17

我遇到了这个错误。我看到原因是因为有时返回的平均值是 0.00，从数据的角度来看这是准确的。此 SQL 查询工作正常，但那是因为它自动输入 0.00。

LINQ 抱怨，所以我尝试使用 DefaultIfEmpty() 但它说它期待我的 ViewModel。

Dim ticketCounts = From t In queue _
   Where _
    (t.StatusId = 2) And _
    (t.CloseDate.Year = Convert.ToDateTime(DateTime.Now).Year) And _
    (t.ResolutionDays > 0)
   Group t By _
    Column1 = CType(t.CloseDate.Month, Integer), _
    Column2 = CType(t.CloseDate.ToString("MMMM"), String) _
    Into g = Group _
   Order By Column1 _
   Select _
    Id = Column1, _
    Month = Column2, _
    Critical = g.Where(Function(t) t.PriorityId = 1).DefaultIfEmpty().Average(Function(t) t.ResolutionDays), _
    High = g.Where(Function(t) t.PriorityId = 2).DefaultIfEmpty().Average(Function(t) t.ResolutionDays), _
    Normal = g.Where(Function(t) t.PriorityId = 3).DefaultIfEmpty().Average(Function(t) t.ResolutionDays), _
    Low = g.Where(Function(t) t.PriorityId = 4).DefaultIfEmpty().Average(Function(t) t.ResolutionDays), _
    Total = g.Where(Function(t) t.Id <> Nothing).DefaultIfEmpty().Average(Function(t) t.ResolutionDays)

更新！ 这是我需要 VB 做的同样事情的 SQL 查询。

SELECT
    DATENAME(MONTH,t.CloseDate) AS 'Month',
    AVG(CASE WHEN (t.PriorityId = 1) THEN CAST(t.ResolutionDays AS Decimal(18, 2)) ELSE 0 END) AS 'Critical',
    AVG(CASE WHEN (t.PriorityId = 2) THEN CAST(t.ResolutionDays AS Decimal(18, 2)) ELSE 0 END) AS 'High',
    AVG(CASE WHEN (t.PriorityId = 3) THEN CAST(t.ResolutionDays AS Decimal(18, 2)) ELSE 0 END) AS 'Normal',
    AVG(CASE WHEN (t.PriorityId = 4) THEN CAST(t.ResolutionDays AS Decimal(18, 2)) ELSE 0 END) AS 'Low',
    AVG(CAST(t.ResolutionDays AS Decimal(18, 2))) AS 'Monthly Average'
FROM
    tblMaintenanceTicket t
WHERE
    t.StatusId = 2
    AND YEAR(t.CloseDate) = year(getdate())
GROUP BY 
    MONTH(t.CloseDate),
    DATENAME(MONTH,t.CloseDate)
ORDER BY
    MONTH(t.CloseDate)
4

3 回答 3

24 
Critical = g.Where(Function(t) t.PriorityId = 1).Select(Function(t) t.ResolutionDays).DefaultIfEmpty().Average()

参考： http: //geekswithblogs.net/SoftwareDoneRight/archive/2011/02/15/fixing-linq-error-sequence-contains-no-elements.aspx

于 2014-12-22T10:49:51.333 回答
13

IEnumerable(Of T)问题是如果输入没有任何元素，所有标量 LINQ 方法（Average、Max 等）都会抛出异常。看起来g.Where调用导致空集合导致异常。

您可能想要做的是编写一个处理空案例并返回默认值的方法。

于 2012-11-08T21:17:56.490 回答
6

这是我使用的辅助方法。

    public static double AverageOrDefault(this IEnumerable<double> enumerable)
    {
        return enumerable.DefaultIfEmpty().Average();
    }

    public static double AverageOrDefault<T>(this IEnumerable<T> enumerable, Func<T, double> selector)
    {
        return enumerable.Select(selector).DefaultIfEmpty().Average();
    }
于 2018-07-09T14:00:55.973 回答