How to create an item in a discussion list
Creating a discussion in discussion list is different from creating a simple list item. The code for creating a discussion item is:
using (SPSite site = new SPSite("SITE URL"))
{
using (SPWeb web = site.OpenWeb())
{
SPList list =web.Lists["LIST TITLE"];
result = SPUtility.CreateNewDiscussion(list.Items, "SUBJECT");
result[SPBuiltInFieldId.Body] = "BODY";
result.Update();
}
}