使用ZXing.dll，下载类库，添加引用即可。

命名空间：using ZXing;

生成二维码：

/// <summary>
/// 生成二维码图片
/// </summary>
/// <param name="strMessage">要生成二维码的字符串</param>
/// <param name="width">二维码图片宽度</param>
/// <param name="height">二维码图片高度</param>
/// <returns></returns>
private Bitmap GetQRCodeByZXingNet(String strMessage, Int32 width, Int32 height)
{
    Bitmap result = null;
    try
    {
        BarcodeWriter barCodeWriter = new BarcodeWriter();
        barCodeWriter.Format = BarcodeFormat.QR_CODE;
        barCodeWriter.Options.Hints.Add(EncodeHintType.CHARACTER_SET, "UTF-8");
        barCodeWriter.Options.Hints.Add(EncodeHintType.ERROR_CORRECTION, ZXing.QrCode.Internal.ErrorCorrectionLevel.H);
        barCodeWriter.Options.Height = height;
        barCodeWriter.Options.Width = width;
        barCodeWriter.Options.Margin = 0;
        ZXing.Common.BitMatrix bm = barCodeWriter.Encode(strMessage);
        result = barCodeWriter.Write(bm);
    }
    catch (Exception ex)
    {
        //异常输出
    }
    return result;
}

读取二维码：

///
/// 解码二维码
///
/// 待解码的二维码图片
/// 扫码结果
private string DecodeQrCode(Bitmap barcodeBitmap)
{
     BarcodeReader reader = new BarcodeReader();
     reader.Options.CharacterSet = "UTF-8";
     var result = reader.Decode(barcodeBitmap);
     return (result == null) ? null : result.Text;
}