Create a thumbnail in C# · oneimage cookbook

Call Image.Thumbnail with the file and the largest width you want, then save. It is a resize with the defaults a published copy needs: the photo is turned upright, never enlarged, converted to sRGB and stripped of EXIF, so no GPS position leaks. The program below uses a public test photo that is stored sideways, the way phones store many portraits.

Terminal

dotnet new console -o Thumbnail && cd Thumbnail
dotnet add package RecursiveAI.OneImage --version 1.10.4
repo=https://raw.githubusercontent.com/recurser/exif-orientation-examples
curl -L -o phone.jpg $repo/master/Portrait_6.jpg

OneImage needs the .NET 10 SDK. Put this in Program.cs and run dotnet run:

using OneImage;
 
ImageInfo stored = Image.Identify("phone.jpg");
Console.WriteLine($"phone.jpg: stored {stored.Width}x{stored.Height}, "
                + $"orientation {stored.Metadata.Orientation}");
 
// At most 400 px wide, upright, sRGB, EXIF/XMP/IPTC removed.
Image.Thumbnail("phone.jpg", 400).Save("thumb.jpg");
 
// A 200x200 square that fills the box, for avatars and grids, as WebP.
Image.Thumbnail("phone.jpg", 200, 200, fit: Fit.Cover).Save("square.webp");
 
foreach (string file in new[] { "thumb.jpg", "square.webp" })
{
    ImageInfo info = Image.Identify(file);
    string exif = info.Metadata.ReadExif() is null ? "no EXIF" : "EXIF kept";
    long kb = new FileInfo(file).Length / 1024;
    Console.WriteLine($"{file}: {info.Width}x{info.Height}, {exif}, {kb} KB");
}

Text

phone.jpg: stored 1800x1200, orientation 6
thumb.jpg: 400x600, no EXIF, 33 KB
square.webp: 200x200, no EXIF, 3 KB

The photo is stored as 1800×1200 with an EXIF orientation of 6 (“rotate 90° to show”), and both thumbnails come out upright. In order, Thumbnail applies the orientation, resizes (decoding a JPEG at 1/2, 1/4 or 1/8 scale when that is enough), converts to sRGB with a compact embedded profile, and removes EXIF, XMP and IPTC. The output format follows the extension.

Thumbnails for a folder

Image.TryLoad returns false for files that aren’t images or are damaged, so one bad file doesn’t stop the batch:

using OneImage;
 
Directory.CreateDirectory("photos");
File.Copy("phone.jpg", "photos/phone.jpg", overwrite: true);
File.WriteAllText("photos/notes.txt", "not an image");
Directory.CreateDirectory("thumbs");
 
foreach (string path in Directory.EnumerateFiles("photos"))
{
    if (!Image.TryLoad(path, out Image? image))
    {
        Console.WriteLine($"skipped {Path.GetFileName(path)}");
        continue;
    }
    string name = Path.GetFileNameWithoutExtension(path) + ".webp";
    string target = Path.Combine("thumbs", name);
    Image.Thumbnail(image, 320).Save(target);
    Console.WriteLine($"{target}: {Image.Identify(target).Width} px wide");
}

Text

skipped notes.txt
thumbs/phone.webp: 320 px wide

Thumbnail also takes a Stream or a byte[], for uploads and blob storage.

Options

height: and fit: set a box: Fit.Cover fills it and crops, with anchor: Anchor.Smart to keep the busiest part; Fit.Pad fits and pads.
upscale: true allows enlarging a source smaller than the box.
keepMetadata: true keeps EXIF, XMP and IPTC.

Limits

It never enlarges by default. A 300 px source asked for at 400 stays 300 px wide. For an exact size, use Resize.
It strips metadata. Copyright and caption fields in EXIF or IPTC go too, unless you pass keepMetadata: true.
SVG, PDF and PSD need their add-on package registered first; see SVG to PNG and PDF to image.

To serve thumbnails from URLs like photo.jpg?width=400, see Resize images on the fly in ASP.NET Core. Every parameter is in the guide.