刷新DropdownList數據後，選擇一個新項目

Question

我有兩個下拉列表，一個是狀態，一個是城市。當選擇一個州時，城市下拉列表將被填充。此時，您可以選擇一個城市，但是如果您選擇一個城市並返回並選擇其他狀態，城市下拉列表將由兩個州的城市填充。我該如何清除第一個州的城市？

這裏是背後

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 

代碼你也可以在控制設置AppendDataboundItems = 'false'。