Question

我正在尝试使用适当的值在DropDownList ddlMealType中选择正确的选项，具体取决于DropDownList ddlMeals中的选择。这在我手动选择Meal时工作正常，但在最初加载页面时却没有（因为ddlMeal的selectedIndex = -1）。

因此我尝试将所选索引设置为Page_Load列表中的第一个Meal，但是在后续行添加断点时，我可以看到SelectedIndex的值仍为-1。是不可能以编程方式设置dropdownList的SelectedIndex属性？

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            ddlMeals.SelectedIndex = 0;
            ddlMeals_SelectedIndexChanged(this, EventArgs.Empty);
        }
   }

    protected void ddlMeals_SelectedIndexChanged(object sender, EventArgs e)
    {
        // Fetch details for selected Meal
        SqlDataReader reader = null;
        String ConnectString = System.Configuration.ConfigurationManager.ConnectionStrings["Kunskapshjulet"].ConnectionString;
        SqlConnection connection1 = new SqlConnection(ConnectString);
        SqlCommand selectCommand = new SqlCommand("SELECT MealType FROM Meals WHERE MealID = " + ddlMeals.SelectedValue, connection1);
        try
        {
            connection1.Open();
            reader = selectCommand.ExecuteReader();

            reader.Read();
            string strMealtype = reader[0].ToString();
                ddlMealTypes2.SelectedValue = reader[0].ToString();
        }


        <asp:DropDownList ID="ddlMeals" runat="server" OnSelectedIndexChanged="ddlMeals_SelectedIndexChanged"
                      AutoPostBack="True" DataSourceID="SqlMealsPerUser" DataTextField="MealName" DataValueField="MealID" Width="180px">
        </asp:DropDownList>

Answer 1

在设置PageLoad中的SelectedIndex

之前，需要将数据绑定到DropDownList

Answer 2

在您的aspx代码中，SqlMealsPerUser是您的数据源ID。它有什么价值吗？我相信只有在Page_Load后面的代码完成后，数据才会绑定到控件上。

在这种情况下，您可以使用

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            ddlMeals.DataSource=BindData(); // this function gets the data u need to bind to your drop down.
            ddlMeals.DataBind();
            if(ddlMeals.Items.Count > 0)
            {
                ddlMeals.SelectedIndex = 0;
                ddlMeals_SelectedIndexChanged(null, EventArgs.Empty);
            }
        }
    }

Answer 3

Para hacer que funcione el SelectedIndex de un DropDownList zh C＃，hay que llamar el evento de SelectIndexChanged de la siguiente manera：

  protected void Page_Load(object sender, EventArgs e)
  {
     if (!IsPostBack)
     {
        ddlMeals.DataBind();
        if(ddlMeals.Items.Count > 0)
        {
            ddlMeals.SelectedIndex = 0;
            ddlMeals_SelectedIndexChanged(null, EventArgs.Empty);
        }
  }

Saludos !!

在Page_Load中设置DropDownList.SelectedIndex

3 个答案: