how to close the file which is using by other process

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C#

Ia developing a windows application. In this for sorting. first I am reading all the lines of file and at the last writing all lines to that file but it is giving me error"this pocess can not access the file because its is used by another process"
Please help me I am new to c#. trying to learn.

try
{
string[] input_lines = File.ReadAllLines("C:\\maq\\" + main1.text + "\\" + main1.text + ".txt");
              List<string>  input_lines_list  = input_lines.ToList();
                input_lines_list.RemoveAt(0);
                input_lines_list = input_lines_list.OrderBy((s) => s.Split(',')[0]).ThenByDescending((s) => DateTime.ParseExact(s.Split(',')[1], "yyyyMMdd", null)).ToList();
                int j = 0;
                while (j < input_lines_list.Count - 1)
                {
                    string[] line_detail = input_lines_list[j].Split(',');
                    string[] next_line_detail = input_lines_list[j + 1].Split(',');
                    if (line_detail[2] == line_detail[3] && line_detail[3] == line_detail[4] && line_detail[4] == line_detail[5] && line_detail[5] == input_lines_list[j + 1].Split(',')[5])
                    {
                        input_lines_list.RemoveAt(j);
                        continue;
                    }
                    else if (line_detail[0] == next_line_detail[0] && line_detail[2] == next_line_detail[2] && line_detail[3] == next_line_detail[3] && line_detail[4] == next_line_detail[4] && line_detail[5] == next_line_detail[5])
                    {
                        input_lines_list.RemoveAt(j + 1);
                        continue;
                    }
                    else if (line_detail[0] == next_line_detail[0] && line_detail[1] == next_line_detail[1])
                    {
                        input_lines_list.RemoveAt(j + 1);
                        continue;
                    }
                    j += 1;
                }
          }
          catch(Exception ex)
          {
                  MessageBox.Show(ex.Message);
          }

                input_lines_list.Insert(0, iplut_lines[0])

            File.WriteAllText("C:\\maq\\" + main1.text + "\\" + main1.text + ".txt", "\n");
            System.IO.File.WriteAllLines("C:\\maq\\" + main1.text + "\\" + main1.text + ".txt", input_lines_list);
 }