ZXing .NET 的项目主页
https://github.com/micjahn/ZXing.Net

代码基本上抄袭自下面两篇文章 XD
http://www.cnblogs.com/tianma3798/p/5426869.html
http://www.cnblogs.com/tianma3798/p/5426880.html

仅作参数优化，更加实用和简便一点

Shared Function MakeQR(ByVal qrtext As String, Optional ByVal width As Integer = 800, Optional ByVal height As Integer = 800, Optional ByVal margin As Integer = 1) As Bitmap
    Dim writer As New ZXing.BarcodeWriter
    writer.Format = ZXing.BarcodeFormat.QR_CODE

    Dim opt As New ZXing.QrCode.QrCodeEncodingOptions
    opt.DisableECI = True '设置为True才可以调整编码
    opt.CharacterSet = "UTF-8" '文本编码，建议设置为UTF-8
    opt.Width = width   '宽度
    opt.Height = height '高度
    opt.Margin = margin '边距，貌似不是像素格式，因此不宜设置过大

    writer.Options = opt

    Return writer.Write(qrtext)
End Function

Shared Function MakeQR(ByVal qrtext As String, ByVal logo As Bitmap, Optional ByVal width As Integer = 800, Optional ByVal height As Integer = 800, Optional ByVal margin As Integer = 1) As Bitmap
    If logo Is Nothing Then
        Return MakeQR(qrtext, width, height, margin)
    End If

    Dim writer As New ZXing.MultiFormatWriter
    Dim hint As New Dictionary(Of ZXing.EncodeHintType, Object)()
    hint.Add(ZXing.EncodeHintType.CHARACTER_SET, "UTF-8")
    hint.Add(ZXing.EncodeHintType.MARGIN, margin)
    hint.Add(ZXing.EncodeHintType.ERROR_CORRECTION, ZXing.QrCode.Internal.ErrorCorrectionLevel.H)

    ' 生成二维码
    Dim bm As ZXing.Common.BitMatrix = writer.encode(qrtext, ZXing.BarcodeFormat.QR_CODE, width, height, hint)
    Dim barcodeWriter = New ZXing.BarcodeWriter()
    Dim bmp As Bitmap = barcodeWriter.Write(bm)

    '获取二维码实际尺寸（去掉二维码两边空白后的实际尺寸）
    Dim rectangle As Integer() = bm.getEnclosingRectangle()

    '计算插入图片的大小和位置
    Dim middleW As Integer = Math.Min((rectangle(2) / 3.5), logo.Width)
    Dim middleH As Integer = Math.Min((rectangle(3) / 3.5), logo.Height)
    Dim middleL As Integer = (bmp.Width - middleW) / 2
    Dim middleT As Integer = (bmp.Height - middleH) / 2

    '将img转换成bmp格式，否则后面无法创建Graphics对象
    Dim bmpimg As New Bitmap(bmp.Width, bmp.Height, Imaging.PixelFormat.Format32bppArgb)

    Using g As Graphics = Graphics.FromImage(bmpimg)
        g.InterpolationMode = Drawing2D.InterpolationMode.HighQualityBicubic
        g.SmoothingMode = Drawing2D.SmoothingMode.HighQuality
        g.CompositingQuality = Drawing2D.CompositingQuality.HighQuality
        g.DrawImage(bmp, 0, 0)
    End Using

    '将二维码插入图片
    Using myGraphic As Graphics = Graphics.FromImage(bmpimg)
        '白底
        myGraphic.FillRectangle(Brushes.White, middleL, middleT, middleW, middleH)
        myGraphic.DrawImage(logo, middleL, middleT, middleW, middleH)
    End Using

    bmp.Dispose()

    Return bmpimg

End Function

Shared Function ReadQR(ByVal bmp As Bitmap) As String
    Dim reader As New ZXing.BarcodeReader
    reader.Options.CharacterSet = "UTF-8"

    Dim ret As ZXing.Result = reader.Decode(bmp)
    If ret Is Nothing Then
        Return Nothing
    Else
        Return ret.Text
    End If
End Function