//select record(s) to data rows
DataRow[] childRow = dt.Select("PersonId = 4");
//if at least one record is found
if(childRow.Any())
{
//copy rows to a datatable
DataTable childDt = childRows.CopyToDataTable();
}
var test = from row in dt.AsEnumerable()
where row.Field<int>("PersonId") == 4
select new {First = row.Field<string>("FirstName"), Last = row.Field<string>("LastName")};
something like
PersonID = dt.Rows(0).Columns(0)
LastName = dt.Rows(0).Columns(1)
....