title: How can I send WM_DROPFILES from C#?
body: <p>I want to use C# code to simulate a user dragging and dropping a file onto a control in a separate process.  As a stepping stone to this goal, I am trying to send a WM_DROPFILES message to my own TextBox and verifying that the DragDrop event is triggered.</p>
  
  <p>With the code below in a Form containing a single TextBox and two Buttons, clicking on button1 successfully sets the text of textBox1 to "Hello world".  So, it seems that I'm using SendMessage correctly and am able to supply arguments via pointers.  Dragging and dropping a file from Windows Explorer onto textBox1 displays the MessageBox, so textBox1 is set up to receive drag-dropped files correctly.  However, when I click button2, nothing happens.  Why don't I see a MessageBox when I click button2?</p>
  
  <pre><code>using System;
  using System.Data;
  using System.Linq;
  using System.Runtime.InteropServices;
  using System.Windows.Forms;
  
  namespace StackOverflow
  {
      public partial class BadDragDrop : Form
      {
          #region WINAPI
  
          [Serializable]
          [StructLayout(LayoutKind.Sequential)]
          struct POINT
          {
              public Int32 X;
              public Int32 Y;
          }
  
          [Serializable]
          [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Ansi)]
          class DROPFILES
          {
              public Int32 size;
              public POINT pt;
              public Int32 fND;
              public Int32 WIDE;
          }
  
          const uint WM_DROPFILES = 0x0233;
          const uint WM_SETTEXT = 0x000C;
  
          [DllImport("Kernel32.dll", SetLastError = true)]
          static extern int GlobalLock(IntPtr Handle);
  
          [DllImport("Kernel32.dll", SetLastError = true)]
          static extern int GlobalUnlock(IntPtr Handle);
  
          [DllImport("user32.dll", SetLastError = true)]
          static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
  
          #endregion
  
          public BadDragDrop()
          {
              InitializeComponent();
              textBox1.AllowDrop = true;
          }
  
          private void button1_Click(object sender, EventArgs e)
          {
              string textToSet = "Hello world\0";
              IntPtr p = Marshal.AllocHGlobal(textToSet.Length);
              Marshal.Copy(textToSet.Select(c =&gt; (byte)c).ToArray(), 0, p, textToSet.Length);
              int success = GlobalUnlock(p);
              SendMessage(textBox1.Handle, WM_SETTEXT, IntPtr.Zero, p);
              Marshal.FreeHGlobal(p);
          }
  
          private void button2_Click(object sender, EventArgs e)
          {
              string filePath = @"C:\Windows\win.ini" + "\0\0";
  
              DROPFILES s = new DROPFILES()
              {
                  size = Marshal.SizeOf&lt;DROPFILES&gt;(),
                  pt = new POINT() { X = 10, Y = 10 },
                  fND = 0,
                  WIDE = 0,
              };
  
              int wparamLen = s.size + filePath.Length;
  
              IntPtr p = Marshal.AllocHGlobal(wparamLen);
              int iSuccess = GlobalLock(p);
  
              Marshal.StructureToPtr(s, p, false);
              Marshal.Copy(filePath.Select(c =&gt; (byte)c).ToArray(), 0, p + s.size, filePath.Length);
  
              iSuccess = GlobalUnlock(p);
  
              var verify = new byte[wparamLen];
              Marshal.Copy(p, verify, 0, wparamLen);
  
              var ipSuccess = SendMessage(textBox1.Handle, WM_DROPFILES, p, IntPtr.Zero);
              Marshal.FreeHGlobal(p);
          }
  
          private void textBox1_DragDrop(object sender, DragEventArgs e)
          {
              MessageBox.Show(this, "Drag drop!");
          }
  
          private void textBox1_DragOver(object sender, DragEventArgs e)
          {
              e.Effect = DragDropEffects.Copy;
          }
      }
  }
  </code></pre>