最初のフォームからダイアログとしてもう一つのフォーム

using System;
using System.Windows.Forms;
 
namespace Basic
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            // フォームのインスタンス作成
 
            using (Form2 form2 = new Form2())
            {
                // ダイアログとして表示
                DialogResult result = form2.ShowDialog(this);
                if (result == DialogResult.OK)
                {
                    Console.WriteLine("OK");
                }
                if (result == DialogResult.Cancel)
                {
                    Console.WriteLine("Cancel");
                }
 
            }
        }
 
        private void button2_Click(object sender, EventArgs e)
        {
            // 閉じる
            this.Close();
        }
    }
}

using System;
using System.Windows.Forms;
 
namespace Basic
{
        public partial class Form2 : Form
        {
                public Form2()
                {
                        InitializeComponent();
                }
 
                private void button1_Click(object sender, EventArgs e)
                {
                        this.Close();
                }
        }
}

フォーム間のコントロール参照

using System;
using System.Windows.Forms;
 
namespace Basic
{
        public partial class Form2 : Form
        {
                public Form2()
                {
                        InitializeComponent();
                }
 
                private void button1_Click(object sender, EventArgs e)
                {
                        ((Form1)Owner).textBox1.Text = "フォーム間のコントロール参照";
 
                        this.Close();
                }
        }
}

Form1 から Form2 にデータを渡す場合は、Form2 のコンストラクタに引数を作成する

Form2 に AcceptButton を設定