using System;
 
    public partial class Form1 : Form
    {
        Class1 class1;
        public Form1()
        {
            InitializeComponent();
            //出力用テキストボックスを引数で処理用クラスに渡す。
            class1 = new Class1(outputTextBox);
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            class1.test();
 
        }
    }
 
 
 
        //処理用クラス
    class Class1
    {
        TextBox textBox;
        public Class1(TextBox textBox)
        {
            //Form1クラスから出力用のテキストボックスを受け取る。
            this.textBox = textBox;
        }
        public void test()
        {
            textBox.AppendText("処理しています……" + "\r\n");
            for (int i = 5; i >= 1; i--)
            {
                textBox.AppendText("あと" + i + "秒" + "\r\n");
                Thread.Sleep(1000);
            }
            textBox.AppendText("処理完了！");
        }
    }

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