LINQ查询在同一查询中需要升序或降序

您可以创建自己的可重用扩展方法,该方法将执行以下操作:

public static IOrderedQueryable<TSource> OrderBy<TSource, TKey>
    (this IQueryable<TSource> source,
     Expression<Func<TSource, TKey>> keySelector,
     bool ascending)
{
     return ascending ? source.OrderBy(keySelector)
          : source.OrderByDescending(keySelector);
}

同样地,对于 ThenBy :

public static IOrderedQueryable<TSource> ThenBy<TSource, TKey>
    (this IOrderedQueryable<TSource> source,
     Expression<Func<TSource, TKey>> keySelector,
     bool ascending)
{
     return ascending ? source.ThenBy(keySelector)
          : source.ThenByDescending(keySelector);
}

您可以将查询拆分为若干位,并使用控制流逻辑。linqtosql将神奇地构造正确的查询,就像您在一行中键入了所有查询一样!这样做的原因是,在您请求数据之前,查询不会发送到数据库,而是作为表达式存储。

var linq = new NorthwindDataContext();
var query = linq.Customers
    .Where(c => c.ContactName.StartsWith("a"))
    .SelectMany(cus=>cus.Orders);

IOrderedQueryable<Order> query2;
if (useAscending) {
    query2 = query.OrderBy(ord => ord.OrderDate);
} else {
    query2 = query.OrderByDescending(ord => ord.OrderDate);
}

var query3 = query2.Select(ord => ord.CustomerID);

return from T in bk.anbarsabts
       where T.kalaname == str
       orderby T.date descending
       select new { T.date, T.kalaname, T.model, T.tedad };

对于数字等,你通常可以对“排序变量”求反。

与 DateTime Timespan .

好吧,如果你有一个条件,你决定顺序是上升还是下降,你可以用这个

var query1 = linq.Customers
.Where(c => c.ContactName.StartsWith("a"))
.SelectMany(cus=>cus.Orders)

if(SortAscending)
   query1 = query1.OrderBy(ord => ord.OrderDate);
else
   query1 = query1.OrderByDescending(ord => ord.OrderDate);

var query2 = query1.Select(ord => ord.CustomerID);