SendMessage not work well

[Sunzhuokai]

I use SendMessage to send WM_COPYDATA message to B process(WinForm) in A process(Flutter). the B process(Winform) can receive message sometime but not everytime!

[Sunzhuokai]

the core code：

 onTap: () {
          var tet = "test";
          var u = tet.toNativeUtf8();
          SendMessage(handle, 0x004A, 0, u.address);
          free(u);
        },

protected override void DefWndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case WM_COPYDATA:
                    this.label3.Text = DateTime.Now.ToFileTimeUtc().ToString();
                    var s = Marshal.PtrToStringAuto(m.LParam);
                    var bytes = Encoding.Unicode.GetBytes(s);
                    var str = Encoding.Default.GetString(bytes);
                    this.textBox1.Text = str;
                    m.Result = new IntPtr(1);
                    break;
                default:
                    base.DefWndProc(ref m);
                    break;
            }
        }

[ilopX]

Hi @Sunzhuokai!
Try this example.

dart code

import 'dart:ffi';

import 'package:ffi/ffi.dart';
import 'package:win32/win32.dart';

void main() {
  int hwnd = FindWindow(nullptr, TEXT('My win form'));
  sendText(hwnd, 'Hello from dart code');
}

void sendText(int hwnd, String text) {
  var uniText = text.toNativeUtf16();

  final copy = calloc<COPYDATASTRUCT>()
    ..ref.dwData = UNICODE_STRING_TYPE
    ..ref.cbData = uniText.byteLength
    ..ref.lpData = uniText;

  SendMessage(hwnd, WM_COPYDATA, 0, copy.address);

  free(uniText);
  free(copy);
}

extension StringUtf16Pointer on Pointer<Utf16> {
  int get byteLength {
    return length * 2 + 1;
  }
}

const WM_COPYDATA = 0x004A;
const UNICODE_STRING_TYPE = 666;

class COPYDATASTRUCT extends Struct {
  @IntPtr()
  external int dwData;

  @DWORD()
  external int cbData;

  external Pointer lpData;
}

c# code

    public partial class Form1 : Form
    {
        const int WM_COPYDATA = 0x004A;
        IntPtr UNICODE_STRING_TYPE = (IntPtr)666;

        [StructLayout(LayoutKind.Sequential)]
        struct COPYDATASTRUCT
        {
            public IntPtr dwData;
            public int cbData;
            public IntPtr lpData;
        }

        public Form1()
        {
            InitializeComponent();
            Text = "My win form";
        }

        protected override void WndProc(ref Message m)
        {
            switch (m.Msg)
            {
                case WM_COPYDATA:
                    var copy = (COPYDATASTRUCT)m.GetLParam(typeof(COPYDATASTRUCT));
                    if (copy.dwData == UNICODE_STRING_TYPE) {
                        var text = Marshal.PtrToStringUni(copy.lpData);
                        textBox1.AppendText(text + "\n");
                    }
                    break;
                default:
                    base.DefWndProc(ref m);
                    break;
            }
        }
    }