Generate sequence in Linq
Size of initial list in the following code is less:

var x = Enumerable.Range(1, 10)
                  .Select(x_ =&gt; x_ * 10)
                  .Concat(new[] {25})
                  .Distinct()
                  .OrderBy(x_ =&gt; x_)
                  .ToList();


Otherwise your original code is perfect.