I have made my ovn extension in VS2010 it's contains

Reminder.cs

namespace EmailReminderShop
{
    public class Reminder
    {
        public static string SendMail(int NodeId, string Email, int ReminderEmailNodeId)
        {
            string returnText = "testc";

            ReminderBLL BLL = new ReminderBLL();
            ReminderItem ReminderToInsert = new ReminderItem();

            ReminderToInsert.email = Email;
            ReminderToInsert.nodeId = NodeId;

            bool rowExist = BLL.InsertReminder(ReminderToInsert);

            if (rowExist)
            {
                returnText = "Du er allerede tilmeldt en reminder på dette produkt";
            }
            else
            {
                returnText = "Du er nu tilmeldt en reminder på dette produkt og vil modtage en E-mail når vi igen har varen på lager.";
            }

            return returnText;
        }

    }
}

ReminderBLL.cs
ReminderDAL.cs
ReminderItem.cs

In the xsltconfig i have added:

<ext assembly="EmailReminderShop" type="EmailReminderShop.Reminder" alias="myExt">
  </ext>

in the xslt i use

<xsl:value-of select="myExt:SendMail('1204', '[email protected]', '1115')"/>

when i try to save it i get this

System.NullReferenceException: Object reference not set to an instance of an object.
at EmailReminderOccShop.ReminderDAL..ctor()
at EmailReminderOccShop.ReminderBLL..ctor()
at EmailReminderOccShop.Reminder.SendMail(Int32 NodeId, String Email, Int32 ReminderEmailNodeId)

but why???