public WaveIn waveSource = null;
public WaveFileWriter waveFile = null;

private void StartBtn_Click(object sender, EventArgs e)
{
    StartBtn.Enabled = false;
    StopBtn.Enabled = true;

    waveSource = new WaveIn();
    waveSource.WaveFormat = new WaveFormat(44100, 1);

    waveSource.DataAvailable += new EventHandler<WaveInEventArgs>(waveSource_DataAvailable);
    waveSource.RecordingStopped += new EventHandler<StoppedEventArgs>(waveSource_RecordingStopped);

    waveFile = new WaveFileWriter(@"C:\Temp\Test0001.wav", waveSource.WaveFormat);

    waveSource.StartRecording();
}

private void StopBtn_Click(object sender, EventArgs e)
{
    StopBtn.Enabled = false;

    waveSource.StopRecording();
}

void waveSource_DataAvailable(object sender, WaveInEventArgs e)
{
    if (waveFile != null)
    {
        waveFile.Write(e.Buffer, 0, e.BytesRecorded);
        waveFile.Flush();
    }
}

void waveSource_RecordingStopped(object sender, StoppedEventArgs e)
{
    if (waveSource != null)
    {
        waveSource.Dispose();
        waveSource = null;
    }

    if (waveFile != null)
    {
        waveFile.Dispose();
        waveFile = null;
    }

    StartBtn.Enabled = true;
}