C#

-----
Form1
-----
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.Sql;
using System.Data.SqlClient;

namespace WindowsFormsApp1
{
    public partial class Form1 : Form
    {
        // string LCA = "Data Source=localhost;Initial Catalog=LerarenCijferApplicatie;Integrated Security=True";


        public Form1()
        {
            InitializeComponent();
            PasswordTXT.PasswordChar = '*';


        }

        private void button1_Click(object sender, EventArgs e)
        {
            string Username = this.UsernameTXT.Text;
            string Password = this.PasswordTXT.Text;
            try
            {
                string LCAConnection = "Data Source=localhost;Initial Catalog=LerarenCijferApplicatie;Integrated Security=True";
                SqlConnection LCA = new SqlConnection(LCAConnection);
                SqlCommand LCACOM = new SqlCommand("SELECT * FROM LoginData where Username='" + Username + "' and Password='" + Password + "';",LCA);
                SqlDataReader LCAReader;

                LCA.Open();
                LCAReader = LCACOM.ExecuteReader();

                int count = 0;

                while (LCAReader.Read())
                {
                    count = count + 1;
                }

                if (count == 1)
                {
                    MessageBox.Show("U bent nu ingelogd, " + UsernameTXT.Text);
                    this.Visible = false;
                    Form2 InvoerForm = new Form2("Welkom, " + UsernameTXT.Text);
                    InvoerForm.ShowDialog();
                    //this.Hide();
                    //From2 f2 = new Form2();
                    //f2.ShowDialog();
                }
                else if (count > 1)
                {
                    MessageBox.Show("duplicate username and password");
                }
                else
                {
                    MessageBox.Show("Gebruikersnaam en/of wachtwoord is incorrect");
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }

            //this.Visible = false;
            //Form2 P = new Form2();
            //P.ShowDialog();

        }
    }
}
-----
Form2
-----
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.Sql;
using System.Data.SqlClient;

namespace WindowsFormsApp1
{
    public partial class Form2 : Form
    {
        SqlConnection sc = new SqlConnection();
        //int x = Int32.Parse(CijferBOX.Text);


        public Form2(string Username)
        {
            InitializeComponent();
            UsernameLBL.Text = Username;


        }

        private void Form2_Load(object sender, EventArgs e)
        {
            sc.ConnectionString = ("Data Source=localhost; Initial Catalog = LerarenCijferApplicatie; Integrated Security=True;");
        }

        private void InvoerBTN_Click(object sender, EventArgs e)
        {

            try
            {
                int x = Int32.Parse(CijferBOX.Text);

                SqlCommand Invoer = new SqlCommand();
                sc.Open();

                Invoer.Connection = sc;

                Invoer.CommandText = (@"INSERT INTO ResultaatData (Voornaam, Achternaam, Vak, Cijfer)
            VALUES ('" + VoornaamBOX.Text + "','" + AchternaamBOX.Text + "','" + VakBOX.Text + "','" + x + "');");
                Invoer.ExecuteNonQuery();

                sc.Close();

                VoornaamBOX.Text = "";
                AchternaamBOX.Text = "";
                VakBOX.Text = "";
                CijferBOX.Text = "";

                MessageBox.Show("De gegevens zijn opgeslagen");
            }
            catch { MessageBox.Show("Er is iets fout gegaan bij het versturen"); }

        }

        private void BekijkBTN_Click(object sender, EventArgs e)
        {
            try
            {
                Form3 BekijkApplicatie = new Form3();
                BekijkApplicatie.ShowDialog();
            }
            catch { MessageBox.Show("Er is iets fout gegaan"); }
        }
    }
}
-----
Form3
-----
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.Data.Sql;
using System.Data.SqlClient;

namespace WindowsFormsApp1
{
    public partial class Form3 : Form
    {

        string connectionstring = @"Data Source=localhost; Initial Catalog=LerarenCijferApplicatie;Integrated Security=True;";

        public Form3()
        {
            InitializeComponent();
        }

        private void Form3_Load(object sender, EventArgs e)
        {
            //    ResultaatGridView.DataSource = ResultaatLijst();

            using (SqlConnection sc = new SqlConnection(connectionstring))
            {
                sc.Open();
                SqlDataAdapter sda = new SqlDataAdapter("SELECT * FROM ResultaatData", sc);
                DataTable dt = new DataTable();
                sda.Fill(dt);

                ResultaatGridView.DataSource = dt;
            }

        }

        private void AfsluitBTN_Click(object sender, EventArgs e)
        {
            try
            {
                this.Hide();
            }
            catch { MessageBox.Show("Er is iets fout gegaan bij het sluiten van de applicatie"); }
        }

        //  private DataTable ResultaatLijst()
        //  {
        //      DataTable dtResultaatLijst = new DataTable();

        //      return dtResultaatLijst;

        //}
    }
}