Determine whether an image is PNG despite .BMP extension

Question

I want to check if images in a directory are of type png but with extension .bmp. The following determines whether it is a .bmp extension

 string x = Path.GetExtension(file);

From this we establish that its extension is .bmp. Now the problem comes in checking if it is in a png format. I am stuck on this part.

The reason why I am doing this is because I want to have my images transparent and .bmp images don't work so well with that. Thank you!

See the accepted answer and first comment: http://stackoverflow.com/questions/670546/determine-if-file-is-an-image — George Johnston, Jun 03 '13 at 20:24
@GeorgeJohnston Even if the file is with an extension .bmp, the last 8 bits will be that answer that is in the other post? Interesting — haysam, Jun 03 '13 at 20:27
@haysam the first 8 bytes, not the last 8 bits, but yes - the extension is just part of the filename, it's just convention that bitmaps are '.bmp' and PNGs are '.png' — Blorgbeard, Jun 03 '13 at 20:29

score 11 · Answer 1 · answered Jun 03 '14 at 03:36

11

The above answer is incorrect, the code should be:

var header = new byte[4];
using (var fs = new FileStream(filename))
{
    fs.Read(header, 0, 4);
}

var strHeader = Encoding.ASCII.GetString(header);
return strHeader.ToLower().EndsWith("png");

answered Jun 03 '14 at 03:36

Andrew dh

881
9
19

score 0 · Answer 2 · answered Jun 06 '17 at 20:31

0

We can check the file extension with this

 Byte[] imageBase64 = ....

 var encodedFile = Encoding.ASCII.GetString(imageBase64);
 return encodedFile.ToLower().StartsWith("?png", StringComparison.InvariantCultureIgnoreCase);

answered Jun 06 '17 at 20:31

Manni Dula

49
4

score 0 · Answer 3 · answered May 18 '20 at 19:56

Here is another take that I personally like since you don't have to check against strings (using the System.Drawing Library).

using (var fs = new FileStream(filename))
{
    var fsImage = System.Drawing.Image.FromStream(fs);
    if (fsImage.RawFormat == System.Drawing.Imaging.ImageFormat.Jpeg)
    {
       // Do something with Jpegs
    }
    else if (fsImage.RawFormat == System.Drawing.Imaging.ImageFormat.Png)
    {
       // Do something with Pngs
    }
}

Reading the whole image is more expensive than reading the header. — Victor Zakharov, May 18 '20 at 20:24

score -1 · Accepted Answer · answered Jun 03 '13 at 20:31

-1

Read the first 4 bytes of the file:

byte[] b = new byte[4];
using (var fs = new FileStream(filename))
{
    fs.Read(b, 0, 4);
}
if (b.ToString().Contains("PNG"))
{
    // this is a png file
}

answered Jun 03 '13 at 20:31

klugerama

3,312
19
25

Yes, you can't always trust the filename extension. You should *always* check the file signature (first few magic bytes) to make sure the file is actually the type it claims to be. – David R Tribble Jun 03 '13 at 23:56
-1 this is incorrect - answr below by @Andrew dh works – lstanczyk Jun 26 '14 at 17:51

Determine whether an image is PNG despite .BMP extension

4 Answers4