How to replace "," with "." at the end line of a file?

Question

I wrote below code to reformat a source text file(columns_original) and write to an output file(output.ctl). It is working but what I should do is, in the last line of output file created by below code, there is a "," at the end of the file and I want to change it to "." In which part should I embed this code? I added this at the end but I am getting an error "An unhandled exception of type 'System.IO.IOException' occurred in mscorlib.dll Output file is being used by another program"

// I am trying to replace "," with "." at the last line of the text I created      above and I created below code.

// I am trying to replace "," with ")" at the last line of the text I created above and 
using (FileStream aFile2 = new FileStream(path, FileMode.Append, FileAccess.Write))
using (StreamWriter sw2 = new StreamWriter(aFile2))
{
    var lastLine = File.ReadLines(path).Last();
    lastLine = lastLine.Replace(",", ".");
    sw2.WriteLine(lastLine);
}

MCVE:

 // I ADDED ABOVE CODE TO BELOW WORKING PART AND I AM GETTING ERROR MENTIONED IN THE POST  


using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace CTL_CMD_Creator
{
class Program
{
    static void Main(string[] args)
    {
        string ColChangable1 = "VARCHAR2";
        string ColChangable2 = "CHAR";
        string ColChangable3 = "NUMBER";
        string ColChangable4 = "DATE";

        string path = @"C:\Users\***\Desktop\output.ctl";

        StreamReader reader = File.OpenText(@"C:\Users\***\Desktop\Columns_Original.txt");
        string line;

        using (FileStream aFile = new FileStream(path, FileMode.Append, FileAccess.Write))
        {
            using (StreamWriter sw = new StreamWriter(aFile))
            {
                while ((line = reader.ReadLine()) != null)
                {                       

                    string[] tokens = line.Split(new char[] { ' ', '\t', '(', ')', ',' }, StringSplitOptions.RemoveEmptyEntries);


                    if (tokens[1].Equals(ColChangable1) || tokens[1].Equals(ColChangable2))
                    {
                        sw.WriteLine(tokens[0] + "\t" + "\"TRIM(:" + tokens[0] + ")\",");
                    }

                    else if (tokens[1].Equals(ColChangable3))
                    {
                        sw.WriteLine(tokens[0] + "\t" + ",");
                    }
                    else if (tokens[1].Equals(ColChangable4))
                    {
                        sw.WriteLine(tokens[0] + "\t" + "DATE" + ",");
                    }                        
                }
            }                
        }
    }
}
}

Answer 1

If I understand your problem, a simpler way would be to use System.IO.File. Demo on .NetFiddle:

using System;
using System.IO;
using System.Linq;

public class Program
{
    public static void Main(string[] args)
    {
        // init
        var path = "output.ctl";
        var content =
            "Hello and Welcome" + Environment.NewLine
            + "Hello and Welcome" + Environment.NewLine
            + ",";
        File.WriteAllText(path, content);

        var text = File.ReadAllLines(path); // read the file as string[]

        foreach (var line in text) // print the file
            Console.WriteLine(line);

        text[text.Length - 1] = text.Last().Replace(",", "."); // replace

        File.WriteAllLines(path, text); // overwrite or write to a new file

        string[] lines2 = File.ReadAllLines(path); // read again
        foreach (var line in lines2) // then print to show the difference
            Console.WriteLine(line);
    }
}

output:

Hello and Welcome
Hello and Welcome
,
Hello and Welcome
Hello and Welcome
.