using (Tiff tiff = Tiff.Open(@"TestN41E071_dem.tif", r"))
{
  int width   = tiff.GetField(TiffTag.IMAGEWIDTH)[0].ToInt();
  int height  = tiff.GetField(TiffTag.IMAGELENGTH)[0].ToInt();
  double dpiX = tiff.GetField(TiffTag.XRESOLUTION)[0].ToDouble();
  double dpiY = tiff.GetField(TiffTag.YRESOLUTION)[0].ToDouble();

  byte[] scanline        = new byte[tiff.ScanlineSize()];
  ushort[] scanline16Bit = new ushort[tiff.ScanlineSize() / 2];

  for (int i = 0; i < height; i++)
  {
    tiff.ReadScanline(scanline, i); //Loading ith Line
    MultiplyScanLineAs16BitSamples(scanline, scanline16Bit, 16,i);
  }
}

private static void MultiplyScanLineAs16BitSamples(byte[] scanline, ushort[] temp, ushort factor,int row)
{
  if (scanline.Length % 2 != 0)
  {
    // each two bytes define one sample so there should be even number of bytes
    throw new ArgumentException();
  }

  Buffer.BlockCopy(scanline, 0,   temp, 0, scanline.Length);

  for (int i = 0; i < temp.Length; i++)
  {
    temp[i] *= factor;
    MessageBox.Show("Row:"+row.ToString()+"Column:"+(i/2).ToString()+"Value:"+temp[i].ToString());
  }
}

int buffersize = 1000000;
  using (Tiff tiff = Tiff.Open(geotifffile, "r"))
  {
    int nooftiles = tiff.GetField(TiffTag.TILEBYTECOUNTS).Length;
    int width = tiff.GetField(TiffTag.TILEWIDTH)[0].ToInt();
    int height = tiff.GetField(TiffTag.TILELENGTH)[0].ToInt();
    byte[] buffer = new byte[buffersize];

    for (int i = 0; i < nooftiles; i++)
    {
      int size = tiff.ReadEncodedTile(i, buffer, 0, buffersize);
      float[,] data = new float[width, height];
      Buffer.BlockCopy(buffer, 0, data, 0, size); // Convert byte array to x,y array of floats (height data)
      // Do whatever you want with the height data (calculate hillshade images etc.)
    }
  }