Question

我正在填写数据库中的下拉列表，但我想将“选择项目”添加到下拉列表中，并将其设为默认选项：我试图设置writerddl.SelectedValue = "please...";，但运行时错误说：

'writerddl'有一个无效的SelectedValue，因为它没有存在于项目列表中。参数名称：值

这是我的代码：

protected void Page_Load(object sender, EventArgs e)
{
writerddl.SelectedValue = "please...";
if (!IsPostBack)
  {
   writerddl.DataSource = DS.show_all_writers();
   writerddl.DataValueField = "writerid";
   writerddl.DataTextField = "writersname";            
   writerddl.DataBind();
  }
}

Answer 1

试试这段代码。

protected void Page_Load(object sender, EventArgs e)
{       
   if (!IsPostBack)
   {
       writerddl.DataSource = DS.show_all_writers();
       writerddl.DataValueField = "writerid";
       writerddl.DataTextField = "writersname";            
       writerddl.DataBind();
       writerddl.Items.insert(0, new ListItem("Please select",""));
   }
}

Answer 2

删除它：

writerddl.SelectedValue = "please...";

并使用此：

//....    
writerddl.DataBind();
writerddl.Items.Insert(0, "please...");
//...

更新：

使用像这样的Clear（）方法：

protected void Page_Load(object sender, EventArgs e)
{  
   writerddl.Items.Clear();
   writerddl.DataSource = DS.show_all_writers();
   writerddl.DataValueField = "writerid";
   writerddl.DataTextField = "writersname";            
   writerddl.DataBind();
   writerddl.Items.Insert(0, "please...");
}

Answer 3

使用以下代码。请注意，选定值行在下方移动，这会产生差异。希望下拉列表中存在值为“please ...”的项目。

 protected void Page_Load(object sender, EventArgs e)
    {           
        if (!IsPostBack)
        {
            writerddl.DataSource = DS.show_all_writers();
            writerddl.DataValueField = "writerid";
            writerddl.DataTextField = "writersname";
            writerddl.DataBind();
            writerddl.SelectedValue = "please...";
        }

    }

更新 “希望带有价值的物品”请...“存在于下拉列表中”将此行更改为“希望带有值的项目”请...“存在于绑定源中”。

更新1： 如果来源没有项目“请...”，这里是以下代码。 protected void Page_Load（object sender，EventArgs e） {

    if (!IsPostBack)
    {
        writerddl.Items.Add(new ListItem("Select An Item", "please..."));
        writerddl.DataSource = DS.show_all_writers();
        writerddl.DataValueField = "writerid";
        writerddl.DataTextField = "writersname";
        writerddl.DataBind();
        writerddl.SelectedValue = "please...";
    }

}

设置从数据库填充的下拉列表的默认值

3 个答案: