为什么serialport 控制使用readto之后close()报错

我的代码如下 
private void Form2_Load(object sender, EventArgs e) 
        { 

            label7.Text="登陆单位机构代码："+strB; 
            this.label1.AutoSize = true; 
            this.label1.AutoEllipsis = true; 
            serialPort1.Close(); 
          
        } 

        private void SetLabel(string text) 
        { 
            if (label1.InvokeRequired) 
            { 
                SetlabelCallback d = new SetlabelCallback(SetLabel); 
                this.Invoke(d, new object[] { text }); 
            } 
            else 
            { 
                label1.Text = text; 
                textBox1.Clear(); 
                textBox2.Clear(); 
                textBox3.Clear(); 
                richTextBox1.Clear(); 
                richTextBox2.Clear(); 
                string[] sArray=label1.Text.Trim().Split('#'); 
                label1.Text = ""; 
                if (sArray[0].Length == 9) 
                { 
                    textBox1.Text = sArray[0]; 
                    textBox2.Text = sArray[1]; 
                    textBox3.Text = sArray[2]; 
                    richTextBox1.Text = sArray[3]; 
                    richTextBox2.Text = sArray[4]; 
                }                
                  
                else 
                { 
                    label1.Text = ""; 
                    MessageBox.Show("请重新扫描正确的二维条码！"); 
                } 
                
                
            } 
        } 

        private void serialPort1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e) 
        { 
            str = ""; 
            try 
            { 
                str = serialPort1.ReadTo("@#");                
                SetLabel(str);                
            } 
            catch 
            { 
                MessageBox.Show("读取数据出错！");                
            } 
              
        } 
private void button1_Click(object sender, EventArgs e) 
        { 
            if (comboBox1.Text.Trim().Length == 0) 
            { MessageBox.Show("请选择端口号"); } 
            else 
            { 
                this.serialPort1.Close(); 
                this.serialPort1.PortName = comboBox1.Text; 
                this.serialPort1.RtsEnable = true; 
                this.serialPort1.Encoding = System.Text.Encoding.Default; 
                try 
                { 
                    this.serialPort1.Open(); 
                    MessageBox.Show("端口打开成功！"); 
                    button1.Enabled = false;                    
                } 
                catch 
                { 
                    MessageBox.Show("您选择的com口不存在！"); 
                } 
            } 
        }        
private void button3_Click(object sender, EventArgs e) 
      { 
          { 
              if (serialPort1.IsOpen) 
              { 
                  serialPort1.Close();
                  MessageBox.Show("关闭成功！"); 
              } 
              
          } 
      } 


用readto 读取数据后，用button3关闭com口，结果总是报错 --------------------编程问答-------------------- 为啥没人回帖子

补充：.NET技术 ,  C#