids unnecessary work.

List<int>[]  a = new List<int>[100];

You still would have to allocate each individual list in the array before you can use it though:

for (int i = 0; i < a.Length; i++)
    a[i] = new List<int>();

use

List<int>[] a = new List<int>[100];

List<List<int>> Registers = new List<List<int>>() { };

for (int j = 0; j < 5; j++)
{
    List<int> data_1 = new List<int>();
    List<int> data_2 = new List<int>();

    data_1.Add(j * 5);
    Registers.Add(data_1);

    data_2.Add(10 + j + 1);
    data_2.Add(10 + j + 2);
    Registers.Add(data_2);

    //if you change or clear data_1 or data_2 then Registers will be change
    //data_1.Clear();
    //data_2.Clear();
}