Question

这可能是一个简单的问题，但我正在使用DropDownList在C＃中开发一个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，以便应该出现的DropDownLists数量是动态的，它给了我错误;

“名称'ddl'不存在于目前的背景。“

出现此错误的原因是“ddl”的多个实例=计数器数。那么我该如何返回多个“ddl”呢？就像这种方法应该具有哪种返回类型？我该如何返回这些值？

我需要动态的原因是我需要为他们选择的任何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

看起来你试图在函数结束时返回下拉列表，但是它在for循环的范围内声明了。尝试在函数开头创建一个下拉数组（在for循环之前），然后在循环内设置数组然后返回它。您还需要更改函数声明以支持一系列下拉。

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

为什么不直接返回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;
}

如何在ASP.NET中动态返回DropDownList选择？

2 个答案: