格式化DataRow.Item时抛出System.InvalidCastException的原因？

环境 #

VS 2022 Community 17.5.3，控制台应用

问题代码 #

Module Module1
   Sub Main()
      Dim DT1 As DataTable = New DataTable("DT1")

      ' 为DataTable添加列
      Dim FieldType As Type = GetType(System.Int64)
      Dim MyColumn As DataColumn = New DataColumn("Col1", FieldType)
      DT1.Columns.Add(MyColumn)

      ' 添加行并赋值
      Dim NewRow As DataRow = DT1.NewRow
      Dim RowArray() As Object = New Object() {1234}
      NewRow.ItemArray = RowArray
      DT1.Rows.Add(NewRow)

      ' 显示行中Col1的格式化值
      Console.WriteLine("Col1 Value = " & NewRow.Item("Col1"))
      Console.WriteLine("Col1 Type = " & NewRow.Item("Col1").GetType.ToString)
      Console.WriteLine("Col1 Formatted Value = " & NewRow.Item("Col1").ToString("N0"))
   End Sub
End Module

输出及异常 #

执行输出 #

Col1 Value = 1234
Col1 Type = System.Int64

异常信息 #

System.InvalidCastException
  HResult=0x80004002
  Message=Conversion from string "N0" to type 'Integer' is not valid.
  Source=Microsoft.VisualBasic
  StackTrace:
   at Microsoft.VisualBasic.CompilerServices.Conversions.ToInteger(String Value) in f:\dd\vb\runtime\msvbalib\Helpers\Conversions.vb:line 788
   at ConsoleTest.Module1.Main() in C:\Data\VB.NET\ConsoleTest\ConsoleTest\Module1.vb:line 1434

  This exception was originally thrown at this call stack:
    Microsoft.VisualBasic.CompilerServices.Conversions.ParseDouble(String, System.Globalization.NumberFormatInfo) in Conversions.vb
    Microsoft.VisualBasic.CompilerServices.Conversions.ParseDouble(String) in Conversions.vb
    Microsoft.VisualBasic.CompilerServices.Conversions.ToInteger(String) in Conversions.vb

Inner Exception 1:
FormatException: Input string was not in a correct format.

原因分析 #

核心问题在于**NewRow.Item("Col1")返回的是Object类型**：

• 编译阶段，编译器只能识别Object自带的无参数ToString()方法，无法直接调用Int64实现的ToString(String)重载（该重载来自IFormattable接口）。
• VB.NET编译器会将Object.ToString("N0")误解析为调用全局的Microsoft.VisualBasic.Strings.ToString函数，这个函数的重载要求传入Integer类型参数，因此会尝试将字符串"N0"转换为整数，转换失败后抛出InvalidCastException。

而当你先将NewRow.Item("Col1")转换为Int64变量后，编译时类型明确为Int64，编译器会正确匹配Int64.ToString(String)重载，自然能生成格式化后的结果。

解决办法 #

方法1：显式转换为Int64后调用ToString #

' 使用CType转换
Console.WriteLine("Col1 Formatted Value = " & CType(NewRow.Item("Col1"), Int64).ToString("N0"))

' 或使用DirectCast转换
Console.WriteLine("Col1 Formatted Value = " & DirectCast(NewRow.Item("Col1"), Int64).ToString("N0"))

方法2：使用String.Format自动处理格式化 #

Console.WriteLine("Col1 Formatted Value = " & String.Format("{0:N0}", NewRow.Item("Col1")))

内容的提问来源于stack exchange，提问作者BandannaMan