问题描述
从 DataTable 中提取数据时抛出异常“Object cannot be cast from dbnull to other types”。显然,下面的语法不是“dbnull 感知”。如果在转换过程中发现 dbnull,有没有办法防止抛出此异常?
Dim arrayOfDoubles()() As Double = DT.AsEnumerable().Select(Function(x) {Convert.Todouble(x("Age")),Convert.Todouble(x("Weight")),Convert.Todouble(x("Location"))}).ToArray()
此外,我实际上是从我知道是双精度的列名的字符串数组开始的。那么有没有办法将所有请求的列名连接到一个查询字符串中,然后将其注入到 Select 语句中?以这种方式,我将只获得所有列都不是 dbnull 的行的双精度数组。
解决方法
如果我明白你需要这样的东西
在 Where
子句中,您可以测试您的字段是否有 DbNull
值。
Dim arrayOfDoubles()() As Double = DT.AsEnumerable().
Where(Function(f) f("Age") IsNot DBNull.Value AndAlso f("Weight") IsNot DBNull.Value).
Select(Function(x) {Convert.ToDouble(x("Age")),Convert.ToDouble(x("Weight")),Convert.ToDouble(x("Location"))}).ToArray()
无需在每个子句(在 where 和 select)中进行具体说明,您可以使用一些集中的“技巧”来简化您的代码,如下所示:
Dim DT As DataTable = New DataTable
DT.Columns.Add("Age",GetType(Double))
DT.Columns.Add("Weight",GetType(Double))
DT.Columns.Add("Location",GetType(Double))
DT.Columns.Add("Moves",GetType(Double))
For i As Integer = 0 To 10
Dim dr As DataRow = DT.NewRow
If i Mod 2 = 0 Then
dr.ItemArray = {i,DBNull.Value,i + 2,DBNull.Value}
Else
dr.ItemArray = {i,i + 1,i + 3}
End If
DT.Rows.Add(dr)
Next
'============================================================================
Dim flds As List(Of String) = {"Age","Weight","Location","Moves"}.ToList
Dim arrayOfDoubles()() As Double = DT.AsEnumerable().
Where(Function(f) flds.Where(Function(el) f(el) IsNot DBNull.Value).Count = flds.Count).
Select(Function(x) flds.Select(Function(el) Convert.ToDouble(x(el))).ToArray).ToArray
'====================================================================================
For Each el In arrayOfDoubles
Debug.Print("Age: " & el(flds.IndexOf("Age")).ToString)
Debug.Print("Weight: " & el(flds.IndexOf("Weight")).ToString)
Debug.Print("Location: " & el(flds.IndexOf("Location")).ToString)
Debug.Print("Location: " & el(flds.IndexOf("Moves")).ToString)
Next