RichTextBox in WPF does not have an property as .Lines?

RichTextBox is a FlowDocument type and that does not have a Lines property. What you are doing seems like a good solution. You may want to use IndexOf instead of split.

You can also add an extension method like the article suggests:

public static long Lines(this string s)
{
    long count = 1;
    int position = 0;
    while ((position = s.IndexOf('\n', position)) != -1)
        {
        count++;
        position++;         // Skip this occurance!
        }
    return count;
}

I know I'm very late to the party, but I came up with another reliable and reusable solution using RTF parsing.

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Idea

In RTF, every paragraph ends with a \par. So e.g. if you enter this text

Lorem ipsum
Foo
Bar

in a RichTextBox, it will internally be stored as (very very simplified)

\par
Lorem ipsum\par
Foo\par
Bar\par

therefore, it is a quite reliable method to simply count the occurrences of those \par commands. Note though that there is always 1 more \par than there are actual lines.

---

Usage

Thanks to extension methods, my proposed solution can simply be used like this:

int lines = myRichTextBox.GetLineCount();

where myRichTextBox is an instance of the RichTexBox class.

---

Code

public static class RichTextBoxExtensions
{
    /// <summary>
    /// Gets the content of the <see cref="RichTextBox"/> as the actual RTF.
    /// </summary>
    public static string GetAsRTF(this RichTextBox richTextBox)
    {
        using (MemoryStream memoryStream = new MemoryStream())
        {
            TextRange textRange = new TextRange(richTextBox.Document.ContentStart, richTextBox.Document.ContentEnd);
            textRange.Save(memoryStream, DataFormats.Rtf);
            memoryStream.Seek(0, SeekOrigin.Begin);

            using (StreamReader streamReader = new StreamReader(memoryStream))
            {
                return streamReader.ReadToEnd();
            }
        }
    }

    /// <summary>
    /// Gets the content of the <see cref="RichTextBox"/> as plain text only.
    /// </summary>
    public static string GetAsText(this RichTextBox richTextBox)
    {
        return new TextRange(richTextBox.Document.ContentStart, richTextBox.Document.ContentEnd).Text;
    }

    /// <summary>
    /// Gets the number of lines in the <see cref="RichTextBox"/>.
    /// </summary>
    public static int GetLineCount(this RichTextBox richTextBox)
    {
        // Idea: Every paragraph in a RichTextBox ends with a \par.

        // Special handling for empty RichTextBoxes, because while there is
        // a \par, there is no line in the strict sense yet.
        if (String.IsNullOrWhiteSpace(richTextBox.GetAsText()))
        {
            return 0;
        }

        // Simply count the occurrences of \par to get the number of lines.
        // Subtract 1 from the actual count because the first \par is not
        // actually a line for reasons explained above.
        return Regex.Matches(richTextBox.GetAsRTF(), Regex.Escape(@"\par")).Count - 1;
    }
}