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//For complete examples and data files, please go to https://github.com/groupdocsmetadata/GroupDocs_Metadata_NET
// path to the image
string imagePath = @"C:\\image.jpg";
string base64String;

// use System.Drawing to get image base64 string
using ( Image image = Image.FromFile(imagePath))
{
    using (MemoryStream m = new MemoryStream())
    {
        image.Save(m, image.RawFormat);
        byte[] imageBytes = m.ToArray();

        // Convert byte[] to Base64 String
        base64String = Convert.ToBase64String(imageBytes);
    }
}

// xmp thumbnail width
int thumbnailWidth = 100;

// xmp thumbnail height
int thumbnailHeight = 50;

// create xmp thumbnail 
Thumbnail thumbnail = new Thumbnail(thumbnailWidth, thumbnailHeight);

// add provide image base64 string
thumbnail.ImageBase64 = base64String;