Question

I have a TextBlock bound to a property. However, I’d like to bold certain words within the text. What is the easiest way to do this? FYI, I’m new to WPF.

Answer 1

Bolding individual words involves actually creating more inline elements, so you can’t just bind a string to the TextBlock’s Text and do this.

What I’ve done for this in the past is created a subclass of TextBlock which has a custom property that I bind to. When this property is bound I clear the Inlines of the base TextBlock and then use an algorithm that parses the new string value creating either plain Runs, Bolds, Hyperlinks etc.

Here’s some sample code which I wrote for my experimental Twitter client which detects URLs, emails and @ pattern and creates hyperlinks for them. Regular text is inlined as normal runs:

[ContentProperty("StatusText")]
public sealed class StatusTextBlock : TextBlock
{
    #region Fields

    public static readonly DependencyProperty StatusTextProperty = DependencyProperty.Register(
                                                                                    "StatusText", 
                                                                                          typeof(string),
                                                                                    typeof(StatusTextBlock),
                                                                                    new FrameworkPropertyMetadata(string.Empty, FrameworkPropertyMetadataOptions.None, StatusTextBlock.StatusTextPropertyChangedCallback, null));
    private static readonly Regex UriMatchingRegex = new Regex(@"(?<url>[a-zA-Z]+:\/\/[a-zA-Z0-9]+([\-\.]{1}[a-zA-Z0-9]+)*\.[a-zA-Z]{2,5}(:[0-9]{1,5})?([a-zA-Z0-9_\-\.\~\%\+\?\=\&\;\|/]*)?)|(?<emailAddress>[^\s]+@[a-zA-Z0-9]+([\-\.]{1}[a-zA-Z0-9]+)*\.[a-zA-Z]{2,5})|(?<toTwitterScreenName>\@[a-zA-Z0-9\-_]+)", RegexOptions.Compiled);

    #endregion

    #region Constructors

    public StatusTextBlock()
    {
    }

    #endregion

    #region Type specific properties

    public string StatusText
    {
        get
        {
            return (string)this.GetValue(StatusTextBlock.StatusTextProperty);
        }

        set
        {
            this.SetValue(StatusTextBlock.StatusTextProperty, value);
        }
    }

    #endregion

    #region Helper methods

    internal static IEnumerable<Inline> GenerateInlinesFromRawEntryText(string entryText)
    {
        int startIndex = 0;
        Match match = StatusTextBlock.UriMatchingRegex.Match(entryText);

        while(match.Success)
        {
            if(startIndex != match.Index)
            {
                yield return new Run(StatusTextBlock.DecodeStatusEntryText(entryText.Substring(startIndex, match.Index - startIndex)));
            }

            Hyperlink hyperLink = new Hyperlink(new Run(match.Value));

            string uri = match.Value;

            if(match.Groups["emailAddress"].Success)
            {
                uri = "mailto:" + uri;
            }
            else if(match.Groups["toTwitterScreenName"].Success)
            {
                uri = "http://twitter.com/" + uri.Substring(1);
            }

            hyperLink.NavigateUri = new Uri(uri);

            yield return hyperLink;

            startIndex = match.Index + match.Length;

            match = match.NextMatch();
        }

        if(startIndex != entryText.Length)
        {
            yield return new Run(StatusTextBlock.DecodeStatusEntryText(entryText.Substring(startIndex)));
        }
    }

    internal static string DecodeStatusEntryText(string text)
    {
        return text.Replace("&gt;", ">").Replace("&lt;", "<");
    }

    private static void StatusTextPropertyChangedCallback(DependencyObject target, DependencyPropertyChangedEventArgs eventArgs)
    {
        StatusTextBlock targetStatusEntryTextBlock = (StatusTextBlock)target;

        targetStatusEntryTextBlock.Inlines.Clear();

        string newValue = eventArgs.NewValue as string;

        if(newValue != null)
        {
            targetStatusEntryTextBlock.Inlines.AddRange(StatusTextBlock.GenerateInlinesFromRawEntryText(newValue));
        }
    }

    #endregion
}