How can I check if username already exists in the file

Question

In a class, I am entering data to create an account. Now every data that is entered by the user is saved in a file. Now for the Username, I want to check if the username already exists the file. How can I do this?

      public void CreateAccount()
      {

        Console.WriteLine("-=-=-=-=-=-=-=-=-=-=-=-=-=-");
        Console.WriteLine("Enter a Username: ");

        Client usernameField = new Client("Username")
        {
            UsernameField = Console.ReadLine()
        };


        string filePath = "C:\\createaccount.txt"

        Client.SerializeData(accountData, filePath);

    public static void SerializeData(List<Client> userToSerialize, string 
    filePath)
    {
        FileReadWrite<Client>.SerializeData(userToSerialize, filePath);
    }

    public static List<Client> DeserializeData<Client>(string filePath)
    {
        return FileReadWrite<Client>.DeserializeData<Client>(filePath);
    }  

Answer 1

You need to deserialize the existing data stored in the file.

var listOfClients = Client.DeserilizeData(fileName);

This would give you a list of existing users. Then you can use Linq to check if Username already exists

var exists = listOfclients.Any(x=>x.UsernameField.Equals(newUserName));

If the user name is case-insensitive, you need to use

var exists = listOfclients.Any(x=>x.UsernameField.Equals(newUserName,StringComparison.OrdinalIgnoreCase));

Update based on your code.

Console.WriteLine("-=-=-=-=-=-=-=-=-=-=-=-=-=-");
Console.WriteLine("Enter a Username: ");

Client usernameField = new Client("Username")
{
    UsernameField = Console.ReadLine()
};
var listOfClients = Client.DeserilizeData(fileName);
var exists = listOfclients.Any(x=>x.UsernameField.Equals(usernameField.UsernameField ));