I tried to extend a jagged array with
public World( ushort worldMatrice)
// OldWith = 864
ushort OldWidth = (ushort)worldMatrice.GetLength(0);
// Extend the matrice to (1024) => only the first level 
worldMatrice.ToList().Add(new ushort[1024- OldWidth]);
// NewWidth = 864 , and should be 1024 ...
ushort NewWidth = worldMatrice.getLenght(0);
Array.AddRange() doesn't change the dimensions of an Array, and Array.Length will always return the max number of elements the array can hold, not the total number of non-null elements inside it.
If you're looking to change the dimensions of the Array, you're likely going to need to transfer the values from the old array to a new Array with dimensions you want.
int newArray = new int; Array.Copy(oldArray, newArray, oldArray.Length);
To get the number of non-null elements in the array, use something like
int count = array.Count(s => s != null);