Drag-and-drop modifies read-only WinForm container (plus other peculiarities of drag-and-drop)

Question

Sampling drag-and-drop between WinForm RichTextBoxes within one application and between them and external applications revealed two interesting observations (item 1 certainly appears to be a bug; in a perfect world, item 2 would probably be as well):

1. Some drag-and-drop operations delete the dragged text from the source container, whether or not it is set to read-only. (Thanks to Mark Morgan for first noticing this in his bug report on my open-source site.)
2. Whether text is retained or deleted from a source container is inconsistent among different applications.

I could not find any definitive reference indicating what drag-and-drop behavior is supposed to be. The closest I found was on page 476 of the Windows User Experience Interaction Guidelines (for Vista): "Dragging and dropping: Object is moved or copied to the drop target". Well, that certainly aligns with my observations; some applications move the object, others copy it!

The questions: I would like to find a workaround for item 1 above; it irks me that a read-only container is not inviolate! As a secondary question, I am wondering if someone has a reference to how drag-and-drop is supposed to behave? When is it a move and when is it a copy?

My sample WinForm application (code below) contains two RichTextBox controls, the left one being read-only (call this RTB1) and initialized with some text; the right one (RTB2) being read/write so it may receive text. Both have drag-and-drop enabled for the test. Here are the combinations I tested; notice that in each grouping there is at least one "odd-man-out" :

---

1. From RTB1 to RTB2: move
2. From RTB1 to other RTB (external): move
3. From RTB1 to WordPad: copy
4. From RTB1 to Word2003: move
5. From RTB1 to Outlook2003: copy

6. From RTB1 to Firefox3.0: copy

   ---

7. From RTB2 to other RTB (external): move

8. From RTB2 to WordPad: copy
9. From RTB2 to Outlook2003: copy

10. From RTB2 to Firefox3.0: copy

    ---

11. From Outlook2003 to RTB2: move

12. From WordPad to RTB2: move
13. From Word2003 to RTB2: move
14. From other RTB (external) to RTB2: move

15. From Firefox3.0 to RTB2: copy

    ---

16. From Word2003 to Outlook2003: copy

17. From Outlook2003 to Word2003 : move

    ---

Tests run on WinXP.
Test app compiled with .NET 2.0 (tried a couple with .NET 3.5 with the same results).

---

Here is the sample application:

using System;
using System.Windows.Forms;

namespace RichTextBoxTest
{
    static class Program
    {
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }
    }

    partial class Form1 : Form
    {

        private RichTextBox richTextBox1 = new RichTextBox();
        private RichTextBox richTextBox2 = new RichTextBox();

        public Form1()
        {
            InitializeComponent();
        }

        private System.ComponentModel.IContainer components = null;

        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        private void InitializeComponent()
        {
            this.SuspendLayout();
            // 
            // richTextBox1
            // 
            this.richTextBox1.EnableAutoDragDrop = true;
            this.richTextBox1.Location = new System.Drawing.Point(34, 25);
            this.richTextBox1.ReadOnly = true;
            this.richTextBox1.Size = new System.Drawing.Size(122, 73);
            this.richTextBox1.Text = "some stuff here";
            // 
            // richTextBox2
            // 
            this.richTextBox2.EnableAutoDragDrop = true;
            this.richTextBox2.Location = new System.Drawing.Point(177, 25);
            this.richTextBox2.Size = new System.Drawing.Size(122, 73);
            this.richTextBox2.Text = "";
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F, 13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(338, 122);
            this.Controls.Add(this.richTextBox2);
            this.Controls.Add(this.richTextBox1);
            this.Text = "Form1";
            this.ResumeLayout(false);
        }
    }
}

Answer 1

Having had no tidbits submitted on this I delved further into the issue.

First, I obtained some information from Microsoft (via MSDN support) that standard drag-and-drop behavior does a move while holding down Control with drag-and-drop does a copy.

Next, consider these three modes of operation:

1. User can edit text.
2. User can move text (via drag-and-drop).
3. Application can change text programmatically.

According to Microsoft, setting read-only disables only item (1) ! To also honor read-only for item (2) one must manually code the solution rather than using the read-only property.

Well, to me this is clearly a defect. I believe read-only should disable both (1) and (2). So I submitted an official defect report to Microsoft Connect espousing this opinion. Alas, the response came back as essentially "Thanks, but not important enough issue to fix." Sigh...