For example I have a WPF window:

public class MyWindow : Window
{
    public MyWindow()
    {
        InitializeStyle();
        InitializeContent();
        InitializeSizePosition();
    }

    // ...
}

Now I want to render this window into a bitmap before showing it in foreground.

I tried the following things:

1. Using RenderTargetBitmap

   var window = new MyWindow();
   RenderTargetBitmap bitmap = new((int)window.Width, (int)window.Height, 96, 96, PixelFormats.Pbgra32);
   bitmap.Render(window);
   // window.Show();
   

   I get a full black bitmap. Same thing happens if I call these functions inside the window (bitmap.Render(this)), but it a just render a component of the window (e.g. a Grid), it works fine.

2. Ensuring window handle

   As someone suggested, I tried WindowInteropHelper to ensure window handle, but it doesn't work.

   var window = new MyWindow();
   var helper = new WindowInteropHelper(window);
   helper.EnsureHandle();
   
   RenderTargetBitmap bitmap = new((int)window.Width, (int)window.Height, 96, 96, PixelFormats.Pbgra32);
   bitmap.Render(window);
   // window.Show();
   

3. Call Measure(), Arrange() & UpdateLayout() manually

   I called these methods both inside & outside the window class before getting bitmap, it doesn't work either, even with a fixed window size.

I want to know if there's a way to solve my problem or workaround?