How do I reliably get an image dimensions in .NET without loading the image?

Question

I know how to get the size (x,y) of an image

Image.FromFile("cat.jpg").Size

BUT that requires loading the image from memory.

When I view the images in Windows Explorer it shows me the size.

• How do I access that size?
• Is it reliable for all images? Does Windows explorer need to have 'seen' the image first for the size to be available. I don't want to upload to a server and have no metadata available before I've loaded it in internet explorer.

Edit: I changed the subject from 'size' to 'dimensions' to clarify which size i wanted.

Answer 1

Doing this usually involves actually opening the file and reading only its header (the first few bytes) to find its format and the size. You don't need to read the whole file to find out it's size. Sometimes the size is even in a specific offset in the file so finding the size is only reading 8 bytes.

You can use filemon.exe to actually find out what windows explorer itself does to find the size.

Answer 2

If you want to get the image dimensions (width and hight) with out loading the image, you need to read some part of jpeg file yourself as below.

Here is an out of the box method for you :)

public static Size GetJpegImageSize(string filename) {
    FileStream stream=null;
    BinaryReader rdr=null;
    try {
     stream=File.OpenRead(filename);
     rdr=new BinaryReader(stream);
     // keep reading packets until we find one that contains Size info
     for(; ; ) {
      byte code=rdr.ReadByte();
      if(code!=0xFF) throw new ApplicationException(
        "Unexpected value in file "+filename);
      code=rdr.ReadByte();
      switch(code) {
       // filler byte
       case 0xFF:
        stream.Position--;
        break;
       // packets without data
       case 0xD0: case 0xD1: case 0xD2: case 0xD3: case 0xD4: 
       case 0xD5: case 0xD6: case 0xD7: case 0xD8: case 0xD9:
        break;
       // packets with size information
       case 0xC0: case 0xC1: case 0xC2: case 0xC3:
       case 0xC4: case 0xC5: case 0xC6: case 0xC7:
       case 0xC8: case 0xC9: case 0xCA: case 0xCB:
       case 0xCC: case 0xCD: case 0xCE: case 0xCF:
        ReadBEUshort(rdr);
        rdr.ReadByte();
        ushort h=ReadBEUshort(rdr);
        ushort w=ReadBEUshort(rdr);
        return new Size(w, h);
       // irrelevant variable-length packets
       default:
        int len=ReadBEUshort(rdr);
        stream.Position+=len-2;
        break;
      }
     }
    } finally {
     if(rdr!=null) rdr.Close();
     if(stream!=null) stream.Close();
    }
}

private static ushort ReadBEUshort(BinaryReader rdr) {
    ushort hi=rdr.ReadByte();
    hi<<=8;
    ushort lo=rdr.ReadByte();
    return (ushort)(hi|lo);
}

This is not my code, got this example some time back in code project. I copied and and saved it to my utility snippets, don't remember the link though.

Answer 3

You can just use Image.FromStream(Stream, bool, bool) with "false" 2nd and 3rd parameter to avoid loading file into memory

Answer 4

Reading Image Headers to Get Width and Height

Answer 5

From the top of my head - LoadImage() has an override that specifies progress callback. Maybe you can use that to cancel loading after header was read?