Extract thumbnail image from a file without reading the whole file

[MrPolite]

this is the VB.NET version of the article I posted here in the C# CodeBank. I wrote the VB code by decompiling the C# code I already had, so no comments for the code. And some parts may be slightly weird but it works


You could use Image.GetThumbnailImage() to get a thumbnail of an already loaded image, but if you have a lot of images and you do that, it'd be fairly slow because you would have to load all the images.
Alternatively most images taken with cameras have a thumbnail of the image stored in the image metadata, and that can be accessed without loading the image as a whole.
If you have .NET 1.1 installed then you can do this:

' Please do not remove :)
' Written by Kourosh Derakshan
'
 
Imports System.IO
Imports System.Drawing.Imaging
 
Private Const THUMBNAIL_DATA As Integer = 20507
 
    Private Function GetThumbnail(ByVal path As String) As Image
        Dim stream1 As FileStream = File.OpenRead(path)
        Dim image1 As Image = Image.FromStream(stream1, False, False)
        Dim flag1 As Boolean = False
        Dim num1 As Integer
        For num1 = 0 To image1.PropertyIdList.Length - 1
            If (image1.PropertyIdList(num1) = Me.THUMBNAIL_DATA) Then
                flag1 = True
                Exit For
            End If
        Next num1
        If Not flag1 Then
            Return Nothing
        End If
        Dim item1 As PropertyItem = image1.GetPropertyItem(Me.THUMBNAIL_DATA)
        stream1.Close()
        image1.Dispose()
        Dim buffer1 As Byte() = item1.Value
        Dim stream2 As New MemoryStream(buffer1.Length)
        stream2.Write(buffer1, 0, buffer1.Length)
        Return Image.FromStream(stream2)
    End Function

If you want support for .NET 1.0 also, you can try this code below

' Please do not remove :)
' Written by Kourosh Derakshan
'
 
Imports System
Imports System.IO
Imports System.Drawing
Imports System.Drawing.Imaging
Imports System.Runtime.InteropServices
 
Public MustInherit Class ExifThumbReader
    <DllImport("gdiplus.dll", CharSet:=CharSet.Unicode, ExactSpelling:=True)> _
    Private Shared Function GdipDisposeImage(ByVal image As IntPtr) As Integer
    End Function
 
    <DllImport("gdiplus.dll", CharSet:=CharSet.Unicode, ExactSpelling:=True)> _
    Friend Shared Function GdipGetPropertyItem(ByVal image As IntPtr, ByVal propid As Integer, ByVal size As Integer, ByVal buffer As IntPtr) As Integer
    End Function
 
    <DllImport("gdiplus.dll", CharSet:=CharSet.Unicode, ExactSpelling:=True)> _
    Friend Shared Function GdipGetPropertyItemSize(ByVal image As IntPtr, ByVal propid As Integer, <Out()> ByRef size As Integer) As Integer
    End Function
 
    <DllImport("gdiplus.dll", CharSet:=CharSet.Unicode, ExactSpelling:=True)> _
    Friend Shared Function GdipLoadImageFromFile(ByVal filename As String, <Out()> ByRef image As IntPtr) As Integer
    End Function
 
    Private Const THUMBNAIL_DATA As Integer = 20507
 
 
    Public Shared Function ReadThumb(ByVal imagePath As String) As Image
        Dim image1 As Image
        Dim ptr1 As IntPtr = IntPtr.Zero
        Dim buffer As IntPtr = IntPtr.Zero
        Dim ret As Integer = ExifThumbReader.GdipLoadImageFromFile(imagePath, ptr1)
        Try
            Dim thumbSize As Integer
            If (ret <> 0) Then
                Throw ExifThumbReader.createException(ret)
            End If
            ret = ExifThumbReader.GdipGetPropertyItemSize(ptr1, ExifThumbReader.THUMBNAIL_DATA, thumbSize)
            If (ret = 19) Then
                Return Nothing
            End If
            If (ret <> 0) Then
                Throw ExifThumbReader.createException(ret)
            End If
            buffer = Marshal.AllocHGlobal(thumbSize)
            If (buffer.Equals(IntPtr.Zero)) Then
                Throw ExifThumbReader.createException(3)
            End If
            ret = ExifThumbReader.GdipGetPropertyItem(ptr1, ExifThumbReader.THUMBNAIL_DATA, thumbSize, buffer)
            If (ret <> 0) Then
                Throw ExifThumbReader.createException(ret)
            End If
            image1 = ExifThumbReader.convertFromMemory(buffer)
        Finally
            If (Not buffer.Equals(IntPtr.Zero)) Then
                Marshal.FreeHGlobal(buffer)
            End If
            ExifThumbReader.GdipDisposeImage(ptr1)
        End Try
        Return image1
    End Function
 
    Private Shared Function createException(ByVal gdipErrorCode As Integer) As Exception
        Select Case gdipErrorCode
            Case 1
                Return New ExternalException("Gdiplus Generic Error", -2147467259)
            Case 2
                Return New ArgumentException("Gdiplus Invalid Parameter")
            Case 3
                Return New OutOfMemoryException("Gdiplus Out Of Memory")
            Case 4
                Return New InvalidOperationException("Gdiplus Object Busy")
            Case 5
                Return New OutOfMemoryException("Gdiplus Insufficient Buffer")
            Case 7
                Return New ExternalException("Gdiplus Generic Error", -2147467259)
            Case 8
                Return New InvalidOperationException("Gdiplus Wrong State")
            Case 9
                Return New ExternalException("Gdiplus Aborted", -2147467260)
            Case 10
                Return New FileNotFoundException("Gdiplus File Not Found")
            Case 11
                Return New OverflowException("Gdiplus Over flow")
            Case 12
                Return New ExternalException("Gdiplus Access Denied", -2147024891)
            Case 13
                Return New ArgumentException("Gdiplus Unknown Image Format")
            Case 18
                Return New ExternalException("Gdiplus Not Initialized", -2147467259)
            Case 20
                Return New ArgumentException("Gdiplus Property Not Supported Error")
        End Select
        Return New ExternalException("Gdiplus Unknown Error", -2147418113)
    End Function
 
 
 
 
    Private Shared Function convertFromMemory(ByVal thumbData As IntPtr) As Image
        Dim prop As propertyItemInternal = CType(Marshal.PtrToStructure(thumbData, GetType(propertyItemInternal)), propertyItemInternal)
        Dim buffer As Byte() = prop.Value
        Dim stream As New MemoryStream(buffer.Length)
        stream.Write(buffer, 0, buffer.Length)
        Return Image.FromStream(stream)
    End Function
 
 
 
 
 
 
 
    <StructLayout(LayoutKind.Sequential)> _
    Private Class propertyItemInternal
 
        Public ReadOnly Property Value() As Byte()
            Get
                Dim buffer1 As Byte() = New Byte(Me.len - 1) {}
                Marshal.Copy(Me.valuePtr, buffer1, 0, Me.len)
                Return buffer1
            End Get
        End Property
 
 
 
 
        Public id As Integer
        Public len As Integer
        Public type As Short
        Public valuePtr As IntPtr
    End Class
End Class

I've tested both codes above and they worked for images with and without thumbnails. But I havent gone through the code line by line... I just some minor issues from decompiling if you want to read the comments see the C# post