c# Add a Column to end of CSV file

Question

Trying to add a column to the end of a cvs file, which will just count up the number of lines (or can all be the same number it doesn\'t really mater) as the main aim is to

Answer 1

Why so many materializations ReadAllLines(), .ToList()? Why not just

String filePath = @"C:/CSV/test.csv";

var csv = File.ReadLines(filePath) // not AllLines
  .Select((line, index) => index == 0 
     ? line + ",Column 4"
     : line + "," + index.ToString())
  .ToList(); // we should write into the same file, that´s why we materialize

File.WriteAllLines(filePath, csv);