Image.RawFormat to detect stream's format?

Question

Hello,

There's one MemoryStream which is made from a file and it can be in any format, to check if it's an image (just well known most used ones) is using Image.FromStream and Image.RawFormat a reliable method?

If so, here's my function:

Dim img As Image = Nothing
Try
	img = Image.FromStream(inputStream)
	If img IsNot Nothing Then
		Select Case img.RawFormat
			Case Imaging.ImageFormat.Bmp, Imaging.ImageFormat.Emf, etc...
				Return True
			Case Else
				Return False
		End Select
	End If
Catch Exception As Exception
	Return False
End Try

Question: FromStream(Stream, Boolean, Boolean) overload, which has validateImageData, shall be used here?

The docs won't help much:

https://learn.microsoft.com/en-us/dotnet/api/system.drawing.image.fromstream?view=netframework-4.0

Problem:

Error: Operator '=' is not defined for types 'System.Drawing.Imaging.ImageFormat' and 'System.Drawing.Imaging.ImageFormat'.

Can't evaluate the format with Select Case!

Any help is highly appreciated :)

Answer 1

Hi @Sani Love ,

Here is an optional way to check the file format: Checking the file's magic numbers.

Magic numbers are the first bits of a file which uniquely identify the type of file. This makes programming easier because complicated file structures need not be searched in order to identify the file type.

You can find the magic number in the file format you want by searching.

For example, The magic numbers for JPEG are FF, D8, FF, E0, and PNG are 89, 50, 4E, 47.

Imports System.IO

Public Class ImageDetection
    Public Shared Function IsImage(stream As Stream) As Boolean
        Dim buffer As Byte() = New Byte(7) {} ' Read 8 bytes to check magic numbers
        stream.Read(buffer, 0, 8)

        If buffer(0) = &HFF AndAlso buffer(1) = &HD8 AndAlso buffer(2) = &HFF AndAlso buffer(3) = &HE0 Then
            Return True
        End If

        If buffer(0) = &H89 AndAlso buffer(1) = &H50 AndAlso buffer(2) = &H4E AndAlso buffer(3) = &H47 Then
            Return True
        End If

        Return False
    End Function
End Class

Best Regards.

Jiachen Li

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