刷新DropdownList数据后，选择一个新项目

Question

我有两个下拉列表，一个是状态，一个是城市。当选择一个州时，城市下拉列表将被填充。此时，您可以选择一个城市，但是如果您选择一个城市并返回并选择其他状态，城市下拉列表将由两个州的城市填充。我该如何清除第一个州的城市？

这里是背后

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

    End Sub 

    Public Sub ddlState_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) 
     SqlDataSource2.SelectParameters.Clear() 
     SqlDataSource2.SelectParameters.Add(New Parameter("@State", DbType.String, ddlState.SelectedValue)) 
     ddlCity.DataBind() 
     'SqlDataSource2.SelectParameters.Clear() 
     'Dim State As String = ddlState.SelectedItem.Text 
    End Sub 

    Protected Sub ddlCity_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ddlCity.SelectedIndexChanged 
     'Dim City As String = ddlCity.SelectedItem.Text 
    End Sub 

Answer 1

Public Sub ddlState_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs)   
    SqlDataSource2.SelectParameters.Clear()   
    SqlDataSource2.SelectParameters.Add(New Parameter("@State", DbType.String, ddlState.SelectedValue)) 
    ddlCity.Items.Clear(); 
    ddlCity.Items.Add(new ListItem("SELECT ONE", "")) ' If you want it '  
    ddlCity.DataBind() 
End Sub 

代码你也可以在控制设置AppendDataboundItems = 'false'。