如何在 ASP.NET 中动态返回 DropDownList 选择？

Question

这可能是一个简单的问题，但我正在使用 DropDownList 用 C# 开发一个 Web 应用程序。目前它只适用于一个 DropDownList。但是现在我修改了代码，使应该出现的 DropDownLists 的数量是动态的，它给了我错误；

"名称 'ddl' 不存在于 当前上下文。”

出现此错误的原因是 'ddl' = number of counters 的多个实例。那么我该如何返回多个“ddl”呢？比如这个方法应该有什么返回类型？以及如何返回这些值？

我需要动态的原因是我需要为他们选择的 Adventureworks 表中的每一列创建一个 DropDownList。

      private DropDownList CreateDropDownLists()
    {
        for (int counter = 0; counter < NumberOfControls; counter++)
        {
            DropDownList ddl = new DropDownList();
            SqlDataReader dr2 = ADONET_methods.DisplayTableColumns(targettable);
            ddl.ID = "DropDownListID" + (counter + 1).ToString();

            int NumControls = targettable.Length;
            DataTable dt = new DataTable();
            dt.Load(dr2);

            ddl.DataValueField = "COLUMN_NAME";
            ddl.DataTextField = "COLUMN_NAME";
            ddl.DataSource = dt;
            ddl.ID = "DropDownListID 1";
            ddl.SelectedIndexChanged += new EventHandler(ddlList_SelectedIndexChanged);
            ddl.DataBind();

            ddl.AutoPostBack = true;
            ddl.EnableViewState = true; //Preserves View State info on Postbacks
            //ddlList.Style["position"] = "absolute";
            //ddl.Style["top"] = 80 + "px";
            //ddl.Style["left"] = 0 + "px";
            dr2.Close();
        }
        return ddl;
    }

Answer 1

看起来您正试图在函数末尾返回下拉菜单，但它是在 for 循环范围内声明的。尝试在函数的开头（在 for 循环之前）创建一个下拉数组，并将数组设置在循环内，然后返回它。您还需要更改函数声明以支持下拉列表。

private DropDownList[] CreateDropDownLists()
{
    DropDownList[] dropDowns = new DropDownList[NumberOfControls];

    for (int counter = 0; counter < NumberOfControls; counter++)
    {
        DropDownList ddl = new DropDownList();
        SqlDataReader dr2 = ADONET_methods.DisplayTableColumns(targettable);
        ddl.ID = "DropDownListID" + (counter + 1).ToString();

        int NumControls = targettable.Length;
        DataTable dt = new DataTable();
        dt.Load(dr2);

        ddl.DataValueField = "COLUMN_NAME";
        ddl.DataTextField = "COLUMN_NAME";
        ddl.DataSource = dt;
        ddl.ID = "DropDownListID 1";
        ddl.SelectedIndexChanged += new EventHandler(ddlList_SelectedIndexChanged);
        ddl.DataBind();

        ddl.AutoPostBack = true;
        ddl.EnableViewState = true; //Preserves View State info on Postbacks
        //ddlList.Style["position"] = "absolute";
        //ddl.Style["top"] = 80 + "px";
        //ddl.Style["left"] = 0 + "px";
        dr2.Close();

        dropDowns[counter] = ddl;
    }
    return dropDowns;
}

Answer 2

为什么不只返回一个 DropDownList 列表？

private List<DropDownList> CreateDropDownLists()
{
    List<DropDownList> listDDL = new List<DropDownList>();
    for (int counter = 0; counter < NumberOfControls; counter++)
    {
        DropDownList ddl = new DropDownList();

        SqlDataReader dr2 = ADONET_methods.DisplayTableColumns(targettable);
        ddl.ID = "DropDownListID" + (counter + 1).ToString();

        int NumControls = targettable.Length;
        DataTable dt = new DataTable();
        dt.Load(dr2);

        ddl.DataValueField = "COLUMN_NAME";
        ddl.DataTextField = "COLUMN_NAME";
        ddl.DataSource = dt;
        ddl.ID = "DropDownListID 1";
        ddl.SelectedIndexChanged += new EventHandler(ddlList_SelectedIndexChanged);
        ddl.DataBind();

        ddl.AutoPostBack = true;
        ddl.EnableViewState = true; //Preserves View State info on Postbacks
        //ddlList.Style["position"] = "absolute";
        //ddl.Style["top"] = 80 + "px";
        //ddl.Style["left"] = 0 + "px";
        dr2.Close();

        listDDL.Add(ddl);
    }
    return listDDL;
}