Passwords in C#

I have tried it myself. I can get the user input and write it to a file. The problem occurs when I try to read the passwords back to authorize them. I tested it with to user names and passwords. If you put in a matching username and password, it works...but it also works if you put in an existing username and password. I came up with the code myself. so I know it's not the correct way to do it.
Heres my code:

This is from one form that asks for the username and password and then writes it to a file:

private void button1_Click(object sender, EventArgs e)
        {
            string username = textBox1.Text;
            string password = textBox2.Text;
            WriteUserAndPassword(username, password);
        }
        private void WriteUserAndPassword(string user, string pass)
        {
            using (StreamWriter writer = File.AppendText("C:\\pass.txt"))
            {
                writer.WriteLine("username:");
                writer.WriteLine(user);
                writer.WriteLine("password:");
                writer.WriteLine(pass);

                writer.Dispose();
                writer.Close();
             }
        }

This code is for the form that asks the user to login:

private void button1_Click(object sender, EventArgs e)
        {
            using (StreamReader reader = new StreamReader("C:\\pass.txt"))
            {
                string username = null;
                string password = null;
                string content = null;
               
                while (reader.ReadLine() != null)
                {
                    content = reader.ReadLine();
                    if (textBox1.Text == content)
                    {
                        username = content;
                    }

                    reader.ReadLine();
                    content = reader.ReadLine();
                    if (textBox2.Text == content)
                    {
                        password = content;
                    }
                    if ((textBox1.Text == username) && (textBox2.Text == password))
                    {
                        this.Hide();
                        Form2 frm = new Form2();
                        frm.Show();
                       
                    }
                }
                if ((textBox1.Text != username)||(textBox2.Text != password))
                {
                    MessageBox.Show("Username or password is incorrect");
                }
            }
        }

        private void linkLabel1_LinkClicked(object sender, LinkLabelLinkClickedEventArgs e)
        {
            Form3 frm3 = new Form3();
            frm3.Show();
        }
    }

I realize the code is very sloppy. I had a hard time finding good tutorials for this.
Any help will be appreciated.

it is a base use for every application,u can find it easily by google.

but try it youself is the best way