C#读取串口条码扫描枪出现问题

大家救命啊，我的程序在使用扫描枪读取读取qr条码时总出现乱码
代码如下
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.Threading;namespace RSR232
{
    public partial class Form1 : Form
    {
        private delegate void SetlabelCallback(string text);
        private string str = "";        public Form1()
        {
            InitializeComponent();
        }        private void Form1_Load(object sender, EventArgs e)
        {
            this.label1.AutoSize = true;
            this.label1.AutoEllipsis = true;
        }
        private void SetLabel(string text)
        {
            if (label1.InvokeRequired)
            {
                SetlabelCallback d = new SetlabelCallback(SetLabel);
                this.Invoke(d, new object[] { text });                
            }
            else
            {
                label1.Text = text;
                richTextBox1.Text = label1.Text;
            }
        }        private void serialPort1_DataReceived(object sender, System.IO.Ports.SerialDataReceivedEventArgs e)
        {
            try
            {
                str += this.serialPort1.ReadExisting();
                SetLabel(str);
            }
            catch
            {
                MessageBox.Show("请选择你的com端口！");
            }
        }        private void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
        {
            //MessageBox.Show(comboBox1.Text);
            this.serialPort1.Close();
            this.serialPort1.PortName = comboBox1.Text;
            this.serialPort1.RtsEnable = true;
            try
            {
                this.serialPort1.Open();
            }
            catch
            {
                MessageBox.Show("您选择的com口不存在！");
            }
            this.serialPort1.Encoding = Encoding.Default;
        }     }
}