UnityHost.cs

public class UnityHost : HwndHost
{
    private const uint STARTF_USEPOSITION = 0x0004;
    private const uint STARTF_USESIZE = 0x0002;

    private Process _childProcess;
    private HandleRef _childHandleRef;

    public string AppPath { get; set; }
    public HandleRef Child => _childHandleRef;

    protected override HandleRef BuildWindowCore(HandleRef hwndParent)
    {
        var sInfo = new STARTUPINFO { dwFlags = STARTF_USESIZE | STARTF_USEPOSITION };
        var pSec = new SECURITY_ATTRIBUTES { nLength = Marshal.SizeOf<SECURITY_ATTRIBUTES>() };
        var tSec = new SECURITY_ATTRIBUTES { nLength = Marshal.SizeOf<SECURITY_ATTRIBUTES>() };
        var cmdline = $"-parentHWND {hwndParent.Handle}";
        var success = Kernel32.CreateProcess(AppPath, cmdline, ref pSec, ref tSec,
            false, 0, IntPtr.Zero, null, ref sInfo, out var pInfo);
        if (!success) throw new Exception("Failed to create child process");
        _childProcess = Process.GetProcessById(pInfo.dwProcessId);
        while (true)
        {
            var hwndChild = User32.FindWindowEx(hwndParent.Handle, IntPtr.Zero, null, null);
            if (hwndChild != IntPtr.Zero)
            {
                return (_childHandleRef = new HandleRef(this, hwndChild));
            }
            Thread.Sleep(100);
        }
    }

    protected override void DestroyWindowCore(HandleRef hwnd)
    {
        _childProcess.Dispose();
    }
}

static class Kernel32
{
    [DllImport("kernel32.dll", SetLastError = true, CharSet = CharSet.Auto)]
    public static extern bool CreateProcess(
        string lpApplicationName,
        string lpCommandLine,
        ref SECURITY_ATTRIBUTES lpProcessAttributes,
        ref SECURITY_ATTRIBUTES lpThreadAttributes,
        bool bInheritHandles,
        uint dwCreationFlags,
        IntPtr lpEnvironment,
        string lpCurrentDirectory,
        [In] ref STARTUPINFO lpStartupInfo,
        out PROCESS_INFORMATION lpProcessInformation);
}

static class User32
{
    [DllImport("user32.dll")]
    public static extern IntPtr FindWindowEx(IntPtr hParent, IntPtr hChildAfter, string pClassName, string pWindowName);
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct STARTUPINFO
{
    public uint cb;
    public string lpReserved;
    public string lpDesktop;
    public string lpTitle;
    public uint dwX;
    public uint dwY;
    public uint dwXSize;
    public uint dwYSize;
    public uint dwXCountChars;
    public uint dwYCountChars;
    public uint dwFillAttribute;
    public uint dwFlags;
    public short wShowWindow;
    public short cbReserved2;
    public IntPtr lpReserved2;
    public IntPtr hStdInput;
    public IntPtr hStdOutput;
    public IntPtr hStdError;
}

[StructLayout(LayoutKind.Sequential)]
public struct PROCESS_INFORMATION
{
    public IntPtr hProcess;
    public IntPtr hThread;
    public int dwProcessId;
    public int dwThreadId;
}

[StructLayout(LayoutKind.Sequential)]
public struct SECURITY_ATTRIBUTES
{
    public int nLength;
    public IntPtr lpSecurityDescriptor;
    public int bInheritHandle;
}

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct STARTUPINFOEX
{
    public STARTUPINFO StartupInfo;
    public IntPtr lpAttributeList;
}