How to create .ico file with more than 4 bpp?

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C#

icon

API

Hi,
I have problem with saving bitmap as an .ico file. Standard icon.save reduce color depth from 32 to 4 bpp which destroys good look of my picture. Is there any possibility to change that? I´ve tried bitmap save as imageformat.icon but this return gdi++ error.

this is my code:

Image image = Image.FromFile(previewPath);
Image newImage = image.GetThumbnailImage(256, 256, null, new IntPtr());

Stream IconStream = System.IO.File.OpenWrite(iconPath);
Bitmap bitmap = new Bitmap(newImage);

Icon icon = System.Drawing.Icon.FromHandle(bitmap.GetHicon());
icon.Save(IconStream);
icon.Dispose();
bitmap.Dispose();
IconStream.Close();