private IWaveIn waveIn;
        private WaveFileWriter writer;
        /// <summary>
        /// 开始录音
        /// </summary>
        private void StartRecording()
        {
            if (waveIn != null) return;
            waveIn = new WaveIn {WaveFormat = new WaveFormat(8000, 1)};//设置码率
            writer = new WaveFileWriter("test.wav", waveIn.WaveFormat);
            waveIn.DataAvailable += waveIn_DataAvailable;
            waveIn.RecordingStopped += OnRecordingStopped;
            waveIn.StartRecording();
        }
        /// <summary>
        /// 停止录音
        /// </summary>
        private void StopRecording()
        {
            waveIn.StopRecording();
            waveIn.Dispose();
        }
        /// <summary>
        /// 录音中
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void waveIn_DataAvailable(object sender, WaveInEventArgs e)
        {
            if (this.InvokeRequired)
            {
                this.BeginInvoke(new EventHandler<WaveInEventArgs>(waveIn_DataAvailable), sender, e);
            }
            else
            {
                writer.Write(e.Buffer, 0, e.BytesRecorded);
                int secondsRecorded = (int)(writer.Length / writer.WaveFormat.AverageBytesPerSecond);//录音时间获取
                if (secondsRecorded >= 30)
                {
                    StopRecording();
                }

            }
        }
        /// <summary>
        /// 停止录音
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
       private void OnRecordingStopped(object sender, StoppedEventArgs e)
        {
            if (this.InvokeRequired)
            {
                this.BeginInvoke(new EventHandler<StoppedEventArgs>(OnRecordingStopped), sender, e);
            }
            else
            {
                if (waveIn != null) // 关闭录音对象
                {
                    waveIn.Dispose();
                    waveIn = null;
                }
                if (writer != null)//关闭文件流
                {
                    writer.Close();
                    writer = null;
                }
                if (e.Exception != null)
                {
                    MessageBox.Show(String.Format("出现问题 {0}",
                                                  e.Exception.Message));
                }
            }
        }
上面代码

int secondsRecorded = (int)(writer.Length / writer.WaveFormat.AverageBytesPerSecond)
用这句获取录音时间后，
调用停止录音的方法，

    private void StopRecording()
        {
            waveIn.StopRecording(); 这里报错，未将对像引用到实例
            waveIn.Dispose();
        }