如何在 ASP 中获取下拉列表值作为 SQL 命令的数字答案

【问题标题】：How to get dropdownlist value as number to SQL command in ASP如何在 ASP 中获取下拉列表值作为 SQL 命令的数字
【发布时间】：2019-12-16 00:19:00
【问题描述】：

我有一个下拉列表：

<asp:DropDownList ID="DropDownList1" runat="server">
        <asp:ListItem>10</asp:ListItem>
        <asp:ListItem>30</asp:ListItem>
        <asp:ListItem Value="100000">All</asp:ListItem>

后面的代码是：

Private Sub BindAllEmployee()
    Dim CS As String = ConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString
    Dim sNumber As String = DropDownList1.SelectedValue
    Dim iNumber As Integer = Convert.ToInt32(sNumber)
    Using con As SqlConnection = New SqlConnection(CS)

        Using cmd As SqlCommand = New SqlCommand("SELECT TOP '" + iNumber + "' [EmployeeID], [EmployeeCode], [EmployeeName], [Position], [HireDate] FROM [Employee] ORDER BY [EmployeeID] DESC", con)

            Using sda As SqlDataAdapter = New SqlDataAdapter(cmd)
                Dim dte As DataTable = New DataTable()
                sda.Fill(dte)
                GridView1.DataSource = dte
                GridView1.DataBind()
            End Using
        End Using
    End Using
End Sub

我遇到了一个错误：

从字符串“SELECT TOP '”到类型“Double”的转换无效

【问题讨论】：

标签： sql asp.net sql-server vb.net tsql

【解决方案1】：

不应引用 TOP 的参数。所以你可以试试

"SELECT TOP " + iNumber + " [EmployeeID] ...

相反。

但实际上也不需要将sNumber 转换为整数，因为当您将它包含在 SqlCommand 中时，它只需要再次转换回字符串（因为 SQL 命令文本是一个大字符串）。

其实

"SELECT TOP " + sNumber + " [EmployeeID] ...

会更有意义。

所以这是我认为效果更好的最终版本：

Private Sub BindAllEmployee()
    Dim CS As String = ConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString
    Dim sNumber As String = DropDownList1.SelectedValue

    Using con As SqlConnection = New SqlConnection(CS)
        Dim sql as String = "SELECT TOP" + sNumber + " [EmployeeID], [EmployeeCode], [EmployeeName], [Position], [HireDate] FROM [Employee] ORDER BY [EmployeeID] DESC"

        Using cmd As SqlCommand = New SqlCommand(sql , con)

            Using sda As SqlDataAdapter = New SqlDataAdapter(cmd)
                Dim dte As DataTable = New DataTable()
                sda.Fill(dte)
                GridView1.DataSource = dte
                GridView1.DataBind()
            End Using
        End Using
    End Using
End Sub

【讨论】：

【解决方案2】：

更改查询字符串：

New SqlCommand("SELECT TOP " + iNumber.ToString() + " [EmployeeID], [EmployeeCode], [EmployeeName], [Position], [HireDate] FROM [Employee] ORDER BY [EmployeeID] DESC", con)

您必须将 iNumber 转换为字符串以将其连接起来并删除已建议的单引号。

矿石直接使用sNumber：

New SqlCommand("SELECT TOP " + sNumber + " [EmployeeID], [EmployeeCode], [EmployeeName], [Position], [HireDate] FROM [Employee] ORDER BY [EmployeeID] DESC", con)

【讨论】：

【解决方案3】：

似乎与用于连接的 + 符号有些混淆。我收到 Option Strict 的错误。编译器正在尝试将（算术添加）“SELECT TOP”添加到整数当我将 + 更改为和 & & 符号时，它是 vb.net 中的连接符号，该错误消失了。同时删除单引号。

无需将下拉值更改为数字。它正在字符串中使用，因此要使用字符串连接，字符串是合适的。如果您使用字符串，编译器会将 + 符号解释为连接运算符，但最好始终使用 &。

在 Using 块之外标注 DataTable。这样您就可以在连接关闭后使用它来填充网格。尽快关闭连接。

Private Sub BindAllEmployee()
    Dim dte As DataTable = New DataTable()
    Dim CS As String = ConfigurationManager.ConnectionStrings("ConnectionString").ConnectionString
    Dim sNumber As String = DropDownList1.SelectedValue
    Using con As New SqlConnection(CS)
        Using cmd As New SqlCommand("SELECT TOP " & sNumber & " [EmployeeID], [EmployeeCode], [EmployeeName], [Position], [HireDate] FROM [Employee] ORDER BY [EmployeeID] DESC", con)
            dte.Load(cmd.ExecuteReader)
        End Using
    End Using
    GridView1.DataSource = dte
    GridView1.DataBind()
End Sub

【讨论】：