Get image dimensions in C# · oneimage cookbook

Call Image.Identify on the file: it reads only the headers and returns the width, height, format, frame count and metadata, without decoding any pixels. For the size a phone photo is shown at, take Width and Height from Image.Load, which applies the EXIF orientation. The program below checks NASA’s “Blue Marble” photograph and a public test photo stored sideways.

Terminal

dotnet new console -o ImageSize && cd ImageSize
dotnet add package RecursiveAI.OneImage --version 1.10.4
curl -L -o photo.jpg \
  https://images-assets.nasa.gov/image/as17-148-22727/as17-148-22727~orig.jpg
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;
 
foreach (string file in new[] { "photo.jpg", "phone.jpg" })
{
    // Headers only: fast even for very large files.
    ImageInfo info = Image.Identify(file);
    Console.WriteLine($"{file}: {info.Format.Name}, "
                    + $"stored {info.Width}x{info.Height}, "
                    + $"{info.Frames.Count} frame(s), "
                    + $"orientation {info.Metadata.Orientation}");
 
    // Load is lazy too, and reports the upright size.
    Image image = Image.Load(file);
    Console.WriteLine($"{file}: displayed {image.Width}x{image.Height}");
}

Text

photo.jpg: jpeg, stored 4579x4579, 1 frame(s), orientation 1
photo.jpg: displayed 4579x4579
phone.jpg: jpeg, stored 1800x1200, 1 frame(s), orientation 6
phone.jpg: displayed 1200x1800

Orientations 5 to 8 swap width and height, so a phone photo stored landscape is shown portrait. Use the displayed size for layout, <img> attributes and crop coordinates; use the stored size to check a file against limits.

Check uploads without exceptions

TryIdentify returns false for data that isn’t an image, is damaged, or is over the size limits, so it is a cheap gate for uploads. It takes a path, a Stream or bytes:

using OneImage;
 
byte[][] uploads = [File.ReadAllBytes("photo.jpg"), "not an image"u8.ToArray()];
 
foreach (byte[] bytes in uploads)
{
    if (Image.TryIdentify(bytes, out ImageInfo? info))
    {
        Console.WriteLine($"{info.Format.Name} {info.Width}x{info.Height}");
    }
    else
    {
        Console.WriteLine("rejected: not a supported image");
    }
}

Text

jpeg 4579x4579
rejected: not a supported image

Limits

The format is detected from the content, not the file name. Formats the core doesn’t read, such as HEIC and AVIF, are rejected.
SVG, PSD and PDF report their size only when their add-on format is registered in LoadOptions. An SVG’s size comes from its width, height or viewBox; a PDF page’s from its size in points and the render DPI.
I/O errors still throw. TryIdentify returns false for bad image data, not for a missing file.

For the camera, date and GPS position, see Read EXIF data. Errors and limits are in the guide.