create lines with strings for each blank line until it reaches line 100

Question

I'm new to C# i need help on reading a file that currently has 7 lines of text but I need it to write "Line PlaceHolder" after those 7 lines until it reaches line 100 in the text file. This is what i have so far and i know it's my failed attempt: EDIT: It's good but only issue is an exception is throw that a process is already using the text file, how do I solve this to read/write that file at the same time??

  public void ReadFile()
    {
        if (File.Exists(AccountsFile))
        {
            using (StreamReader Reader = new StreamReader(AccountsFile))
            using (StreamWriter Writer = new StreamWriter((AccountsFile)))
            {
                for (int i = 0; i < 100; i++)
                {
                    string line;
                    if ((line = Reader.ReadLine()) == null)
                    {
                        Writer.WriteLine("Line Placeholder");

                    }
                }
            }
        }
        else
        {
            File.Create(AccountsFile);
        }
    }

Answer 1

You could first read the file contents into an array using File.ReadAllLines, get the array .Length (representing the number of lines in the file), and subtract that number from 100 to see how many lines you need to write. If the number is greater than zero, then create a List<string> with that many empty lines and write those lines to the end of the file using File.AppendAllLines:

// See how many lines we need to add
var newLinesNeeded = 100 - File.ReadAllLines(AccountsFile).Length;

// Add them if needed
if (newLinesNeeded > 0)            
{
    // Create a list of empty lines
    var blankLines = new List<string>();
    for(int i = 0; i < newLinesNeeded; i++)
    {
        blankLines.Add("");
    }

    // Append them to our file
    File.AppendAllLines(AccountsFile, blankLines);
}