Create List in C# and add values to list

You are creating a new list each time you want to add a new item. Furthermore you are not doing anything with the list after that. I guess you want to fill the list and then return it form the method:

private List<dashinfo> OnLoadDashBoardDataCompleted(LoadOperation<dashboardjson> loadOperation)
{
    List<dashinfo> dashinfos = new List<dashinfo>(); // create the list once
    
    foreach (DashboardJson dashjson in loadOperation.Entities)
    {
        if (dashjson.groups.ElementAt(0).Cells.ElementAt(0) != null)
        {
            foreach (var region in loadOperation.Entities)
            {
                foreach (var group in region.groups)
                {
                    // add items to the list
                    dashinfos.Add(new DashInfo { Gid = group.gid, Group = group.Name, Region = group.Region, Duration = group.AvgInf });
                    }
                }
            }
        }
    }

    return dashinfos; // return the created list to the caller
}