C#: Displaying text in richtextbox in reverse order/updwards

Question

I am trying to create a 'log display' using the richtextbox control in C#.NET.

public void logLine(string line)
{
     rtxtLoginMessage.AppendText(line + "\r\n");
} 

Is there a way to display the text in reverse order/upwards? (where the newest log and date will be displayed at the top)

Your help is much appreciated.

Answer 1

Short Answer

You want to set the selection to 0 and then set the SelectedText property.

public void logLine(string line)
{
    rtxtLoginMessage.Select(0, 0);    
    rtxtLoginMessage.SelectedText = line + Environment.NewLine;
} 

Long Answer

How did I work this out?

Using Reflector, search for the RichTextBox control and find the AppendText method (follow the base types to TextBoxBase). Have a look at what it does (below for convenience).

public void AppendText(string text)
{
    if (text.Length > 0)
    {
        int num;
        int num2;
        this.GetSelectionStartAndLength(out num, out num2);
        try
        {
            int endPosition = this.GetEndPosition();
            this.SelectInternal(endPosition, endPosition, endPosition);
            this.SelectedText = text;
        }
        finally
        {
            if ((base.Width == 0) || (base.Height == 0))
            {
                this.Select(num, num2);
            }
        }
    }
}

You will see that it finds the end position, sets the internal selection, and then sets the SelectedText to the new value. To insert text at the very beginning you simply want to find the start position instead of the end position.

Now, so you do not have repeat this piece of code for each time you want to prefix text you can create an Extension Method.

public static void PrependText(this TextBoxBase textBox, string text)
{
    if (text.Length > 0)
    {
        var start = textBox.SelectionStart;
        var length = textBox.SelectionLength;

        try
        {
            textBox.Select(0, 0);
            textBox.SelectedText = text;
        }
        finally
        {
            if (textBox.Width == 0 || textBox.Height == 0)
                textBox.Select(start, length);
        }
    }
}

Note: I'm only using the Try/Finally block to match the implementation of AppendText. I'm not certain as why we would want to restore the initial selection if the Width or Height is 0 (if you do know why, please leave a comment as I'm interested in finding out).

Also, there is some contention to using "Prepend" as the opposite for "Append" as the direct English definition is confusing (do a Google search - there are several posts on the topic). However, if you look at the Barron's Dictionary of Computer Terms it has become an accepted use.

HTH,

Dennis