private void button1_Click(object sender, EventArgs e)
    {
        con = new SqlConnection(@"Data Source=MARIA-PC;Initial Catalog=Account;Integrated Security=True");
        SqlCommand cmd1 = new SqlCommand("SELECT * FROM [dbo].[Cont] WHERE Username = @Username and Password = @Password;", con);
        cmd1.Parameters.AddWithValue("@Username", this.Username.Text);
        cmd1.Parameters.AddWithValue("@Password", this.Password.Text);
        cmd1.Connection = con;
        con.Open();

        DataSet ds = new DataSet();
        SqlDataAdapter da = new SqlDataAdapter(cmd1);
        da.Fill(ds);
        con.Close();

        bool loginSuccessful = ((ds.Tables.Count > 0) && (ds.Tables[0].Rows.Count > 0));

        if (loginSuccessful )
        {
                MessageBox.Show("You logged in successfully!","Success!");
                this.Visible = false;
                f3.ShowDialog();
                this.Visible = true;
        }
        else
        {
            MessageBox.Show("Invalid username or password!", "Error!");
        }

    }