LINQ Convert and Filter List Part 2
FieldDefEnum1 : FieldDef

List<FileDef> fieldDefs

public List<FieldDefEnum1> FieldDefsEnum1
{
    get
    {
        return FieldDefs.OfType<FieldDefEnum1>().ToList().Where(fd => fd.SysCus == enumSysCus.Cus).ToList();
    }
}

FieldDefs.OfType<FieldDefEnum1>().Where(fd => fd.SysCus == enumSysCus.Cus).ToList();