How do I calculate the maximum scroll value RichTextBox?

Question

There is a RichTextBox with scrollbars disabled. Tried to find out the maximum scroll value, using:

• GetScrollPos - returns 0;
• GetScrollPosInfo - returns 0;
• GetScrollRange - returns 100;
• https://techarks.ru/qa/csharp/poluchit-polosu-prokrutki-q-I4/ - allows you to know only the pitch (px), if you scroll to the end will show the maximum, if you want to know it in the beginning - nothing (requires scrolling to the end);
• float heightLine = targetCtrl.Font.GetHeight() / 3; Maximum = (int)Math.Ceiling(targetCtrl.GetPreferredSize(targetCtrl.Size).Height - targetCtrl.Height + heightLine); - at the beginning the maximum is correct, but as you add or change the size of the elements you get a value that is smaller than the real maximum.

Answer 1

The purpose of all the calculations presented below is: to determine the maximum scroll value of the RichTextBox for working with the custom scrollbar.

Useful materials / tips:

1. To find out the value that the standard scrollbar receives while scrolling, use the Microsoft Spy++ program;
2. Absolute Difference Calculator: https://calculatorpack.com/absolute-difference-calculator.

Determine the maximum vertical scroll value:

NOTE: (code works correctly if you don't scale the RichTextBox contents).

private void DetermineVMax(RichTextBox RTB) {      
  // Y position of the first character
  int y1 = RTB.GetPositionFromCharIndex(0).Y;

  // Y position of the last character
  int y2 = RTB.GetPositionFromCharIndex(RTB.TextLength).Y;

  // First option: The height of the line - returns 43
  double heightLine = Math.Ceiling(RTB.Font.GetHeight());

  // Second option: The height of the line - returns 45
  // NOTE: GetFirstCharIndexFromLine( NUMBER )
  // 0 is the first line, 1 is the second line, etc.
  //int index = targetCtrl.GetFirstCharIndexFromLine(1);
  //int heightLine = targetCtrl.GetPositionFromCharIndex(index).Y;

  // Absolute difference between the position of the 1st and the last characters
  int absoluteDifference = Math.Abs(y1 - y2);

  // RichTextBox height
  int heightRtb = RTB.ClientSize.Height;

  // Maximum vertical scroll value
  // NOTE: if you don't add a line height, the RTB content will not be displayed in full
  int max = absoluteDifference - heightRtb + (int)heightLine;
}

Thanks to the stack overflow response (Size RichTextBox according to contents), a another solution was found for calculating the maximum value of vertical / horizontal scrolling (taking into account the scaling inside the RichTextBox).

Order of actions

1. Sign the RichTextBox to "ContentsResized" and "Resize" events;
2. Use the code below.

Determine the maximum vertical scroll value:

int heightContentRtb, widthContentRtb;

private void DetermineVMax(RichTextBox RTB) {
    int heightContainerRtb = RTB.ClientSize.Height;
    int max = heightContentRtb - heightContainerRtb + 1;
}    

Determine the maximum horizontal scroll value:

private void DetermineHMax(RichTextBox RTB) {
    int widthContainerRtb = RTB.ClientSize.Width;
    int max = widthContentRtb - widthContainerRtb + 1;
}

Code inside the "ContentsResized" and "Resize" events

private void richTextBox1_ContentsResized(object sender, ContentsResizedEventArgs e) {
    //heightContentRtb = e.NewRectangle.Height; DetermineVMax(richTextBox1);
    widthContentRtb = e.NewRectangle.Width; DetermineHMax(richTextBox1);
}

private void richTextBox1_Resize(object sender, EventArgs e) {
    //DetermineVMax(richTextBox1);
    DetermineHMax(richTextBox1);
}