Controlling the Length of a RichTextBox in C#

Introduction

I use TextBox controls in background forms for logging application events. Because the applications run for days at a time and many events are recorded, the TextBoxes need to be pruned to avoid using too much memory.

Recently, I switched to RichTextBox controls so I could use colored text, and found that my trim routine lost the text colors and other formatting. The method presented here solves that problem.

Using the code

Here is my new logging routine:

/// <summary>
/// timestamp the message and add to the textbox.
/// To prevent runtime faults should the amount
/// of data become too large, trim text when it reaches a certain size.
/// </summary>
/// <param name="text"></param>
public void AppendTrace(string text, Color textcolor)
{
    // keep textbox trimmed and avoid overflow
    // when kiosk has been running for awhile

    Int32 maxsize = 1024000;
    Int32 dropsize = maxsize / 4;

    if (richTextBox_RTCevents.Text.Length > maxsize)
    {
        // this method preserves the text colouring
        // find the first end-of-line past the endmarker

        Int32 endmarker = richTextBox_RTCevents.Text.IndexOf('\n', dropsize) + 1;
        if (endmarker < dropsize)
            endmarker = dropsize;

        richTextBox_RTCevents.Select(0, endmarker);
        richTextBox_RTCevents.Cut();
    }

    try
    {
        // trap exception which occurs when processing
        // as application shutdown is occurring

        richTextBox_RTCevents.SelectionStart = richTextBox_RTCevents.Text.Length;
        richTextBox_RTCevents.SelectionLength = 0;
        richTextBox_RTCevents.SelectionColor = textcolor;
        richTextBox_RTCevents.AppendText(
          System.DateTime.Now.ToString("HH:mm:ss.mmm") + " " + text);
    }
    catch (Exception ex)
    {
    }
}

It is called like this:

AppendTrace("some text" + Environment.NewLine, Color.Blue);

Points of Interest

The SelectionStart and SelectionLength properties are reset prior to setting the SelectionColor and appending the text; otherwise, the line added, sometimes, did not have the right color.

History

• March 11, 2009 - Original submission.