Question

我是JOIN两个DataTales（通过解析两个Excel电子表格填充）。这些Excel电子表格包含并非始终具有值的列TOTAL_DOLLARS和TOTAL_UNITS。因此，当我尝试运行query.CopyToDataTable()

时，我的程序会抛出以下错误

DataSet does not support System.Nullable<>

Dim query = From c In dtDollars.AsEnumerable() _
   Join r In dtUnits.AsEnumerable() _
   On c.Field(Of String)("UPC") Equals r.Field(Of String)("UPC") _
   Select _
   New _
   With {.UPC = r.Field(Of String)("UPC"), .WIC_NUMBER = c.Field(Of String)("WIC_NUMBER"), _
   .WAG_ITEM_DESC = c.Field(Of String)("WAG_ITEM_DESC"), _
   .WAG_BRAND = c.Field(Of String)("WAG_BRAND"), .UOM = c.Field(Of String)("UOM"), _
   .GSK_PROMO_GRP = c.Field(Of String)("GSK_PROMO_GRP"), _
   .TOTAL_DOLLARS = c.Field(Of Decimal?)("TOTAL_DOLLARS"), _
   .TOTAL_UNITS = r.Field(Of Integer?)("TOTAL_UNITS"), _
   .WKND_DATE = c.Field(Of DateTime)("WKND_DATE")}

    myResultTable = query.CopyToDataTable()

我该怎么做才能解决这个问题？有没有办法可以将这些列的默认值设置为0 if = DBNull.Value？

以下是我Modules方法使用的Extensions和CopyToDataTable()：

Public Module CustomLINQtoDataSetMethods
    <Extension()> _
    Public Function CopyToDataTable(Of T)(ByVal source As IEnumerable(Of T)) As DataTable
        Return New ObjectShredder(Of T)().Shred(source, Nothing, Nothing)
    End Function

    <Extension()> _
    Public Function CopyToDataTable(Of T)(ByVal source As IEnumerable(Of T), ByVal table As DataTable, ByVal options As LoadOption?) As DataTable
        Return New ObjectShredder(Of T)().Shred(source, table, options)
    End Function

End Module

Module DataSetLinqOperators
  ''' <summary>
  ''' Creates a <see cref="DataTable"/> that contains the data from a source sequence.
  ''' </summary>
  ''' <remarks>
  ''' The initial schema of the DataTable is based on schema of the type T. All public property and fields are turned into DataColumns.
  ''' If the source sequence contains a sub-type of T, the table is automatically expanded for any addition public properties or fields.
  ''' </remarks>
  <Extension()> _
  Public Function ToDataTable(Of T)(ByVal source As IEnumerable(Of T)) As DataTable
    Return New ObjectShredder(Of T)().Shred(source, Nothing, Nothing)
  End Function

  ''' <summary>
  ''' Loads the data from a source sequence into an existing <see cref="DataTable"/>.
  ''' </summary>
  ''' <remarks>
  ''' The schema of <paramref name="table" /> must be consistent with the schema of the type T (all public property and fields are mapped to DataColumns).
  ''' If the source sequence contains a sub-type of T, the table is automatically expanded for any addition public properties or fields.
  ''' </remarks>
  <Extension()> _
  Public Function LoadSequence(Of T)(ByVal source As IEnumerable(Of T), ByVal table As DataTable, ByVal options As System.Nullable(Of LoadOption)) As DataTable
    If table Is Nothing Then
      Throw New ArgumentNullException("table")
    End If
    Return New ObjectShredder(Of T)().Shred(source, table, options)
  End Function
End Module

此外，我正在使用ObjectShredder类。

非常感谢任何帮助！

Answer 1

试试这个：

.TOTAL_DOLLARS = If( (c.Field(Of Decimal?)("TOTAL_DOLLARS")).HasValue, _
                     (c.Field(Of Decimal?)("TOTAL_DOLLARS"), 0))

这有点尴尬，但它应该有用。

加入两个DataTables - DataSet不支持System.Nullable＆lt;＆gt;

1 个答案: