直接上代码:

string sendData = "0012345678";
string recvData;
DateTime NowTime;
DateTime WaitTime;
bool TimeOver;

// 数据写入串口
serialPort1.Write(sendData);
// 等待3秒
WaitTime = DateTime.Now.AddSeconds(3);
TimeOver = false;

do
{
    NowTime = DateTime.Now;
    if (NowTime > WaitTime)
    {
        TimeOver = true;
    }
    // 退出条件:读取到了硬件发送过来的数据或超过等待时间
} while (serialPort1.BytesToRead > 10 || TimeOver == true);
// 从串口读取数据
recvData = serialPort1.ReadExisting();
MessageBox.Show(recvData);