WPF TextBlock Highlighter

public static readonly DependencyProperty SelectionProperty =
    DependencyProperty.RegisterAttached("Selection", typeof(string), typeof(TextBlockHighlighter),
        new PropertyMetadata(new PropertyChangedCallback(SelectText)));

private static void SelectText(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    if (d == null) return;
    if (!(d is TextBlock)) throw new InvalidOperationException("Only valid for TextBlock");

    TextBlock txtBlock = d as TextBlock;
    string text = txtBlock.Text;
    if (string.IsNullOrEmpty(text)) return;

    string highlightText = (string)d.GetValue(SelectionProperty);
    if (string.IsNullOrEmpty(highlightText)) return;

    int index = text.IndexOf(highlightText, StringComparison.CurrentCultureIgnoreCase);
    if (index < 0) return;

    Brush selectionColor = (Brush)d.GetValue(HighlightColorProperty);
    Brush forecolor = (Brush)d.GetValue(ForecolorProperty);

    txtBlock.Inlines.Clear();
    while (true)
    {
        txtBlock.Inlines.AddRange(new Inline[] {
                 new Run(text.Substring(0, index)),
                 new Run(text.Substring(index, highlightText.Length)) {Background = selectionColor,
                     Foreground = forecolor}
        });

        text = text.Substring(index + highlightText.Length);
        index = text.IndexOf(highlightText, StringComparison.CurrentCultureIgnoreCase);

        if (index < 0)
        {
            txtBlock.Inlines.Add(new Run(text));
            break;
        }
    }
}

Public Shared ReadOnly SelectionProperty As DependencyProperty =
        DependencyProperty.RegisterAttached("Selection", GetType(String), GetType(TextBlockHighlighter),
                                            New PropertyMetadata(New PropertyChangedCallback
                                           (AddressOf SelectText)))

Private Shared Sub SelectText(d As DependencyObject, e As DependencyPropertyChangedEventArgs)
    If d Is Nothing Then Exit Sub
    If TypeOf d IsNot TextBlock Then Throw New InvalidOperationException("Only valid for textBlock")

    Dim txtBlock As TextBlock = CType(d, TextBlock)
    Dim text As String = txtBlock.Text
    If String.IsNullOrEmpty(text) Then Exit Sub

    Dim highlightText As String = CStr(d.GetValue(SelectionProperty))
    If String.IsNullOrEmpty(highlightText) Then Exit Sub

    Dim index = text.IndexOf(highlightText, StringComparison.CurrentCultureIgnoreCase)
    If index < 0 Then Exit Sub

    Dim selectionColor As Brush = CType(d.GetValue(HighlightColorProperty), Brush)
    Dim forecolor As Brush = CType(d.GetValue(ForecolorProperty), Brush)

    txtBlock.Inlines.Clear()
    While True
        txtBlock.Inlines.AddRange(New Inline() {
                                      New Run(text.Substring(0, index)),
                                      New Run(text.Substring(index, highlightText.Length)) _
                                      With {.Background = selectionColor, .Foreground = forecolor}
                                      })

        text = text.Substring(index + highlightText.Length)
        index = text.IndexOf(highlightText, StringComparison.CurrentCultureIgnoreCase)

        If index < 0 Then
            txtBlock.Inlines.Add(New Run(text))
            Exit While
        End If
    End While
End Sub

public static readonly DependencyProperty HighlightColorProperty =
    DependencyProperty.RegisterAttached("HighlightColor", typeof(Brush), typeof(TextBlockHighlighter),
        new PropertyMetadata(Brushes.Yellow, new PropertyChangedCallback(HighlightText)));
...

public static readonly DependencyProperty ForecolorProperty =
    DependencyProperty.RegisterAttached("Forecolor", typeof(Brush), typeof(TextBlockHighlighter),
        new PropertyMetadata(Brushes.Black, new PropertyChangedCallback(HighlightText)));

Public Shared ReadOnly HighlightColorProperty As DependencyProperty =
    DependencyProperty.RegisterAttached("HighlightColor", GetType(Brush), GetType(TextBlockHighlighter),
                                        New PropertyMetadata(Brushes.Yellow,
                                                             New PropertyChangedCallback
                                                             (AddressOf SelectText)))
...

Public Shared ReadOnly ForecolorProperty As DependencyProperty =
    DependencyProperty.RegisterAttached("Forecolor", GetType(Brush), GetType(TextBlockHighlighter),
                                        New PropertyMetadata(Brushes.Black,
                                                             New PropertyChangedCallback
                                                             (AddressOf SelectText)))