How to set the TAB width in a Windows Forms TextBox control?

Question

Given a WinForms TextBox control with MultiLine = true and AcceptsTab == true, how can I set the width of the tab character displayed?

I want to use this as a quick and dirty script input box for a plugin. It really doesn't need to be fancy at all, but it would be nice if tabs were not displayed as 8 characters wide...

Answer 1

I think sending the EM_SETTABSTOPS message to the TextBox will work.

// set tab stops to a width of 4 private const int EM_SETTABSTOPS = 0x00CB;  [DllImport("User32.dll", CharSet = CharSet.Auto)] public static extern IntPtr SendMessage(IntPtr h, int msg, int wParam, int[] lParam);  public static void SetTabWidth(TextBox textbox, int tabWidth) {     Graphics graphics = textbox.CreateGraphics();     var characterWidth = (int)graphics.MeasureString("M", textbox.Font).Width;     SendMessage         ( textbox.Handle         , EM_SETTABSTOPS         , 1         , new int[] { tabWidth * characterWidth }         ); } 

This can be called in the constructor of your Form, but beware: Make sure InitializeComponents is run first.

• Link at MSDN
• Here is another link

Answer 2

I know you are using a TextBox currently, but if you can get away with using a RichTextBox instead, then you can use the SelectedTabs property to set the desired tab width:

richTextBox.SelectionTabs = new int[] { 15, 30, 45, 60, 75}; 

Note that these offsets are pixels, not characters.

Answer 3

The example offered is incorrect.

The EM_SETTABSTOPS message expects the tab sizes to be specified in dialog template units and not in pixels. After some digging around, it appears that a dialog template unit equals to 1/4th the average width of the window's character. So you'll need to specify 8 for 2 characters long tabs, 16 for four charachters, and so on.

So the code can be simplified as:

public static void SetTabWidth(TextBox textbox, int tabWidth)
{
    SendMessage(textbox.Handle, EM_SETTABSTOPS, 1, 
            new int[] { tabWidth * 4 });
}

Answer 4

With the use of extension methods, you can add a new method to the TextBox control class. This is my implementation (including an additional extension method that gives you the coordinates for the current location of the insert caret) from what I gathered from the previous contributors above:

using System;
using System.Drawing;
using System.Windows.Forms;
using System.Runtime.InteropServices;

namespace Extensions
{
    public static class TextBoxExtension
    {
        private const int EM_SETTABSTOPS = 0x00CB;

        [DllImport("User32.dll", CharSet = CharSet.Auto)]
        private static extern IntPtr SendMessage(IntPtr h, int msg, int wParam, int[] lParam);

        public static Point GetCaretPosition(this TextBox textBox)
        {
            Point point = new Point(0, 0);

            if (textBox.Focused)
            {
                point.X = textBox.SelectionStart - textBox.GetFirstCharIndexOfCurrentLine() + 1;
                point.Y = textBox.GetLineFromCharIndex(textBox.SelectionStart) + 1;
            }

            return point;
        }

        public static void SetTabStopWidth(this TextBox textbox, int width)
        {
            SendMessage(textbox.Handle, EM_SETTABSTOPS, 1, new int[] { width * 4 });
        }
    }
}