How can I show scrollbars on a System.Windows.Forms.TextBox only when the text doesn't fit?

Question

For a System.Windows.Forms.TextBox with Multiline=True, I'd like to only show the scrollbars when the text doesn't fit.

This is a readonly textbox used only for display. It's a TextBox so that users can copy the text out. Is there anything built-in to support auto show of scrollbars? If not, should I be using a different control? Or do I need to hook TextChanged and manually check for overflow (if so, how to tell if the text fits?)

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Not having any luck with various combinations of WordWrap and Scrollbars settings. I'd like to have no scrollbars initially and have each appear dynamically only if the text doesn't fit in the given direction.

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@nobugz, thanks, that works when WordWrap is disabled. I'd prefer not to disable wordwrap, but it's the lesser of two evils.

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@André Neves, good point, and I would go that way if it was user-editable. I agree that consistency is the cardinal rule for UI intuitiveness.

Answer 1

I also made some experiments, and found that the vertical bar will always show if you enable it, and the horizontal bar always shows as long as it's enabled and WordWrap == false.

I think you're not going to get exactly what you want here. However, I believe that users would like better Windows' default behavior than the one you're trying to force. If I were using your app, I probably would be bothered if my textbox real-estate suddenly shrinked just because it needs to accomodate an unexpected scrollbar because I gave it too much text!

Perhaps it would be a good idea just to let your application follow Windows' look and feel.

Answer 2

Play a bit with the combination of the Multiline, WordWrap and ScrollBars property and see what happens. Windows does normally just what I want.

Answer 3

This class inherits TextBox. Whenever text is changed it checks for fit to enable/disable scrollbars.

using System;
using System.Drawing;
using System.Windows.Forms;

public class MyTextBox : TextBox {
  private bool mScrollbars;
  public MyTextBox() {
    this.Multiline = true;
    this.ReadOnly = true;
  }
  private void checkForScrollbars() {
    bool scroll = false;
    int cnt = this.Lines.Length;
    if (cnt > 1) {
      int pos0 = this.GetPositionFromCharIndex(this.GetFirstCharIndexFromLine(0)).Y;
      if (pos0 >= 32768) pos0 -= 65536;
      int pos1 = this.GetPositionFromCharIndex(this.GetFirstCharIndexFromLine(1)).Y;
      if (pos1 >= 32768) pos1 -= 65536;
      int h = pos1 - pos0;
      scroll = cnt * h > (this.ClientSize.Height - 6);  // 6 = padding
    }
    if (scroll != mScrollbars) {
      mScrollbars = scroll;
      this.ScrollBars = scroll ? ScrollBars.Vertical : ScrollBars.None;
    }
  }

  protected override void OnTextChanged(EventArgs e) {
    checkForScrollbars();
    base.OnTextChanged(e);
  }

  protected override void OnClientSizeChanged(EventArgs e) {
    checkForScrollbars();
    base.OnClientSizeChanged(e);
  }
}

Answer 4

I came across this question when I wanted to solve the same problem.

The easiest way to do it is to change to System.Windows.Forms.RichTextBox. The ScrollBars property in this case can be left to the default value of RichTextBoxScrollBars.Both, which indicates "Display both a horizontal and a vertical scroll bar when needed." It would be nice if this functionality were provided on TextBox.

Answer 5

There's an extremely subtle bug in nobugz's solution that results in a heap corruption, but only if you're using AppendText() to update the TextBox.

Setting the ScrollBars property from OnTextChanged will cause the Win32 window (handle) to be destroyed and recreated. But OnTextChanged is called from the bowels of the Win32 edit control (EditML_InsertText), which immediately thereafter expects the internal state of that Win32 edit control to be unchanged. Unfortunately, since the window is recreated, that internal state has been freed by the OS, resulting in an access violation.

So the moral of the story is: don't use AppendText() if you're going to use nobugz's solution.

Answer 6

That solution doesn't work. I think it may have to do with a non monotype font. The closest I got was using g.drawstring, but that seems to not display empty lines the same way (it uses more blank space to display empty lines)