MFCの部品をWindows.Forms.Formに埋め込む ref: https://qiita.com/snipsnipsnip/items/19045907ae96121e8310

attach.cpp

/** WinFormsのhwndにattachし、newしたcwndを返す */
extern "C" __declspec(dllexport) CWnd* PASCAL attach(HWND hwnd) {
    auto w = std::make_unique<CWnd>();

    if (!w->Attach(hwnd)) { return nullptr; }

    // ... wに好きな部品を置く ...

    return w.release();
}

/** attachで作ったcwndをdetachとdeleteする */
extern "C" __declspec(dllexport) void PASCAL detach(Pair *p) {
    auto w = std::unique_ptr<CWnd>(cwnd);
    w->Detach();
}

SomeMFCControl.cs

public class GuestControl : Control
{
    /// <summary>アタッチしたMFCのCWnd</summary>
    private IntPtr CWnd { get; }

    [DllImport("attach")]
    private static extern IntPtr attach(IntPtr hwnd);
    [DllImport("attach")]
    private static extern void detach(IntPtr hwnd);

    public GuestControl()
    {
        CWnd = attach(Handle);
    }

    protected override void Dispose(bool disposing)
    {
       if (disposing)
       {
          detach(CWnd);
       }

       base.Dispose(disposing);
    }
}