Windows Forms Drag and Drop Functionality

Question

I appear to have all of the things necessary for Drag-n-Drop to be working, yet my code is not called when I attempt to drag something from one control to another.

• I created a Test Class to use to create data:

  class TestClass
  {
  
      public TestClass()
      {
          Name = "Blank";
          Id = -1;
      }
  
      public String Name { get; set; }
  
      public int Id { get; set; }
  
      public object Data { get; set; }
  
      public override String ToString()
      {
          return String.Format("{0}: {1}", Id, Name);
      }
  
  }
  

• I created an empty form with 2 List View controls, listView1 and listView2:

• I created event handlers for all of the Drag-n-Drop methods and put breakpoints on them:

  private void Form_DragDrop(object sender, DragEventArgs e)
  {
      e.Effect = DragDropEffects.Copy;
      if (e.Data.GetDataPresent(typeof(TestClass)))
      {
          var item = e.Data.GetData(typeof(TestClass)) as TestClass;
          if (item != null)
          {
              // ?
          }
      }
  }
  
  private void Form_DragEnter(object sender, DragEventArgs e)
  {
      e.Effect = DragDropEffects.Copy;
      if (sender.Equals(listView2))
      {
          e.Effect = DragDropEffects.All;
          if (e.Data != null)
          {
              var format = GetString(e);
              Console.WriteLine("{0} has a(n) {1} entering it.", listView2.Name, format);
          }
      }
  }
  
  private void Form_DragLeave(object sender, EventArgs e)
  {
      if (sender.Equals(listView2))
      {
  
      }
  }
  
  private void Form_DragOver(object sender, DragEventArgs e)
  {
      // Code Project Article 9017 says DragOver fires repeatedly, even if the mouse does not move.
      if ((e.X != _lastX) || (e.Y != _lastY))
      {
          _lastX = e.X;
          _lastY = e.Y;
          if (e.Data.GetDataPresent(typeof(TestClass)))
          {
  
          }
          else
          {
              e.Effect = DragDropEffects.None;
          }
      }
  }
  
  private void Form_GiveFeedback(Object sender, GiveFeedbackEventArgs e)
  {
      if (sender.Equals(listView2))
      {
          // ?
      }
  }
  
  private void Form_QueryContinueDrag(object sender, QueryContinueDragEventArgs e)
  {
      var listView = sender as ListView;
      if (listView != null)
      {
  
      }
  }
  
  private String GetString(DragEventArgs e)
  {
      var result = String.Empty;
      if ((e.AllowedEffect & DragDropEffects.Copy) == DragDropEffects.Copy)
      {
          var formats = e.Data.GetFormats();
          foreach (var format in formats)
          {
              Console.WriteLine("Data is in the format of [{0}].", format);
              result = format;
          }
      }
      return result;
  }
  

• I wired both controls up after the constructor's InitializeComponent();:

  public ListViewsForm()
  {
      InitializeComponent();
      // Setup ListView 1:
      listView1.AllowDrop = true;
      listView1.DragDrop += Form_DragDrop;
      listView1.DragEnter += Form_DragEnter;
      listView1.DragLeave += Form_DragLeave;
      listView1.DragOver += Form_DragOver;
      listView1.GiveFeedback += Form_GiveFeedback;
      listView1.QueryContinueDrag += Form_QueryContinueDrag;
      listView1.Dock = DockStyle.Fill;
      listView1.View = View.List;
      // Setup ListView 2:
      listView2.AllowDrop = true;
      listView2.DragDrop += Form_DragDrop;
      listView2.DragEnter += Form_DragEnter;
      listView2.DragLeave += Form_DragLeave;
      listView2.DragOver += Form_DragOver;
      listView2.GiveFeedback += Form_GiveFeedback;
      listView2.QueryContinueDrag += Form_QueryContinueDrag;
      listView2.Dock = DockStyle.Fill;
      // Create some data:
      var group1 = new ListViewGroup("Known Colors");
      foreach (KnownColor known in Enum.GetValues(typeof(KnownColor)))
      {
          var item = new TestClass()
          {
              Name = known.ToString(),
              Id = (int)known,
              Data = known,
          };
          var color = Color.FromKnownColor(known);
          var lvi = new ListViewItem(item.ToString(), group1) {
              BackColor = Color.FromArgb(color.ToArgb() ^ 0xffffff),
              ForeColor = color,
              Tag = item,
          };
          listView1.Items.Add(lvi);
      }
  }
  

Whenever I run the project, everything loads up fine, but any attempt to drag an item from the LEFT side to the RIGHT side does nothing. None of my break points are hit. Nothing happens.

What have I left out?

Answer 1

You are missing a handy event and the method that starts the dragging operation:

listView1.ItemDrag += listView1_ItemDrag;

void listView1_ItemDrag(object sender, ItemDragEventArgs e) {
  DoDragDrop(e.Item, DragDropEffects.Copy);
}