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// Displaying text in an empty ListBox
// https://www.cyotek.com/blog/displaying-text-in-an-empty-listbox
// Copyright © 2018 Cyotek Ltd. All Rights Reserved.

// This work is licensed under the Creative Commons Attribution 4.0 International License.
// To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

// Found this example useful? 
// https://www.paypal.me/cyotek

using System;
using System.ComponentModel;
using System.Diagnostics;
using System.Drawing;
using System.Windows.Forms;

namespace ListBoxEmptyTextDemo
{
  public class ListBox : System.Windows.Forms.ListBox
  {
    #region Constants

    private static readonly object _eventEmptyTextChanged = new object();

    private const int WM_ERASEBKGND = 20;

    private const int WM_PAINT = 15;

    #endregion

    #region Fields

    private string _emptyText;

    #endregion

    #region Constructors

    public ListBox()
    {
      _emptyText = string.Empty;
    }

    #endregion

    #region Events

    /// <summary>
    /// Occurs when the EmptyText property value changes
    /// </summary>
    [Category("Property Changed")]
    public event EventHandler EmptyTextChanged
    {
      add { this.Events.AddHandler(_eventEmptyTextChanged, value); }
      remove { this.Events.RemoveHandler(_eventEmptyTextChanged, value); }
    }

    #endregion

    #region Properties

    [Category("Appearance")]
    [DefaultValue("")]
    [Editor("System.ComponentModel.Design.MultilineStringEditor, System.Design, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a", "System.Drawing.Design.UITypeEditor, System.Drawing, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")]
    public string EmptyText
    {
      get { return _emptyText; }
      set
      {
        if (_emptyText != value)
        {
          _emptyText = value;

          this.OnEmptyTextChanged(EventArgs.Empty);
        }
      }
    }

    #endregion

    #region Methods

    /// <summary>
    /// Raises the <see cref="EmptyTextChanged" /> event.
    /// </summary>
    /// <param name="e">The <see cref="EventArgs" /> instance containing the event data.</param>
    protected virtual void OnEmptyTextChanged(EventArgs e)
    {
      EventHandler handler;

      this.Invalidate();

      handler = (EventHandler)this.Events[_eventEmptyTextChanged];

      handler?.Invoke(this, e);
    }

    protected virtual void OnWmPaint(ref Message m)
    {
      this.DrawText();
    }

    [DebuggerStepThrough]
    protected override void WndProc(ref Message m)
    {
      base.WndProc(ref m);

      // ReSharper disable once ConvertIfStatementToSwitchStatement
      if (m.Msg == WM_PAINT)
      {
        this.OnWmPaint(ref m);
      }
      else if (m.Msg == WM_ERASEBKGND && this.ShouldDrawEmptyText())
      {
        this.ClearBackground();
      }
    }

    private void ClearBackground()
    {
      using (Graphics g = this.CreateGraphics())
      {
        g.Clear(this.BackColor);
      }
    }

    private void DrawText()
    {
      if (this.ShouldDrawEmptyText())
      {
        TextFormatFlags flags;

        flags = TextFormatFlags.ExpandTabs | TextFormatFlags.HorizontalCenter | TextFormatFlags.NoPrefix | TextFormatFlags.WordBreak | TextFormatFlags.WordEllipsis | TextFormatFlags.VerticalCenter;

        using (Graphics g = this.CreateGraphics())
        {
          TextRenderer.DrawText(g, _emptyText, this.Font, this.ClientRectangle, this.ForeColor, this.BackColor, flags);
        }
      }
    }

    private bool ShouldDrawEmptyText()
    {
      return this.Items.Count == 0 && !string.IsNullOrEmpty(_emptyText) && !this.DesignMode;
    }

    #endregion
  }
}