sample.cs

var apng = new APNG("a.png");
var bitmaps = apng.ToBitmapSources();

// Save to PNG files
foreach (var (bitmap, index) in bitmaps.Select((item, index) => (item, index)))
{
    using (var fileStream = new FileStream($"{index}.png", FileMode.Create))
    {
        var encoder = new PngBitmapEncoder();
        encoder.Frames.Add(BitmapFrame.Create(bitmap));
        encoder.Save(fileStream);
    }
}