Public Class acThumbnailReader

    Private Structure BITMAPINFOHEADER
        Dim biSize As Integer
        Dim biWidth As Integer
        Dim biHeight As Integer
        Dim biPlanes As Short
        Dim biBitCount As Short
        Dim biCompression As Integer
        Dim biSizeImage As Integer
        Dim biXPelsPerMeter As Integer
        Dim biYPelsPerMeter As Integer
        Dim biClrUsed As Integer
        Dim biClrImportant As Integer
    End Structure

    Private Structure RGBQUAD
        Dim rgbBlue As Byte
        Dim rgbGreen As Byte
        Dim rgbRed As Byte
        Dim rgbReserved As Byte
    End Structure

    Private Structure IMGREC
        Dim bytType As Byte
        Dim lngStart As Integer
        Dim lngLen As Integer
    End Structure

    Public Shared Function GetThumbnail(ByVal strFile As String, Optional ByVal boolRetainBackColor As Boolean = True, Optional ByVal boolSaveToFile As Boolean = False, Optional ByVal strSaveName As String = "") As Bitmap
        Dim bmp As New Bitmap(1, 1, System.Drawing.Imaging.PixelFormat.Format8bppIndexed)
        Dim bytCnt As Byte
        Dim bytBMPBuff() As Byte
        Dim lngImgLoc As Integer
        Dim lngFile As Integer
        Dim lngCurLoc As Integer
        Dim lngY As Integer
        Dim lngX As Integer
        Dim lngColor As Integer
        Dim lngCnt As Integer
        Dim intCnt As Short
        Dim udtRec As IMGREC
        Dim udtColors() As RGBQUAD
        Dim udtColor As RGBQUAD
        Dim udtHeader As BITMAPINFOHEADER
        Dim intRed As Short
        Dim intGreen As Short
        Dim intBlue As Short
        On Error GoTo Err_Control
        If Len(Dir(strFile)) > 0 Then
            lngFile = FreeFile()
            FileOpen(lngFile, strFile, OpenMode.Binary)
            Seek(lngFile, 14)
            FileGet(lngFile, lngImgLoc)
            Seek(lngFile, lngImgLoc + 17)
            lngCurLoc = Seek(lngFile)
            Seek(lngFile, lngCurLoc + 4)
            FileGet(lngFile, bytCnt)
            If bytCnt > 1 Then
                For intCnt = 1 To bytCnt
                    FileGet(lngFile, udtRec)
                    If udtRec.bytType = 2 Then
                        Seek(lngFile, udtRec.lngStart + 1)
                        FileGet(lngFile, udtHeader)
                        ReDim bytBMPBuff(udtRec.lngLen)
                        If udtHeader.biBitCount = 8 Then
                            ReDim udtColors(256)
                            FileGet(lngFile, udtColors)
                            Seek(lngFile, udtRec.lngStart)
                            FileGet(lngFile, bytBMPBuff)
                            bmp = New Bitmap(udtHeader.biWidth, udtHeader.biHeight)
                            For lngY = 1 To udtHeader.biHeight
                                For lngX = udtHeader.biWidth To 1 Step -1
                                    lngColor = bytBMPBuff(UBound(bytBMPBuff) - lngCnt)
                                    udtColor = udtColors(lngColor)
                                    intRed = CShort(udtColor.rgbRed)
                                    intGreen = CShort(udtColor.rgbGreen)
                                    intBlue = CShort(udtColor.rgbBlue)
                                    lngColor = RGB(intRed, intGreen, intBlue)
                                    If boolRetainBackColor = False Then
                                        If lngColor = Drawing.ColorTranslator.ToOle(Drawing.Color.Black) Then
                                            lngColor = Drawing.ColorTranslator.ToOle(Drawing.Color.White)
                                        ElseIf lngColor = Drawing.ColorTranslator.ToOle(Drawing.Color.White) Then
                                            lngColor = Drawing.ColorTranslator.ToOle(Drawing.Color.Black)
                                        End If
                                    End If
                                    bmp.SetPixel(lngX - 1, lngY - 1, Drawing.ColorTranslator.FromOle(lngColor))
                                    lngCnt = lngCnt + 1
                                Next lngX
                            Next lngY
                        End If
                        Exit For
                    ElseIf udtRec.bytType = 3 Then
                        Exit For
                    End If
                Next intCnt
            End If
        End If
Exit_Here:
        FileClose(lngFile)
        If boolSaveToFile = True Then
            If (strSaveName = "") Then
                Dim fName As String
                fName = IO.Path.GetTempPath & IO.Path.GetFileNameWithoutExtension(IO.Path.GetRandomFileName) & ".bmp"
                bmp.Save(fName)
            Else
                bmp.Save(strSaveName)
            End If
        End If
        Return bmp
        Exit Function
Err_Control:
        Select Case Err.Number
            Case Else
                MsgBox(Err.Description)
                Resume Exit_Here
        End Select
    End Function
End Class

//Copyright 2009 K.E. Blackie
//This work is based in part from a VBA macro
//initially developed by members of www.vbdesign.net
//it may be used freely and may be modified to suit
//your needs as long as credit is given where it is due

using System;
using System.Drawing;
using System.Drawing.Imaging;
using System.IO;
using System.Runtime.InteropServices;
using System.Windows.Forms; 

class acThumbnailReader
{

    [StructLayout(LayoutKind.Sequential)]
    private struct BITMAPINFOHEADER
    {
        public int biSize;
        public int biWidth;
        public int biHeight;
        public short biPlanes;
        public short biBitCount;
        public int biCompression;
        public int biSizeImage;
        public int biXPelsPerMeter;
        public int biYPelsPerMeter;
        public int biClrUsed;
        public int biClrImportant;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct IMGREC
    {
        public byte bytType;
        public int lngStart;
        public int lngLen;
    }

    [StructLayout(LayoutKind.Sequential)]
    private struct RGBQUAD
    {
        public byte rgbBlue;
        public byte rgbGreen;
        public byte rgbRed;
        public byte rgbReserved;
    }
    
    public static Bitmap GetThumbnail(string strFile)
    {
        return GetThumbnail(strFile, true, false, ""); 
    }

    public static Bitmap GetThumbnail(string strFile, bool boolRetainBackColor)
    {
        return GetThumbnail(strFile, boolRetainBackColor, false, "");
    }

    public static Bitmap GetThumbnail(string strFile, bool boolRetainBackColor, bool boolSaveToFile)
    {
        return GetThumbnail(strFile, boolRetainBackColor, boolSaveToFile, "");
    }

    public static Bitmap GetThumbnail(string strFile, bool boolRetainBackColor, bool boolSaveToFile, string strSaveName)
    {
        Bitmap bmp = new Bitmap(1, 1, PixelFormat.Format8bppIndexed);
        byte bytCnt;
        byte[] bytBMPBuff;
        int lngImgLoc;
        FileStream fs = null;
        BinaryReader br = null;
        int lngCurLoc;
        int lngY;
        int lngX;
        int lngColor;
        int lngCnt;
        short intCnt;
        IMGREC udtRec;
        RGBQUAD[] udtColors;
        RGBQUAD udtColor;
        BITMAPINFOHEADER udtHeader;
        short intRed;
        short intGreen;
        short intBlue;
        try
        {
            if (File.Exists(strFile))
            {
                fs = File.OpenRead(strFile);
                br = new BinaryReader(fs);
                fs.Seek(13, SeekOrigin.Begin);
                lngImgLoc = br.ReadInt32();
                fs.Seek(lngImgLoc + 17, SeekOrigin.Begin);
                lngCurLoc = lngImgLoc + 17;
                fs.Seek(lngCurLoc + 3, SeekOrigin.Begin);
                bytCnt = br.ReadByte();
                if (bytCnt > 1)
                {
                    for (intCnt = 0; intCnt < bytCnt; intCnt++)
                    {
                        udtRec.bytType = br.ReadByte();
                        udtRec.lngStart = br.ReadInt32();
                        udtRec.lngLen = br.ReadInt32();
                        if (udtRec.bytType == 2)
                        {
                            fs.Seek(udtRec.lngStart, SeekOrigin.Begin);
                            udtHeader.biSize = br.ReadInt32();
                            udtHeader.biWidth = br.ReadInt32();
                            udtHeader.biHeight = br.ReadInt32();
                            udtHeader.biPlanes = br.ReadInt16();
                            udtHeader.biBitCount = br.ReadInt16();
                            udtHeader.biCompression = br.ReadInt32();
                            udtHeader.biSizeImage = br.ReadInt32();
                            udtHeader.biXPelsPerMeter = br.ReadInt32();
                            udtHeader.biYPelsPerMeter = br.ReadInt32();
                            udtHeader.biClrUsed = br.ReadInt32();
                            udtHeader.biClrImportant = br.ReadInt32();
                            bytBMPBuff = new byte[udtRec.lngLen + 1];
                            if (udtHeader.biBitCount == 8)
                            {
                                udtColors = new RGBQUAD[256];
                                for (int count = 0; count < 256; count++)
                                {
                                    udtColors[count].rgbBlue = br.ReadByte();
                                    udtColors[count].rgbGreen = br.ReadByte();
                                    udtColors[count].rgbRed = br.ReadByte();
                                    udtColors[count].rgbReserved = br.ReadByte();
                                }
                                fs.Seek(udtRec.lngStart - 1, SeekOrigin.Begin);
                                for (int count = 0; count <= udtRec.lngLen; count++)
                                {
                                    bytBMPBuff[count] = br.ReadByte();
                                }
                                bmp = new Bitmap(udtHeader.biWidth, udtHeader.biHeight);
                                lngCnt = 0;
                                for (lngY = 1; lngY <= udtHeader.biHeight; lngY++)
                                {
                                    for (lngX = udtHeader.biWidth; lngX >= 1; lngX--)
                                    {
                                        lngColor = bytBMPBuff[bytBMPBuff.GetUpperBound(0) - lngCnt];
                                        udtColor = udtColors[lngColor];
                                        intRed = Convert.ToInt16(udtColor.rgbRed);
                                        intGreen = Convert.ToInt16(udtColor.rgbGreen);
                                        intBlue = Convert.ToInt16(udtColor.rgbBlue);
                                        lngColor = ColorTranslator.ToOle(Color.FromArgb(intRed, intGreen, intBlue));
                                        if (boolRetainBackColor == false)
                                        {
                                            if (lngColor == ColorTranslator.ToOle(Color.Black))
                                            {
                                                lngColor = ColorTranslator.ToOle(Color.White);
                                            }
                                            else
                                            {
                                                if (lngColor == ColorTranslator.ToOle(Color.White))
                                                {
                                                    lngColor = ColorTranslator.ToOle(Color.Black);
                                                }
                                            }
                                        }
                                        bmp.SetPixel(lngX - 1, lngY - 1, ColorTranslator.FromOle(lngColor));
                                        lngCnt++;
                                    }
                                }
                            }
                            goto Exit_Here;
                        }
                        else
                        {
                            if (udtRec.bytType == 3)
                            {
                                goto Exit_Here;
                            }
                        }
                    }
                }
            }
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message.ToString());
            goto Exit_Here;
        }
    Exit_Here:
        if (br != null)
        {
            br.Close();
            fs.Close();
            fs.Dispose();
        }
        if (boolSaveToFile == true)
        {
            if (strSaveName == "")
            {
                string fName;
                fName = String.Concat(Path.GetTempPath(), Path.GetFileNameWithoutExtension(Path.GetRandomFileName()), ".bmp");
                bmp.Save(fName);
            }
            else
            {
                bmp.Save(strSaveName);
            }
        }
        return bmp;
    }
}