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using System;
using System.Collections;
using System.Drawing;
using System.Windows.Forms;
using System.Windows.Forms.Design;
using System.Windows.Forms.Design.Behavior;

namespace DesignerSnapLinesDemo.Design
{
  internal class BetterTextBoxDesigner : ControlDesigner
  {
    #region Properties

    public override SelectionRules SelectionRules
    {
      get { return SelectionRules.Visible | SelectionRules.Moveable | SelectionRules.LeftSizeable | SelectionRules.RightSizeable; }
    }

    public override IList SnapLines
    {
      get
      {
        IList snapLines;
        int textBaseline;
        SnapLine snapLine;
        BetterTextBox control;

        control = (BetterTextBox)this.Control;

        snapLines = base.SnapLines;
        textBaseline = this.GetFontAscent(control) + 1;

        textBaseline += 2; // offset for the borders of the control

        textBaseline += 2; // inner margin

        snapLine = new SnapLine(SnapLineType.Baseline, textBaseline, SnapLinePriority.Medium);

        // Although Resharper thinks this can be a null value, the default implemention
        // *always* adds snaplines detailing the margins of the control. Therefore I can
        // happily disable this warning - it's not likely the behaviour will change

        // ReSharper disable once PossibleNullReferenceException
        snapLines.Add(snapLine);

        return snapLines;
      }
    }

    #endregion

    #region Methods

private int GetFontAscent(Control control)
{
  using (Graphics graphics = control.CreateGraphics())
  {
    return this.GetFontAscent(graphics, control.Font);
  }
}

private int GetFontAscent(IDeviceContext dc, Font font)
{
  int result;
  IntPtr hDC;
  IntPtr hFont;
  IntPtr hFontDefault;

  hDC = IntPtr.Zero;
  hFont = IntPtr.Zero;
  hFontDefault = IntPtr.Zero;

  try
  {
    NativeMethods.TEXTMETRICW textMetric;

    hDC = dc.GetHdc();

    hFont = font.ToHfont();
    hFontDefault = NativeMethods.SelectObject(hDC, hFont);

    NativeMethods.GetTextMetrics(hDC, out textMetric);

    result = textMetric.tmAscent;
  }
  finally
  {
    if (hFontDefault != IntPtr.Zero)
    {
      NativeMethods.SelectObject(hDC, hFontDefault);
    }

    if (hFont != IntPtr.Zero)
    {
      NativeMethods.DeleteObject(hFont);
    }

    dc.ReleaseHdc();
  }

  return result;
}

    #endregion
  }
}