Determine file type of an image

Question

I'm downloading some images from a service that doesn't always include a content-type and doesn't provide an extension for the file I'm downloading (ugh, don't ask).

What's the best way to determine the image format in .NET?

The application that is reading these downloaded images needs to have a proper file extension or all hell breaks loose.

Answer 1

A probably easier approach would be to use Image.FromFile() and then use the RawFormat property, as it already knows about the magic bits in the headers for the most common formats, like this:

Image i = Image.FromFile("c:\\foo"); if (System.Drawing.Imaging.ImageFormat.Jpeg.Equals(i.RawFormat))      MessageBox.Show("JPEG"); else if (System.Drawing.Imaging.ImageFormat.Gif.Equals(i.RawFormat))     MessageBox.Show("GIF"); //Same for the rest of the formats 

Answer 2

You can use code below without reference of System.Drawing and unnecessary creation of object Image. Also you can use Alex solution even without stream and reference of System.IO.

public enum ImageFormat {     bmp,     jpeg,     gif,     tiff,     png,     unknown }  public static ImageFormat GetImageFormat(Stream stream) {     // see http://www.mikekunz.com/image_file_header.html     var bmp = Encoding.ASCII.GetBytes("BM");     // BMP     var gif = Encoding.ASCII.GetBytes("GIF");    // GIF     var png = new byte[] { 137, 80, 78, 71 };    // PNG     var tiff = new byte[] { 73, 73, 42 };         // TIFF     var tiff2 = new byte[] { 77, 77, 42 };         // TIFF     var jpeg = new byte[] { 255, 216, 255, 224 }; // jpeg     var jpeg2 = new byte[] { 255, 216, 255, 225 }; // jpeg canon      var buffer = new byte[4];     stream.Read(buffer, 0, buffer.Length);      if (bmp.SequenceEqual(buffer.Take(bmp.Length)))         return ImageFormat.bmp;      if (gif.SequenceEqual(buffer.Take(gif.Length)))         return ImageFormat.gif;      if (png.SequenceEqual(buffer.Take(png.Length)))         return ImageFormat.png;      if (tiff.SequenceEqual(buffer.Take(tiff.Length)))         return ImageFormat.tiff;      if (tiff2.SequenceEqual(buffer.Take(tiff2.Length)))         return ImageFormat.tiff;      if (jpeg.SequenceEqual(buffer.Take(jpeg.Length)))         return ImageFormat.jpeg;      if (jpeg2.SequenceEqual(buffer.Take(jpeg2.Length)))         return ImageFormat.jpeg;      return ImageFormat.unknown; }