Title:       ASP.NET Common Web Page Class Library - Part 4
Author:      Eric Woodruff
Email:       Eric@EWoodruff.us
Environment: Visual Studio .NET, IIS, ASP.NET, C#, VB.NET
Keywords:    page, template, render, link, regular expression, validation summary
Level:       Intermediate
Description: A utility class containing some useful features for ASP.NET applications
Section      ASP.NET
SubSection   General

Table of Contents

• Introduction
• HTML Encoding
• Link Encoding
• Converting Validation Messages to Hyperlinks
• Conclusion
• Revision History

This is the fourth in a series of articles on a class library for ASP.NET applications that I have developed. It contains a set of common, reusable page classes that can be utilized in web applications as-is to provide a consistent look, feel, and set of features. New classes can also be derived from them to extend their capabilities. The features are all fairly modular and may be extracted and placed into your own classes too. For a complete list of articles in the series along with a demonstration application and the code for the classes, see Part 1 [^ ].

This article describes the only non-page derived class in the library, PageUtils, along with the remaining methods of the BasePage class that are somewhat similar in nature to those contained in it. PageUtils contains a set of utility functions that you may find useful in any ASP.NET application. Each of the features is described below. The class itself is sealed and all public properties and methods are static. As such, the constructor is declared private as there is no need to instantiate the class.

HTML Encoding

The first method presented is HtmlEncode, which can be called to encode an object for output to an HTML page. It encodes any HTML special characters as literals instead of letting the browser interpret them. In addition, it replaces multiple spaces, tabs, and line breaks with their HTML equivalents thus preserving the layout of the specified text. The size of expanded tab characters can be altered using the TabSize property. Set it to the number of non-breaking spaces that should replace the tab character. The default is four.

If the object is null (Nothing), results in an empty string, or is a single space, a non-breaking space is returned. In conjunction with the above-described behavior, this is useful for displaying database fields that contain HTML special characters, formatting, or nulls such as those with the text or memo data type.

As an added bonus, if the encodeLinks parameter is true, URLs, UNCs, and e-mail addresses are converted to hyperlinks whenever possible using the EncodeLinks method (see below). If false, they are not converted and will be rendered as normal text. As shown below, the code is fairly simple and requires little in the way of additional explanation.

public static string HtmlEncode(Object objText, bool encodeLinks)
{
    StringBuilder sb;
    string text;

    if(objText != null)
    {
        text = objText.ToString();

        if(text.Length != 0)
        {
            // Create tab expansion string if not done already
            if(expandTabs == null)
                expandTabs = new String(' ',
                    PageUtils.TabSize).Replace(" ", " ");

            // Encode the string
            sb = new StringBuilder(
                HttpUtility.HtmlEncode(text), 256);

            sb.Replace("  ", "  ");  // Two spaces
            sb.Replace("\t", expandTabs);
            sb.Replace("\r", "");
            sb.Replace("\n", "<br>");

            text = sb.ToString();

            if(text.Length > 1)
            {
                if(!encodeLinks)
                    return text;

                // Try to convert URLs, UNCs, and e-mail
                // addresses to links.
                return PageUtils.EncodeLinks(text);
            }

            if(text.Length == 1 && text[0] != ' ')
                return text;
        }
    }

    return "&nbsp;";
}

The second method presented is EncodeLinks. This method is called by HtmlEncode but can also be called directly by your code. It takes the passed string and finds all URLs, UNCs, and e-mail addresses and converts them to clickable hyperlinks suitable for rendering in an HTML page. For UNC paths, it will include any text up to the first whitespace character. If the path contains spaces, you can enclose the entire path in angle brackets (i.e., <\\Server\Folder\Name With Spaces>) and the encoder will include all text between the angle brackets in the hyperlink. The angle brackets will not appear in the encoded hyperlink.

public static string EncodeLinks(string text)
{
    // We'll create these on first use and keep them around
    // for subsequent calls to save resources.
    if(reURL == null)
    {
        reURL = new Regex(@"(((file|news|(ht|f|nn)tp(s?))://)|" +
            @"(www\.))+[\w()*\-!_%]+.[\w()*\-/.!_#%]+[\w()*\-/" +
            @".!_#%]*((\?\w+(\=[\w()*\-/.!_#%]*)?)(((&|&(?" +
            @"!\w+;))(\w+(\=[\w()*\-/.!_#%]*)?))+)?)?",
            RegexOptions.IgnoreCase);
        reUNC = new Regex(@"(\\{2}\w+(\\((&.{2,8};|" +
            @"[\w\-\.,@?^=%&:/~\+#\$])*[\w\-\@?^=%&/~\+#\$])?)" +
            @"*)|((\<|\&lt;)\\{2}\w+(\\((&.{2,8};|" +
            @"[\w\-\.,@?^=%&:/~\+#\$ ])*)?)*(\>|\&gt;))",
            RegexOptions.IgnoreCase);
        reEMail = new Regex(@"([a-zA-Z0-9_\-])([a-zA-Z0-9_\-\." +
            @"]*)@(\[((25[0-5]|2[0-4][0-9]|1[0-9][0-9]|[1-9]" +
            @"[0-9]|[0-9])\.){3}|((([a-zA-Z0-9\-]+)\.)+))(" +
            @"[a-zA-Z]{2,}|(25[0-5]|2[0-4][0-9]|1[0-9][0-9]|" +
            @"[1-9][0-9]|[0-9])\])", RegexOptions.IgnoreCase);
        reTSUNC = new Regex(
            @"\.?((&\#\d{1,3}|&\w{2,8});((&\#\d{1,3}|&" +
            @"\w{2,8}))?)+\w*$");

        urlMatchEvaluator = new MatchEvaluator(OnUrlMatch);
        uncMatchEvaluator = new MatchEvaluator(OnUncMatch);
    }

    // Do the replacements
    text = reURL.Replace(text, urlMatchEvaluator);
    text = reUNC.Replace(text, uncMatchEvaluator);
    text = reEMail.Replace(text,
        @"<a href='mailto:$&'>$&</a>");

    return text;
}

As you can see, the method uses regular expressions to search for and replace each URL, UNC, and e-mail address. The expressions used should catch just about all variations of each type. The regular expression objects are created on first use and are kept around for subsequent calls to save a little time. For URLs and UNCs, the following match evaluators handle the actual work of the replacement:

// Replace a URL with a link to the URL.  This checks for a
// missing protocol and adds it if necessary.
private static string OnUrlMatch(Match match)
{
    StringBuilder sb = new StringBuilder("<a href='", 256);
    string url = match.Value;

    // Use default HTTP protocol if one wasn't specified
    if(url.IndexOf("://") == -1)
        sb.Append("http://");

    sb.Append(url);
    sb.Append("' target='_BLANK'>");
    sb.Append(url);
    sb.Append("</a>");

    return sb.ToString();
}

// Replace a UNC with a link to the UNC.  This strips off any
// containing brackets (plain or encoded) and flips the slashes.
private static string OnUncMatch(Match match)
{
    StringBuilder sb = new StringBuilder("<a href='file:", 256);
    string unc = match.Value;

    // Strip brackets if found.  If it has encoded brackets,
    // strip them too.
    if(unc[0] == '<')
        unc = unc.Substring(1, unc.Length - 2);
    else
        if(unc.StartsWith("&lt;"))
            unc = unc.Substring(4, unc.Length - 8);

    // Move trailing special characters outside the link
    Match m = reTSUNC.Match(unc);
    if(m.Success == true)
        unc = reTSUNC.Replace(unc, "");

    sb.Append(unc);
    sb.Append("' target='_BLANK'>");

    // Replace backslashes with forward slashes
    sb.Replace('\\', '/');

    sb.Append(unc);
    sb.Append("</a>");

    if(m.Success == true)
        sb.Append(m.Value);

    return sb.ToString();
}

A regular expression match evaluator is like a callback. Each time the regular expression finds a match, it calls the evaluator. Its job is to take the found text and modify it in any way necessary and then return it to the regular expression so that it can be used to replace the original text. In these two cases, the match evaluators add the anchor tag and ensure that the links are formatted appropriately.

Converting Validation Messages to Hyperlinks

In my applications, I have come to favor the validation summary control to contain all validation error messages generated by the page. It keeps them all in one location and does not adversely affect the layout of the controls in the form when they are made visible. The drawback is that on a form with a large number of controls and validation conditions, it can sometimes be difficult to match each message to its control, especially if the form is long enough to require scrolling around to find it. As such, I have added functionality to the BasePage class to automatically convert all validation control error messages that are set to appear in a validation summary control to clickable hyperlinks that will take you directly to the offending field by giving it the focus.

protected virtual void ConvertValMsgsToLinks()
{
    BaseValidator bv;

    foreach(IValidator val in this.Validators)
    {
        bv = val as BaseValidator;

        if(bv != null && bv.Visible == true &&
          bv.ControlToValidate.Length > 0 &&
          bv.Display == ValidatorDisplay.None)
            bv.ErrorMessage = MakeMsgLink(bv.ControlToValidate,
                bv.ErrorMessage, this.MsgLinkCssClass);
    }
}

A call to ConvertValMsgsToLinks is done as the very first step in the overridden Render method. It iterates over the page's Validators collection. The validator control must be visible, must have its ControlToValidate property set to a control ID, and must have its Display property set to None indicating that it will appear in a validation summary control. If all of the necessary conditions are met, a call is placed to the MakeMsgLink method to convert the error message to a hyperlink.

Note that since this occurs within the rendering step, changes to the error messages are not retained. If the page posts back, the error messages are restored from view state and will be in their non-hyperlink form. When the page renders during the postback, the messages will be converted to hyperlinks again provided that they still meet the necessary conditions. I chose this approach so that it is transparent to users of the class, is non-intrusive, and will not break any code that expects the messages to be in their non-hyperlink form. Derived classes can override this method to extend or suppress this behavior.

Note: If extracting the above method for use in your own classes, be sure to override the page's Render method and call it. If not, the links will not be converted.

public string MakeMsgLink(string id, string msg, string cssClass)
{
    string newClass;

    // Don't bother if it's null, empty, or already in the form
    // of a link.
    if(msg == null || msg.Length == 0 || msg.StartsWith("<a "))
        return msg;

    StringBuilder sb = new StringBuilder(512);

    // Add the anchor tag and the optional CSS class
    sb.Append("<a ");

    newClass = (cssClass == null) ?
        this.MsgLinkCssClass : cssClass;

    if(newClass != null && newClass.Length > 0)
    {
        sb.Append("class='");
        sb.Append(newClass);
        sb.Append("' ");
    }

    // An HREF is included that does nothing so that we can use
    // the hover style to do stuff like underline the link when
    // the mouse is over it.  OnClick performs the action and
    // returns false so that we don't trigger IE's
    // OnBeforeUnload event which may be tied to data change
    // checking code.

    // NOTE: OnPreRender registers the script containing the
    // function.  Tell the function to use the "Find Control"
    // method to locate the ID.  That way, it works for controls
    // embedded in data grids.
    sb.Append("href='javascript:return false;' " +
        "onclick='javascript: return BP_funSetFocus(\"");
    sb.Append(id);
    sb.Append("\", true);'>");
    sb.Append(msg);
    sb.Append("</a>");

    return sb.ToString();
}

The MakeMsgLink method will convert the passed text into a hyperlink that transfers focus to the control with the specified ID. The Set Focus script, described in part one of this series, controls setting the focus to the control. As such, the specified ID can be an exact match or the ending part of an ID (see part one for details). An optional CSS class name can be specified that will be applied to the hyperlink. If null, it uses the one defined by the MsgLinkCssClass property. By default, it is set to the value of the BasePage.MsgLinkCssName constant which is currently set to the style name ErrorMsgLink. The class name should appear in the stylesheet associated with the application. As noted, a dummy href is added to the link so that you can add a hover style to the CSS class. For example, in my applications, the error messages display as normal text and show an underline as the mouse passes over them.

Conclusion

Although small, the PageUtils class contains some very helpful features. The validation message link feature of BasePage can also make the validation summary control more user friendly. Hopefully, you will find this class and the others in the library, or parts of them, as useful as I have.

Revision History