Margem em Textbox de Windows Form

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

public sealed class NativeMethods
{
    public static readonly IntPtr TRUE  = new IntPtr(1);					

    public static readonly IntPtr FALSE = new IntPtr(0);

    public enum WindowsMessages : int
    {
        WM_NCCALCSIZE = 0x0083,
        WM_NCPAINT = 0x0085
    }

    [StructLayout(LayoutKind.Sequential)]

    public struct RECT
    {
        public int left, top, right, bottom;

        public RECT(int left, int top, int right, int bottom)
        {
            this.left   = left;
            this.top    = top;
            this.right  = right;
            this.bottom = bottom;
        }
    }

    public sealed class User32
    {
        [DllImport("user32.dll")]

        public static extern IntPtr GetWindowDC(IntPtr hWnd);
    }
}

public class TextBoxEx : TextBox
{
    protected virtual void OnNcCalcSize(ref Message m)
    {
        if (m.WParam == NativeMethods.FALSE)
        {
            NativeMethods.RECT rect = (NativeMethods.RECT) m.GetLParam(typeof(NativeMethods.RECT));

            // Ajusta o espaçamento à esquerda em 15px.
            rect.left += 15;

            Marshal.StructureToPtr(rect, m.LParam, false);
        }

        base.WndProc(ref m);
    }

    protected virtual void OnNcPaint(ref Message m)
    {
        // Preenche a cor de fundo do controle.
        using (Graphics g = this.CreateNcGraphics())
        {
            g.Clear(base.BackColor);
        }

        base.WndProc(ref m);
    }

    protected virtual Graphics CreateNcGraphics()
    {
        // Cria um objeto Graphics que permite desenhar na non-client area do controle.
        IntPtr hDc = NativeMethods.User32.GetWindowDC(base.Handle);

        if (hDc != IntPtr.Zero)
            return Graphics.FromHdcInternal(hDc);

        return null;
    }

    protected override void WndProc(ref Message m)
    {
        switch (m.Msg)
        {
            case (int) NativeMethods.WindowsMessages.WM_NCCALCSIZE:
                {
                    this.OnNcCalcSize(ref m);
                    break;
                }
            case (int) NativeMethods.WindowsMessages.WM_NCPAINT:
                {
                    this.OnNcPaint(ref m);
                    break;
                }
            default:
                base.WndProc(ref m);
                break;
        }
    }
}