如何从前端将 ASP.net 下拉列表索引设置为 0

Question

我在 ASP.net 页面中有以下代码：

<asp:DropDownList ClientIDMode="Static" ID="ddl1" CssClass="chosen-select le" runat="server" AppendDataBoundItems="true"></asp:DropDownList>
<asp:DropDownList ClientIDMode="Static" ID="ddl2" CssClass="chosen-select le" runat="server" AppendDataBoundItems="true"></asp:DropDownList>
<asp:DropDownList ClientIDMode="Static" ID="ddl3" CssClass="chosen-select le" runat="server" AppendDataBoundItems="true"></asp:DropDownList>

<input type="button" id="ClearForm" value="Clear" class="btn1" />

JQuery 将所有三个下拉列表的索引设置为 0:

$(function () {
    $("body").on("click", "#ClearForm", function (e) {
        e.preventDefault();
        $("select#ddl1, select#ddl2, select#ddl3").prop('selectedIndex', 0); //does not set the selected index to 0
        alert($("select#ddl1").text()); //displays all the options from #ddl1
    });
});

如何修改，使下拉列表索引设置为 0。

HTML 第一个下拉列表的渲染：

<select name="ctl00$BodyPlaceHolder$ddl1" id="ddl1" class="le">
    <option value="Select a State">Select a State</option>
    <option value="Alabama">AL</option>
    <option value="Alaska">AK</option>
    <option value="Arizona">AZ</option>
</select>

我尝试了以下 JQuery 并且 returns null:

alert($("select#ddl1").val());

也尝试了以下方法，但没有用：

$('select').each(function () {
    $(this).find('option:first').prop('selected', 'selected');
});

添加了以下内容JavaScript：

function setSelectedIndex(dropdownlist, selVal) {
    var ddl1 = document.getElementById(dropdownlist);
    alert(ddl1.selectedIndex);
    if (ddl1.length >= selVal) { //if the selected value is greater then 0, the alert is shown but the value is not set back to 0.
        ddl1.selectedIndex = selVal;
        alert("greater or equal");
    }
    else {
        alert("not greater or equal");
    }
}

似乎，如果我删除选择的 Jquery 它工作正常：http://jsfiddle.net/bpbvhsay/

Answer 1

有很多方法可以做到这一点，但这里是 JavaScript 的解决方案：

function setSelectedIndex(dropdownlist, selVal)
    var ddl1 = document.getElementById(dropdownlist);
    //alert(ddl1.selectedIndex); //displays the proper index...
    if(ddl1.length >= selVal)
    {
       ddl1.selectedIndex = selVal;
    }
}

根据您的需要调用上述函数如下：

setSelectedIndex('<%=ddl1.ClientID %>', 2);

UPDATE 正如你所说你已经设置 ClientIDMode，试试下面的更新函数：

function setSelectedIndex(selVal){
    var ddl1 = document.getElementById('ddl1');
    if(ddl1.length >= selVal)
    {
       ddl1.selectedIndex = selVal;
    }
}

并将其命名为：

setSelectedIndex(0);

Answer 2

我使用的是 Chosen Jquery，导致下拉列表无法更新。

这是HTML:

<select name="ctl00$BodyPlaceHolder$ddl1" id="ddl1" class="chose-select le">
    <option value="Select a State">Select a State</option>
    <option value="Alabama">AL</option>
    <option value="Alaska">AK</option>
    <option value="Arizona">AZ</option>
</select>

JavaScript 将所选索引设置为 0:

function setSelectedIndex(dropdownlist, selVal) {
    var ddl1 = document.getElementById(dropdownlist);
    if (selVal < ddl1.selectedIndex) {
        ddl1.selectedIndex = selVal;

        $(ddl1).val(0).trigger('chosen:updated');
    }
}

那就行了。

如何从前端将 ASP.net 下拉列表索引设置为 0

How to set ASP.net dropdownlist index to 0 from the front-end

html

asp.net

jquery

jquery-chosen