Question

Is there a .NET API that generates QR Codes such as this one?

I’d like to display these on pages that I expect my users to print out.

Answer 1

I wrote a basic HTML helper method to emit the correct <img> tag to take advantage of Google’s API. So, on your page (assuming ASPX view engine) use something like this:

<%: Html.QRCodeImage(Request.Url.AbsolutePath) %>
<%: Html.QRCodeImage("Meagre human needs a phone to read QR codes. Ha ha ha.") %>

Or if you want to specify the size in pixels (the image is always square):

<%: Html.QRCodeImage(Request.Url.AbsolutePath, size: 92) %>

Here is the code:

public static class QRCodeHtmlHelper
{
    /// <summary>
    /// Produces the markup for an image element that displays a QR Code image, as provided by Google's chart API.
    /// </summary>
    /// <param name="htmlHelper"></param>
    /// <param name="data">The data to be encoded, as a string.</param>
    /// <param name="size">The square length of the resulting image, in pixels.</param>
    /// <param name="margin">The width of the border that surrounds the image, measured in rows (not pixels).</param>
    /// <param name="errorCorrectionLevel">The amount of error correction to build into the image.  Higher error correction comes at the expense of reduced space for data.</param>
    /// <param name="htmlAttributes">Optional HTML attributes to include on the image element.</param>
    /// <returns></returns>
    public static MvcHtmlString QRCode(this HtmlHelper htmlHelper, string data, int size = 80, int margin = 4, QRCodeErrorCorrectionLevel errorCorrectionLevel = QRCodeErrorCorrectionLevel.Low, object htmlAttributes = null)
    {
        if (data == null)
            throw new ArgumentNullException("data");
        if (size < 1)
            throw new ArgumentOutOfRangeException("size", size, "Must be greater than zero.");
        if (margin < 0)
            throw new ArgumentOutOfRangeException("margin", margin, "Must be greater than or equal to zero.");
        if (!Enum.IsDefined(typeof(QRCodeErrorCorrectionLevel), errorCorrectionLevel))
            throw new InvalidEnumArgumentException("errorCorrectionLevel", (int)errorCorrectionLevel, typeof (QRCodeErrorCorrectionLevel));

        var url = string.Format("http://chart.apis.google.com/chart?cht=qr&chld={2}|{3}&chs={0}x{0}&chl={1}", size, HttpUtility.UrlEncode(data), errorCorrectionLevel.ToString()[0], margin);

        var tag = new TagBuilder("img");
        if (htmlAttributes != null)
            tag.MergeAttributes(new RouteValueDictionary(htmlAttributes));
        tag.Attributes.Add("src", url);
        tag.Attributes.Add("width", size.ToString());
        tag.Attributes.Add("height", size.ToString());

        return new MvcHtmlString(tag.ToString(TagRenderMode.SelfClosing));
    }
}

public enum QRCodeErrorCorrectionLevel
{
    /// <summary>Recovers from up to 7% erroneous data.</summary>
    Low,
    /// <summary>Recovers from up to 15% erroneous data.</summary>
    Medium,
    /// <summary>Recovers from up to 25% erroneous data.</summary>
    QuiteGood,
    /// <summary>Recovers from up to 30% erroneous data.</summary>
    High
}