How to export the PDF pages to images in WinForms FileFormat libraries ?

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
<table width="100%" style="color: #5D6B03" border="1px">
<tr style="width: 100%; background-color: #8EA718; text-align:center">
<td >
<asp:FileUpload ID="FileUpload1" runat="server" />
<asp:Button ID="Button1" runat="server" onclick="Button1_Click"
Text="ExportAsImage" />
</td>
</tr>
</table>
</asp:Content>

protected void Button1_Click(object sender, EventArgs e)
{
if (System.IO.Path.GetExtension(FileUpload1.PostedFile.FileName).Equals(".pdf"))
{
//locate the file path and the path the images to be saved
string imagePath = Server.MapPath("~/Images/");
//Clear the previous pdf file images from the Images folder
CleareFolder(imagePath);
//Get the pdf file stream
Stream pdfStream = FileUpload1.PostedFile.InputStream;
//Create an instance of PdfViewerControl
PdfViewerControl pdfViewercontrol = new PdfViewerControl();
//Load the document in the viewer
pdfViewercontrol.Load(pdfStream);
//Export the pages of the loaded document as bitmap images
System.Drawing.Bitmap[] images = pdfViewercontrol.ExportAsImage(0, pdfViewercontrol.PageCount - 1);
int page = 0;
//Save the images in the imagepath defined earlier
foreach (System.Drawing.Bitmap bmp in images)
{
images[page].Save(imagePath + "exportedPage_" + page.ToString() + ".png", System.Drawing.Imaging.ImageFormat.Png);
page++;
}
}
}
/// <summary>
///Clear the Folder.
/// </summary>
/// <param name="folderName"></param>
///<return></return>
private void ClearFolder(string folderName)
{
DirectoryInfo directoryInfo = new DirectoryInfo(folderName);
foreach (FileInfo fileInfo in directoryInfo.GetFiles())
{
fileInfo.IsReadOnly = false;
fileInfo.Delete();
}
}