Text columnizer

Columnizer.cs

using System.Text;

namespace ConsoleApp1
{
    public static class Columnizer
    {
        public static IEnumerable<string> Columnize(IList<string> headers, IList<string[]> rows, string separator = " ")
        {
            // Start with the lengths of the column headers
            int[] columnWidths = headers
                .Select(h => (h ?? "").Length)
                .ToArray();

            //First pass, get the lengths
            foreach (var row in rows)
            {
                if (columnWidths.Length != row.Length)
                    throw new InvalidOperationException();

                for (int x = 0; x < row.Length; x++)
                {
                    columnWidths[x] = Math.Max(columnWidths[x], (row[x] ?? "").Length);
                }
            }

            var rowTemp = new string[columnWidths.Length];

            //Format the column names
            for(int h = 0; h < columnWidths.Length; h++)
            {
                rowTemp[h] = (headers[h] ?? "").PadRight(columnWidths[h], ' ');
            }

            yield return string.Join(separator, rowTemp);

            //Separator row
            for (int h = 0; h < columnWidths.Length; h++)
            {
                rowTemp[h] = ("").PadRight(columnWidths[h], '-');
            }

            yield return string.Join(separator, rowTemp);

            //Format the body
            foreach (var row in rows)
            {
                for (int x = 0; x < row.Length; x++)
                {
                    rowTemp[x] = (row[x] ?? "").PadRight(columnWidths[x], ' ');
                }

                yield return string.Join(separator, rowTemp); ;
            }
        }
    }
}

Sample.cs

var rows = new List<string[]>
{
    new string[] { "Dean Koontz", "Intensity" },
    new string[] { "Stephen King", "It" },
    new string[] { "James Clear", "Atomic Habits" }
};

var headers = new string[]
{
    "Author",
    "Book"
};

var lines = Columnizer.Columnize(headers, rows);

foreach(var line in lines)
{
    Console.WriteLine(line);
}

SampleOutput.txt

Author       Book
------------ -------------
Dean Koontz  Intensity
Stephen King It
James Clear  Atomic Habits