Vb.net DropDownList的初始值应为空

我有一个Dropdownlist“Servers”，它在表单加载事件期间由下面的代码填充。连接字符串中的数据库有一个服务器名称列表，使用下面的代码填充这些名称时不会出现任何问题

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

    If Not IsPostBack Then
        Populate_Server()
    End If
End Sub

Public Sub Populate_Server()
    Dim query As String = "select ServerName, ServerId from Server_Names"
    BindDropDownList(DropDownList_Server, query, "ServerName", "ServerId", "")
End Sub

接下来，我有另一个下拉列表“实例”。要填充到此ddlist的数据位于另一台服务器/数据库上。因此，根据前面的选择（ddlist_server），我正在填充数据，如下代码所示：

Public Sub Populate_Instance()

    Dim State As String
    Dim ConnectString As String
    Dim SelectSQL As String
    Srv = DropDownList_Instance.SelectedItem.Text
    DropDownList_Instance.Items.Clear()


    ConnectString = "Data Source=" & Srv & ";Initial Catalog=master;Integrated Security=true"

    Using Con As New SqlConnection(ConnectString)
        Con.Open()
        SelectSQL = "Select * from Instance;"

        Dim com As SqlCommand = New SqlCommand(SelectSQL, Con)
        Dim dr As SqlDataReader = com.ExecuteReader()
        Do While dr.Read = True
            DropDownList_Database.Items.Add(dr.GetString(0))
        Loop
        'DropDownList_Database.DataBind()
    End Using
End Sub

<asp:RequiredFieldValidator ID="req1" runat="server" Style="top: 85px;
        left: 378px; position: absolute; height: 20px; width: 160px" Font- Bold="True" Font-Names="Trebuchet M S" Font-Size="Small"
        ControlToValidate="DropDownList_Instance" validationgroup="Info"
        ErrorMessage="Select an Instance" InitialValue=0>
</asp:RequiredFieldValidator>

实例ddlist已成功填充，但第一个值是表中的第一条记录，但我的要求是该值为空。原因是，我想进行服务器端验证，如下代码所示：

Public Sub Populate_Instance()

    Dim State As String
    Dim ConnectString As String
    Dim SelectSQL As String
    Srv = DropDownList_Instance.SelectedItem.Text
    DropDownList_Instance.Items.Clear()


    ConnectString = "Data Source=" & Srv & ";Initial Catalog=master;Integrated Security=true"

    Using Con As New SqlConnection(ConnectString)
        Con.Open()
        SelectSQL = "Select * from Instance;"

        Dim com As SqlCommand = New SqlCommand(SelectSQL, Con)
        Dim dr As SqlDataReader = com.ExecuteReader()
        Do While dr.Read = True
            DropDownList_Database.Items.Add(dr.GetString(0))
        Loop
        'DropDownList_Database.DataBind()
    End Using
End Sub

<asp:RequiredFieldValidator ID="req1" runat="server" Style="top: 85px;
        left: 378px; position: absolute; height: 20px; width: 160px" Font- Bold="True" Font-Names="Trebuchet M S" Font-Size="Small"
        ControlToValidate="DropDownList_Instance" validationgroup="Info"
        ErrorMessage="Select an Instance" InitialValue=0>
</asp:RequiredFieldValidator>

如Pikoh所述，在

ddl_实例

中第一个条目为空或验证

ddl_实例

时，我需要您的帮助

DropDownList_Database.Items.Add("", "0")

添加空白项。

然后更改reuqired字段验证器，使其通过添加来知道初始值

InitialValue="0"

---

只需执行

DropDownList\u Database.Items。在
do While
loop之前添加（“”）

，谢谢你的时间，Pikoh，这很有效。但当ddlist条目为空且单击按钮后，ddl_数据库验证不会显示“选择数据库”消息。请给出建议。将InitialValue=“IDOFBLANKITEMADTED”添加到所需的字段，这样就行了