Подключение библиотеки классов - C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
public partial class InputBox : Form
{
    public string val = null;
 
    public InputBox()
    {
        InitializeComponent();
    }
 
    public InputBox(String info, String title, String defVal)
    {
        InitializeComponent();
 
        this.info.Text = info != null ? info : "Введите данные";
        this.Text = title != null ? title : "Ввод данных";
        val = defVal;
        data.Text = val;
    }
 
    public String getVal()
    {
        return val;
    }
 
    private void accept_Click(object sender, EventArgs e)
    {
        if (data.Text != "")
            val = data.Text;
        Close();
    }
 
    private void cancel_Click(object sender, EventArgs e)
    {
        Close();
    }
}

String npc = new InputBox().showDialog().getVal();