Check out this code..... it might be useful for you.
public partial class ThisAddIn
{
private Outlook.Application outApp;
private Outlook.NameSpace _nameSpace;
private Outlook.MAPIFolder _calendarFolder;
private void ThisAddIn_Startup(object sender, System.EventArgs e)
{
outApp = this.Application;
_nameSpace = outApp.GetNamespace("MAPI");
_calendarFolder =
_nameSpace.GetDefaultFolder(OlDefaultFolders.olFolderCalendar);
_calendarFolder.Items.ItemAdd += new
ItemsEvents_ItemAddEventHandler(Items_ItemAdd);
}
void Items_ItemAdd(object Item)
{
MessageBox.Show("Item Added");
}
private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
{
_calendarFolder.Items.ItemAdd -= new
ItemsEvents_ItemAddEventHandler(Items_ItemAdd);
outApp = null;
}
}
For more details please check out this link..
http://www.codeproject.com/KB/office/outlook_drag_drop_in_cs.aspx
http://forums.asp.net/t/1138955.aspx