Как изменить класс div с помощью ngClass

Просто удалите начальную часть строки «data: image / svg + xml; base64», все после запятой - это фактическое изображение base64, и вы можете сохранить это изображение в .NET следующим образом:

var bytes = Convert.FromBase64String(base64string);
using (var img = new FileStream(filePath, FileMode.Create))
{
    img.Write(bytes ,0, bytes.Length);
    img.Flush();
}

Чтобы помочь уточнить:

    class Program
    {
        static void Main(string[] args)
        {
            string b64 = "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";
            ImageFromBase64(b64);
        }

        public static  void ImageFromBase64(string base64Img)
        {
            var bytes = Convert.FromBase64String(base64Img);
            using (var img = new FileStream(@"result.svg", FileMode.Create))
            {
                img.Write(bytes, 0, bytes.Length);
                img.Flush();
            }
        }
    }

и результирующий результат. svg:

Если вы хотите использовать png, вы можете использовать Inkscape.

Вот ссылка на учебник: http://www.31a2ba2a-b718-11dc-8314-0800200c9a66.com/2012/09/how-to-convert-svg-data- к-Пурнефтегаз-image.html