Table of Contents

Read Image Info Without Decoding

When you are working with uploads, queues, or validation rules, fully decoding every image is often unnecessary work. Image.Identify() and Image.DetectFormat() let you answer the early questions first: what is this file, how large is it, how many frames does it have, what kind of pixel data does it claim to contain, and how much pixel memory might a full decode require?

Read Dimensions, Frame Count, and Pixel Info

Image.Identify() returns an ImageInfo with dimensions, frame count, pixel type, and metadata:

using SixLabors.ImageSharp;

ImageInfo imageInfo = Image.Identify("input.webp");

Console.WriteLine($"{imageInfo.Width}x{imageInfo.Height}");
Console.WriteLine($"Frames: {imageInfo.FrameCount}");
Console.WriteLine($"Bits per pixel: {imageInfo.PixelType.BitsPerPixel}");
Console.WriteLine($"Estimated pixel memory: {imageInfo.GetPixelMemorySize():N0} bytes");

Estimate Pixel Memory Before Decoding

ImageInfo.GetPixelMemorySize() reports the estimated in-memory size of the decoded pixel data represented by the identified image.

using SixLabors.ImageSharp;

ImageInfo imageInfo = Image.Identify("input.gif");
long pixelBytes = imageInfo.GetPixelMemorySize();

if (pixelBytes > 256L * 1024 * 1024)
{
    throw new InvalidOperationException("Image is too large to decode safely.");
}

This is especially useful for upload validation and other untrusted-input workflows. A file can be small on disk but still expand into a very large decoded pixel budget, especially for multi-frame formats such as GIF, animated WebP, or TIFF. If frame metadata is available, the reported size includes all frames.

Inspect the Encoded Pixel Type

ImageInfo.PixelType gives you the encoded pixel characteristics reported by the format metadata. This is more than a single bit-depth number. PixelTypeInfo can tell you whether the source is indexed, grayscale, RGB, alpha-bearing, or higher precision:

using SixLabors.ImageSharp;
using SixLabors.ImageSharp.PixelFormats;

ImageInfo imageInfo = Image.Identify("input.tiff");
PixelTypeInfo pixelType = imageInfo.PixelType;

Console.WriteLine($"Bits per pixel: {pixelType.BitsPerPixel}");
Console.WriteLine($"Color type: {pixelType.ColorType}");
Console.WriteLine($"Alpha: {pixelType.AlphaRepresentation}");

if (pixelType.ComponentInfo is { } componentInfo)
{
    Console.WriteLine($"Components: {componentInfo.ComponentCount}");
    Console.WriteLine($"Max component precision: {componentInfo.GetMaximumComponentPrecision()}");
}

This is especially useful before format conversion, because the same pixel-type information is used by ImageSharp's format-bridging metadata to choose the best destination encoding options the target format can support.

Detect the Encoded Format

If you specifically want to know what encoded format a file contains, use Image.DetectFormat():

using SixLabors.ImageSharp;
using SixLabors.ImageSharp.Formats;

IImageFormat format = Image.DetectFormat("input.bin");

Console.WriteLine(format.Name);

This is useful when file extensions are missing or untrustworthy.

Use DetectFormat() when routing depends only on the encoded format. Use Identify() when you need dimensions, frame count, pixel type, or metadata-driven decisions. DetectFormat() answers a narrower question and does less work.

Use Async APIs

For asynchronous workflows, use IdentifyAsync():

using SixLabors.ImageSharp;

ImageInfo imageInfo = await Image.IdentifyAsync("input.webp");

Console.WriteLine(imageInfo.Width);
Console.WriteLine(imageInfo.Height);

Notes

  • Image.Identify() is usually much cheaper than Image.Load() for inspection-only workflows.
  • ImageInfo.Metadata still gives you access to metadata without allocating a full pixel buffer.
  • ImageInfo.PixelType includes color model, alpha behavior, bit depth, and component precision without decoding the full image.
  • ImageInfo.GetPixelMemorySize() estimates decoded pixel memory before you commit to a full load.
  • Image.DetectFormat() is focused on encoded format detection, while Image.Identify() returns the broader inspection result.
  • Identification is not a replacement for decode-time error handling. It is a cheap preflight step; malformed input can still fail later when pixels are decoded.

For more detail, see Loading, Identifying, and Saving, Working with Metadata, Convert Between Formats, and Pixel Formats.

Practical Guidance

  • Use DetectFormat(...) for routing by encoded format only.
  • Use Identify(...) when dimensions, frame count, pixel type, or metadata affect the decision.
  • Use GetPixelMemorySize() before decoding untrusted or very large inputs.
  • Still handle decode failures; identification is preflight, not a guarantee that the full image is valid.
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