为什么通过“HasRows”测试后出现“无数据”错误？

Question

我有这个代码：

Protected Function GetArgValsForCompanyName(coName As String) As String()
    Dim args(2) As String
    Dim sqlConnection1 As New SqlConnection("SERVER=PLATYPUS42;DATABASE=duckbilldata;UID=durante;PWD=pondscum")
    Dim cmd As New SqlCommand
    Dim reader As SqlDataReader

    cmd.CommandText = "select Unit, MemberNo, CustNo from Customers WHERE CompanyName = @CoName"
    cmd.CommandType = CommandType.Text
    cmd.Parameters.Add("@CoName", SqlDbType.VarChar, 50).Value = coName
    cmd.Connection = sqlConnection1
    sqlConnection1.Open()

    reader = cmd.ExecuteReader()
    If reader.HasRows Then
            args(0) = reader.Item(0).ToString()
            args(1) = reader.Item(1).ToString()
            args(2) = reader.Item(2).ToString()
    End If
    reader.Close()
    sqlConnection1.Close()

    Return args
End Function

...这行失败：

args(0) = reader.Item(0).ToString()

...与：

*System.InvalidOperationException was unhandled
  HResult=-2146233079
  Message=Invalid attempt to read when no data is present.*

它怎么可能是“HasRows”却没有数据存在？

注意：当我尝试这个（而不是使用“0”索引）时，它也会失败并出现同样的错误：

args(0) = reader.Item("Unit").ToString()

更新

已接受的答案在现代应用程序（例如，我的“沙盒”Windows 窗体应用程序）中运行良好，但在旧应用程序中，例如使用 .NET 史前的陈旧、陈旧的网站，它就不行了——“使用”显然无法识别；我明白了：

Server Error in '/EMS/customerreportingnet' Application.
--------------------------------------------------------------------------------

Compilation Error 
Description: An error occurred during the compilation of a resource required to service this request. 

Please review the following specific error details and modify your source code appropriately. 

Compiler Error Message: BC30203: Identifier expected.

Source Error:

Line 90:         Dim args(2) As String
Line 91: 
Line 92:         Using con As New SqlConnection("SERVER=PLATYPUS42;DATABASE=duckbilldata;UID=durante;PWD=pondscum"),
Line 93:               cmd As New SqlCommand("select Unit, MemberNo, CustNo from Customers WHERE 

CompanyName = @CoName", con)
Line 94: 

--------------------------------------------------------------------------------
Version Information: Microsoft .NET Framework Version:2.0.50727.5485; ASP.NET Version:2.0.50727.5491 

Answer 1

正如您在answer 中所说，需要致电.Read。

If reader.HasRows 但不是必需的。 Do While reader.Read 将处理此问题。如果有行则进入循环，否则绕过。

作为补充说明，我认为实现Using 会有所帮助：

Protected Function GetArgValsForCompanyName(coName As String) As String()
    Dim args(2) As String

    Using con As New SqlConnection("SERVER=PLATYPUS42;DATABASE=duckbilldata;UID=durante;PWD=pondscum"),
          cmd As New SqlCommand("select Unit, MemberNo, CustNo from Customers WHERE CompanyName = @CoName", con)

        con.Open()

        cmd.CommandType = CommandType.Text
        cmd.Parameters.Add("@CoName", SqlDbType.VarChar, 50).Value = coName

        Using reader As SqlDataReader = cmd.ExecuteReader

            While reader.Read
                args(0) = reader.Item(0).ToString()
                args(1) = reader.Item(1).ToString()
                args(2) = reader.Item(2).ToString()
            End While

        End Using

    End Using

    Return args
End Function

使用Using，您不必担心调用.Close 或处理对象。我也觉得它读起来更好。

Answer 2

还需要调用“读取”；这有效（可以使用返回的字段名称或其索引）：

If reader.HasRows Then
    Do While reader.Read
        'args(0) = reader.Item(0).ToString()
        args(0) = reader.Item("Unit").ToString()
        args(1) = reader.Item(1).ToString()
        args(2) = reader.Item(2).ToString()
    Loop
End If