How to check if there is a number in a txt file?

Question

C++ / C#

I am making a phone book in C# Windows Forms NET. Unable to check if there is a number in the file. Always says no.
loginUser - text from TextBox

string loginUser = loginField.Text;
            string[] numbers = File.ReadAllLines(@"C:\Sharp_text\number2.txt");
            if(numbers.Contains(loginUser))
            {
                MessageBox.Show("Данные уже существуют!");
            }
            else
            {
                //File.AppendAllText(@"C:\Sharp_text\number2.txt", loginUser + Environment.NewLine);
                MessageBox.Show("Данные успешно добалены");
            }

Answer 1

Through foreach sort through an array and there will be a happiness.
foreach (loginUser in numbers)