asp.net - "Invalid attempt to read when no data is present"

Question

当 ASP.NET 不存在数据时，读取尝试无效

Error is on line 46: Dim requestid = dbreader.GetInt32(1)

我的数据库中有该列的数据，所以我陷入困境。

如果需要，可以发布更多代码。

Imports System.Data.SqlClient

Public Class WebForm1
Inherits System.Web.UI.Page

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    writemystuff()
End Sub

Dim conn As New SqlConnection

Sub CloseDatabase()
    conn.Close()
End Sub

Sub ConnectToDatabase()
    conn.ConnectionString = ConfigurationManager.ConnectionStrings("dbSQL").ConnectionString
    conn.Open()
End Sub


Sub writemystuff()

    'Response.Write(SQL1)

    Dim XMLOutput = "<?xml version=""1.0"" encoding=""utf-8"" ?>"
    'XMLOutput = XMLOutput + "<?xml-stylesheet type=""text/xsl"" href=""XMLSchema1.xslt""?>"
    XMLOutput = XMLOutput + "<requests "
    XMLOutput = XMLOutput + " xmlns = ""http://www.w3schools.com"""
    XMLOutput = XMLOutput + " xmlns:xsi = ""http://www.w3.org/2001/XMLSchema-instance"""
    XMLOutput = XMLOutput + " xsi:schemaLocation=""http://www.w3schools.com XMLSchema1.xsd"">"
    ConnectToDatabase()

    Dim choice = Request("requestchoice")
    Dim SQL1 = "select * from request where r_id = 314"
    If choice = "Request 112" Then
        SQL1 = "select * from request where r_id = 112"
    End If

    Dim cmd = New SqlCommand(SQL1, conn)
    Dim dbreader = cmd.ExecuteReader()
    For Each record In dbreader

    Next

    Dim requestid = dbreader.GetInt32(1)
    XMLOutput = XMLOutput + "<vendor"
    XMLOutput = XMLOutput + " v_name=""" & dbreader.GetString(2) & """"
    XMLOutput = XMLOutput + " v_id=""" & dbreader.GetInt32(0) & """"
    XMLOutput = XMLOutput + " r_id=""" & dbreader.GetInt32(1) & """"
    XMLOutput = XMLOutput + " >"
    Dim SQL2 = "select * from product where r_id = " & requestid

    Dim cmd2 = New SqlCommand(SQL2, conn)

    Dim dbreader2 = cmd2.ExecuteReader()
    For Each record2 In dbreader2
        XMLOutput = XMLOutput + "<product"
        XMLOutput = XMLOutput + " p_name=""" & dbreader2.GetString(3) & """"
        XMLOutput = XMLOutput + " p_id=""" & dbreader2.GetInt32(0) & """"
        XMLOutput = XMLOutput + " cost=""" & dbreader2.GetDecimal(2) & """"
        XMLOutput = XMLOutput + " >"

    Next
    dbreader2.Close()
    XMLOutput = XMLOutput + "</request>"

    CloseDatabase()
    XMLOutput = XMLOutput + "</requests>"
    Response.Write(XMLOutput)
End Sub
End Class

更新了仍然存在问题的代码:

Imports System.Data.SqlClient

Public Class WebForm1
Inherits System.Web.UI.Page

Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
    writemystuff()
End Sub

Dim conn As New SqlConnection

Sub CloseDatabase()
    conn.Close()
End Sub

Sub ConnectToDatabase()
    conn.ConnectionString = ConfigurationManager.ConnectionStrings("dbSQL").ConnectionString
    conn.Open()
End Sub


Sub writemystuff()

    'Response.Write(SQL1)

    Dim XMLOutput = "<?xml version=""1.0"" encoding=""utf-8"" ?>"
    'XMLOutput = XMLOutput + "<?xml-stylesheet type=""text/xsl"" href=""XMLSchema1.xslt""?>"
    XMLOutput = XMLOutput + "<requests "
    XMLOutput = XMLOutput + " xmlns = ""http://www.w3schools.com"""
    XMLOutput = XMLOutput + " xmlns:xsi = ""http://www.w3.org/2001/XMLSchema-instance"""
    XMLOutput = XMLOutput + " xsi:schemaLocation=""http://www.w3schools.com XMLSchema1.xsd"">"
    ConnectToDatabase()

    Dim choice = Request("requestchoice")
    Dim SQL1 = "select * from request where r_id = 314"
    If choice = "Request 112" Then
        SQL1 = "select * from request where r_id = 112"
    End If

    Dim cmd = New SqlCommand(SQL1, conn)
    Dim dbreader = cmd.ExecuteReader()
    For Each record In dbreader


        While dbreader.Read()
            Dim requestid = dbreader.GetInt32(1)
            XMLOutput = XMLOutput + "<vendor"
            XMLOutput = XMLOutput + " v_name=""" & dbreader.GetString(2) & """"
            XMLOutput = XMLOutput + " v_id=""" & dbreader.GetInt32(0) & """"
            XMLOutput = XMLOutput + " r_id=""" & dbreader.GetInt32(1) & """"
            XMLOutput = XMLOutput + " >"
            Dim SQL2 = "select * from product where r_id = " & requestid
        End While

        Dim cmd2 = New SqlCommand(SQL2, conn)

        Dim dbreader2 = cmd2.ExecuteReader()
        While dbreader2.Read()
            XMLOutput = XMLOutput + "<product"
            XMLOutput = XMLOutput + " p_name=""" & dbreader2.GetString(3) & """"
            XMLOutput = XMLOutput + " p_id=""" & dbreader2.GetInt32(0) & """"
            XMLOutput = XMLOutput + " cost=""" & dbreader2.GetDecimal(2) & """"
            XMLOutput = XMLOutput + " >"
        End While

    Next

    XMLOutput = XMLOutput + "</request>"


    CloseDatabase()
    XMLOutput = XMLOutput + "</requests>"
    Response.Write(XMLOutput)
End Sub
End Class

Answer 1

您需要在 SqlDataReader 对象上调用 .Read() 来获取该行的数据，如下所示:

While dbreader2.Read()
    ' Do logic here to get individual values from each row
End While 

更新:

而不是这个:

Dim dbreader2 = cmd2.ExecuteReader()
For Each record2 In dbreader2
    XMLOutput = XMLOutput + "<product"
    XMLOutput = XMLOutput + " p_name=""" & dbreader2.GetString(3) & """"
    XMLOutput = XMLOutput + " p_id=""" & dbreader2.GetInt32(0) & """"
    XMLOutput = XMLOutput + " cost=""" & dbreader2.GetDecimal(2) & """"
    XMLOutput = XMLOutput + " >"

Next

这样做:

Dim dbreader2 = cmd2.ExecuteReader()
While dbreader2.Read()
    XMLOutput = XMLOutput + "<product"
    XMLOutput = XMLOutput + " p_name=""" & dbreader2.GetString(3) & """"
    XMLOutput = XMLOutput + " p_id=""" & dbreader2.GetInt32(0) & """"
    XMLOutput = XMLOutput + " cost=""" & dbreader2.GetDecimal(2) & """"
    XMLOutput = XMLOutput + " >"
End While

---

Note: If you are interested in knowing if the SqlDataReader has any rows or not, then you can check that before looping through the reading of rows, like this:

Dim dbreader2 = cmd2.ExecuteReader()
If dbreader2.HasRows Then
    While dbreader2.Read()
        XMLOutput = XMLOutput + "<product"
        XMLOutput = XMLOutput + " p_name=""" & dbreader2.GetString(3) & """"
        XMLOutput = XMLOutput + " p_id=""" & dbreader2.GetInt32(0) & """"
        XMLOutput = XMLOutput + " cost=""" & dbreader2.GetDecimal(2) & """"
        XMLOutput = XMLOutput + " >"
    End While
Else
    ' Potentially generate a "no data found" message here, etc.

End If

---

更新2:

好的，完整的方法代码应如下所示:

Sub writemystuff()
    'Response.Write(SQL1)

    Dim XMLOutput = "<?xml version=""1.0"" encoding=""utf-8"" ?>"
    'XMLOutput = XMLOutput + "<?xml-stylesheet type=""text/xsl"" href=""XMLSchema1.xslt""?>"
    XMLOutput = XMLOutput + "<requests "
    XMLOutput = XMLOutput + " xmlns = ""http://www.w3schools.com"""
    XMLOutput = XMLOutput + " xmlns:xsi = ""http://www.w3.org/2001/XMLSchema-instance"""
    XMLOutput = XMLOutput + " xsi:schemaLocation=""http://www.w3schools.com XMLSchema1.xsd"">"
    ConnectToDatabase()

    Dim choice = Request("requestchoice")
    Dim SQL1 = "select * from request where r_id = 314"
    If choice = "Request 112" Then
        SQL1 = "select * from request where r_id = 112"
    End If

    Dim cmd = New SqlCommand(SQL1, conn)
    Dim dbreader = cmd.ExecuteReader()
    Dim requestid As Integer
    While dbreader.Read()
        requestid = dbreader.GetInt32(1)
        XMLOutput = XMLOutput + "<vendor"
        XMLOutput = XMLOutput + " v_name=""" & dbreader.GetString(2) & """"
        XMLOutput = XMLOutput + " v_id=""" & dbreader.GetInt32(0) & """"
        XMLOutput = XMLOutput + " r_id=""" & dbreader.GetInt32(1) & """"
        XMLOutput = XMLOutput + " >"
    End While

    Dim SQL2 = "select * from product where r_id = " & requestid
    Dim cmd2 = New SqlCommand(SQL2, conn)
    Dim dbreader2 = cmd2.ExecuteReader()
    While dbreader2.Read()
        XMLOutput = XMLOutput + "<product"
        XMLOutput = XMLOutput + " p_name=""" & dbreader2.GetString(3) & """"
        XMLOutput = XMLOutput + " p_id=""" & dbreader2.GetInt32(0) & """"
        XMLOutput = XMLOutput + " cost=""" & dbreader2.GetDecimal(2) & """"
        XMLOutput = XMLOutput + " >"
    End While
    dbreader2.Close()
    XMLOutput = XMLOutput + "</request>"

    CloseDatabase()
    XMLOutput = XMLOutput + "</requests>"
    Response.Write(XMLOutput)
End Sub