DllImport Error

> Hello friends,
> I need to load an unmanaged dll(which i wrote for license control)
> from system32 folder and then my asp. net project will read the values
> from there.(I'll write the full code to the bottom)
>
> I get an "Object reference is not set an instance of an object
> error"...This code works in my local machine , but in our remote
> machine there was a problem that i told above. I can only fix the
> problem by iisreset , but problem repeats again and again for a while
> later... The problem is in DLL Import section... full code is here
> where it broke...
> i am sure u can help me to solve this problem thanx everybody...
>
> private int CheckLicenceApply()
> {
> PortalSettings portalSettings =
> (PortalSettings) HttpContext.Current.Items
> ["PortalSettings"];
>
> int i= 0;
> // error is in here when trying to load dll
> i = Mydll.LicenceApply
> (portalSettings.PortalTitle,GetLicense().ToString( ) ,
> ProductGroupKey, ProductMagic,
> Request.Url.Host.ToLower());
> return i;
>
> }
>
> public string GetLicense()
> {
> string XMLPath = Server.MapPath(PortalSettings.ApplicationPath) ;
> string XMLLicenseCode="0";
>
> string ConfigFile = XMLPath + "\\web.config";
> XmlDocument xDoc = new XmlDocument();
> xDoc.Load(ConfigFile);
>
> XmlNodeList ndLst;
> ndLst = xDoc.SelectNodes("//appSettings/add");
>
> foreach (XmlNode node in ndLst)
> {
> if (node.Attributes[0].Value == "LicenseNumber")
> {
> XMLLicenseCode = node.Attributes[1].Value ;
> break;
> }
> }
> return XMLLicenseCode;
>
> }
>
> public class Mydll
> {
> [DllImport("Licencontrol.dll" )]
> public static extern ushort LicenceApply(string LicenceName,
> string LicenceCode,string ProductGroupKey,
> uint ProductMagic,string AModifier);
>
> [DllImport("Licencontrol.dll")]
> public static extern ushort ReturnWSCount(string LicenceName,
> string LicenceCode, string ProductGroupKey,
> uint ProductMagic,string AModifier);
>
> [DllImport("Licencontrol.dll")]
> public static extern ushort InfoDemo(string LicenceName,
> string LicenceCode, string ProductGroupKey,
> uint ProductMagic,string AModifier);
>
> [DllImport("Licencontrol.dll")]
> public static extern string ExpireControl(string LicenceName,
> string LicenceCode, string ProductGroupKey,
> uint ProductMagic,string AModifier);
>
> }