C# WinApp ช่วยด้วยครับ อยู่เฉยๆ ผมก็งง กับ checkBox ครับ

 string chk = "";
        private void button1_Click(object sender, EventArgs e)
        {
            if (!string.IsNullOrEmpty(chk))
            {
            
                richTextBox1.Text = chk;
            }
            else
            {
                richTextBox1.Text = "don't have checkBox ";
            }
        }
        void CheckedChanged()
        {

           
            if (checkBox1.Checked && !chk.Contains("checkBox1,"))
            {
                MessageBox.Show("checkBox1,");
                chk += "checkBox1,";
              
            }
            else
            {
                chk= chk.Replace("checkBox1,", "");
            }

            if (checkBox2.Checked && !chk.Contains("checkBox2,"))
            {
                MessageBox.Show("checkBox2,");
                chk += "checkBox2,";
            }
            else
            {
                chk= chk.Replace("checkBox2,", "");
            }

            if (checkBox3.Checked && !chk.Contains("checkBox3,"))
            {
                MessageBox.Show("checkBox3,");
                chk += "checkBox3,";
            }
            else
            {
                chk= chk.Replace("checkBox3,", "");
            }

            if (checkBox4.Checked && !chk.Contains("checkBox4,"))
            {
                MessageBox.Show("checkBox4,");
                chk += "checkBox4,";
            }
            else
            {
                chk= chk.Replace("checkBox4,", "");
            }
        }
    

        private void checkBox1_Click(object sender, EventArgs e)
        {
            CheckedChanged();
        }