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I have a Winforms app containing a RichTextBox.

How can I determine how many lines of text are displayed, currently visible?

Reason: I want to scroll the caret to the middle of the RichTextBox. I can use RichTextBox.ScrollToCaret(), but that puts the caret at the top of the RichTextBox. I figure, If I know how many lines are displayed, I could move the caret "back" n/2 lines, then call ScrollToCaret(), then restore the original caret position.

EDIT:

I found EM_GETLINECOUNT, which I thought was the answer, except the doc says: The EM_GETLINECOUNT message retrieves the total number of text lines, not just the number of lines that are currently visible.

Tantalizingly, there is also EM_GETFIRSTVISIBLELINE, which gets the first visible line, but I couldn't find a GETLASTVISIBLELINE. ??

Related:
How can I scroll the caret to the middle of the RichTextBox?

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  • Just interested - what are you going to do with this information? You do realise that this could be variable, because you can have different fonts and padding throughout the text. Commented Nov 20, 2009 at 22:43
  • It went into building a custom RTB that displays line numbers: stackoverflow.com/questions/2567263/… Commented Apr 20, 2010 at 14:32

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Well, this isn't pretty, but it works for me. Basically I'm checking a point just inside the upper-left corner of the richtextbox and a point just inside the lower left corner of the textbox. You may have to adjust the point coordinates depending on how your richtextbox is displayed. Then I get the character index that is closest to each of those two points, and retrieve the line that it is on.

Dim topIndex As Integer = RichTextBox1.GetCharIndexFromPosition(New Point(1, 1)) Dim bottomIndex As Integer = RichTextBox1.GetCharIndexFromPosition(New Point(1, RichTextBox1.Height - 1)) Dim topLine As Integer = RichTextBox1.GetLineFromCharIndex(topIndex) Dim bottomLine As Integer = RichTextBox1.GetLineFromCharIndex(bottomIndex) Dim numLinesDisplayed As Integer = bottomLine - topLine

I tested it for richtextboxes with multiple sizes of fonts displayed, and it seems to work. I suspect that the answer that is returned will be off (too small) by one line if the last line of displayed text has a lot of white space under it and the next line is almost ready to be displayed. If you have a very tall richtextbox, with many lines, this shouldn't be a problem.

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To display the number of lines that a RichTextBox is capable of displaying, even if there is no text there currently, try:

Dim s As SizeF = TextRenderer.MeasureText("A", rtb.Font, rtb.Size, TextFormatFlags.WordBreak) Dim letterHeight As Integer = CInt(s.Height) Dim displayableLines As Integer = rtb.Height / letterHeight

This uses the height of a test letter 'A' in the current font to find the line height in pixels.

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