Populating a list of integers in .NET
If you're using .Net 3.5, Enumerable.Range is what you need.
Generates a sequence of integral numbers within a specified range.
LINQ to the rescue:
// Adding value to existing list
var list = new List<int>();
list.AddRange(Enumerable.Range(1, x));
// Creating new list
var list = Enumerable.Range(1, x).ToList();
See Generation Operators on LINQ 101