Writing data to CSV, checking for duplicates

Question

I am writing a program that takes user input and writes it to a CSV file. I am also checking to see if what they have input is a duplicate of whatever is in that CSV file. So far, the flow of this code snippet is:

• Get user input ->
• Iterate through the CSV file and assign it to a list ->
• If the list is empty (null) or if it doesn't contain a username ->
• Write data to CSV.
• If user input is a duplicate -> don't write

I'm a bit new to OOP and C# in general, so any tips would be appreciated.

private string writeUser()
{
    string filePath = "usernames.csv";
    List<string> users = new List<string>();

    try
    {
        using (StreamReader sr = new StreamReader(filePath))
        {
            while (sr.Peek() >= 0) // iterate through the csv and add it to our users list
            {
                users.Add(sr.ReadLine());
            }
            sr.Close();
        }
        using (StreamWriter sw = new StreamWriter(filePath, true))
        {
            if (users == null | !users.Contains(this.username)) // write to csv if users is empty, or if it doesnt contain the typed username
            {
                sw.WriteLine(this.username);
                sw.Close();
            }
            else if (users.Contains(this.username)) 
            {
                Console.WriteLine("username already exists");
                sw.Close();
            }
        }
    }
    catch (Exception e)
    {
        Console.WriteLine("Some Error Occured: \n" + e.Message);
        Console.ReadLine();
    }
    return this.username;
}

How can I simplify this/make what I have better?

Answer 1

We can rewrite this to be more declarative. Not to mention StreamReader and StreamWriter are fairly low level types, in most cases there is a different call to make to express your intent better.

private string writeUser()
{
   string filePath = "usernames.csv";
   var found = false;
   //we don't modify the existing list, don't store it
   //if storing, I would use HastSet<> for better duplicate checking
   if(File.Exists(filePath))
   foreach(var user in File.ReadLines(filePath))
   //declare you want to process each line, but not necessarily 
   //read the whole file like below 
   {
       if(user == username)
       {
           found = true;
           break;//found it, stop looping
       }
   }

   if(!found)
   {
       //AppendText will also Create the file for you too if not there
       //skip boolean values that you need to check what the parameter affect
       using (StreamWriter sw = File.AppendText(filePath))
       {
          sw.WriteLine(username);
       }
   }
   return username;
}