Question

我正在尝试实现c＃linq表达式，该表达式将允许我根据年份和月份提取数据。该错误在上下文的where方法中引发。

我目前收到错误消息

System.NotSupportedException: 'Unable to create a constant value of type 'System.Tuple`2[[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089],[System.Int32, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]'. Only primitive types or enumeration types are supported in this context.'

Linq表达式

var periodTuples = period.ToList() // gives you List<KeyValuePair<int, int[]>>
                .SelectMany(kvp =>
                    kvp.Value.Select(it2 => Tuple.Create(kvp.Key, it2))).ToHashSet(); // to optimize .Contains()


benchmark1Returns = GetViewService<MV_INDEX_PERFORMANCE>()
                .Where(x => x.Mtd != null && x.IndexId == benchMark1 && periodTuples.Contains(Tuple.Create(x.PriceDate.Year, x.PriceDate.Month))).Select(x => x.Mtd);

Where方法在我的上下文中定义

public IEnumerable<T> Where(Expression<Func<T, bool>> predicate)
        {
            try
            {
                using (new TimedLogger(_perfLogger, GetCompletedText("Where")))
                {
                    return Authorize(_repo.Where(predicate).ToList(), AuthAccessLevel.Read);
                }
            }
            catch (Exception ex)
            {
                _logger.Error(ex);
                throw;
            }
        }

Answer 1

尝试加入吗？

            var benchmark1Returns = GetViewService<MV_INDEX_PERFORMANCE>()
                .Where(x => x.Mtd != null && x.IndexId == benchMark1)
                .Join(periodTuples,
                    b1r => new { x = b1r.PriceDate.Year, y = b1r.PriceDate.Month },
                    tuple => new { x = tuple.Item1, y = tuple.Item2 },
                    (b, t) => b.Mtd);

Answer 2

EF6不支持（无法转换为SQL）Tuple.Create（和Tuple<>构造函数），内存中集合联接，Contains非基本类型和许多其他类型的内存中集合其他人。

您这种情况下的解决方案是用原始类型表达式替换（int year，int month）对：

int period = 12 * year + (month - 1)

并将其用于过滤，例如

var periods = period.ToList() // gives you List<KeyValuePair<int, int[]>>
    .SelectMany(kvp => kvp.Value, (year, month) => 12 * year + (month - 1));

然后在LINQ查询中

periods.Contains(12 * x.PriceDate.Year + (x.PriceDate.Month - 1))

请注意，在LINQ to Entities中，查询periods不必为HashSet<int>（或List<int>等）-IEnumerable<int>就足够了，因为它将在本地求值一次并转换为SQL IN (val1, val2, ..., valN)标准。

无法创建类型为'System.Tuple的常量值

2 个答案: