private void button2_Click(object sender, EventArgs e)
    {
        Form2 form2 = new Form2();
        form2.Show();

    }

    private void button1_Click(object sender, EventArgs e)
    {
        try
        {
            var sr = new System.IO.StreamReader("C\" + textBox1.Text + "\login.ID");
            username = sr.ReadLine();
            password = sr.ReadLine();
            sr.Close();

            if (username == textBox1.Text && password == textBox2.Text)
                  MessageBox.Show("Log-in Successfull", "Success!");
            else
                 MessageBox.Show("Username or password is wrong! ","Error!");

        }
        catch (System.IO.DirectoryNotFoundException )
        {
            MessageBox.Show("The user doesn't exist!", "Error!");

        }
    }
}

public partial class Form2 : Form
{
    public Form2()
    {
        InitializeComponent();
    }

    private void button2_Click(object sender, EventArgs e)
    {
        Form2 form2 = new Form2();
        form2.Hide();

    }

    private void button1_Click(object sender, EventArgs e)
    {
        try
        {
            var sw = new System.IO.StreamWriter ("C\" + textBox1.Text + "\login.ID");
            sw.Write(textBox1.Text + "n" + textBox2.Text);
            sw.Close();

        }
        catch(System.IO.DriveNotFoundException )

        {
            System.IO.Directory.CreateDirectory("C:\" + textBox1.Text);
            var sw = new System.IO.StreamWriter("C\" + textBox1.Text + "\login.ID");
            sw.Write(textBox1.Text + "n" + textBox2.Text);
            sw.Close();
        }


    }
}