Html.DropDownList()的用法

源码

using System.Collections;
using System.Diagnostics.CodeAnalysis;

namespace System.Web.Mvc
{
    [SuppressMessage("Microsoft.Naming", "CA1710:IdentifiersShouldHaveCorrectSuffix", Justification = "This is a shipped API")]
    public class SelectList : MultiSelectList
    {
        public SelectList(IEnumerable items)
            : this(items, selectedValue: null)
        {
        }

        public SelectList(IEnumerable items, object selectedValue)
            : this(items, dataValueField: null, dataTextField: null, selectedValue: selectedValue)
        {
        }

        /// <summary>
        /// Initializes a new instance of the SelectList class by using the specified items for the list,
        /// the selected value, and the disabled values.
        /// </summary>
        /// <param name="items">The items used to build each <see cref="SelectListItem"/> of the list.</param>
        /// <param name="selectedValue">The selected value. Used to match the Selected property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="disabledValues">The disabled values. Used to match the Disabled property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        public SelectList(IEnumerable items, object selectedValue, IEnumerable disabledValues)
            : this(items,
                   dataValueField: null,
                   dataTextField: null,
                   selectedValue: selectedValue,
                   disabledValues: disabledValues)
        {
        }

        public SelectList(IEnumerable items, string dataValueField, string dataTextField)
            : this(items, dataValueField, dataTextField, selectedValue: null)
        {
        }

        public SelectList(IEnumerable items, string dataValueField, string dataTextField, object selectedValue)
            : base(items, dataValueField, dataTextField, ToEnumerable(selectedValue))
        {
            SelectedValue = selectedValue;
        }

        /// <summary>
        /// Initializes a new instance of the SelectList class by using the specified items for the list,
        /// the data value field, the data text field, the data group field, and the selected value.
        /// </summary>
        /// <param name="items">The items used to build each <see cref="SelectListItem"/> of the list.</param>
        /// <param name="dataValueField">The data value field. Used to match the Value property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="dataTextField">The data text field. Used to match the Text property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="dataGroupField">The data group field. Used to match the Group property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="selectedValue">The selected value. Used to match the Selected property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        public SelectList(IEnumerable items,
                          string dataValueField,
                          string dataTextField,
                          string dataGroupField,
                          object selectedValue)
            : base(items, dataValueField, dataTextField, dataGroupField, ToEnumerable(selectedValue))
        {
            SelectedValue = selectedValue;
        }

        /// <summary>
        /// Initializes a new instance of the SelectList class by using the specified items for the list,
        /// the data value field, the data text field, the selected value, and the disabled values.
        /// </summary>
        /// <param name="items">The items used to build each <see cref="SelectListItem"/> of the list.</param>
        /// <param name="dataValueField">The data value field. Used to match the Value property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="dataTextField">The data text field. Used to match the Text property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="selectedValue">The selected value. Used to match the Selected property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="disabledValues">The disabled values. Used to match the Disabled property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        public SelectList(IEnumerable items,
                          string dataValueField,
                          string dataTextField,
                          object selectedValue,
                          IEnumerable disabledValues)
            : base(items, dataValueField, dataTextField, ToEnumerable(selectedValue), disabledValues)
        {
            SelectedValue = selectedValue;
        }

        /// <summary>
        /// Initializes a new instance of the SelectList class by using the specified items for the list,
        /// the data value field, the data text field, the data group field, the selected value, and the disabled
        /// values.
        /// </summary>
        /// <param name="items">The items used to build each <see cref="SelectListItem"/> of the list.</param>
        /// <param name="dataValueField">The data value field. Used to match the Value property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="dataTextField">The data text field. Used to match the Text property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="dataGroupField">The data group field. Used to match the Group property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="selectedValue">The selected value. Used to match the Selected property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="disabledValues">The disabled values. Used to match the Disabled property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        public SelectList(IEnumerable items,
                          string dataValueField,
                          string dataTextField,
                          string dataGroupField,
                          object selectedValue,
                          IEnumerable disabledValues)
            : base(items, dataValueField, dataTextField, dataGroupField, ToEnumerable(selectedValue), disabledValues)
        {
            SelectedValue = selectedValue;
        }

        /// <summary>
        /// Initializes a new instance of the SelectList class by using the specified items for the list,
        /// the data value field, the data text field, the data group field. the selected value, the disabled values,
        /// and the disabled groups.
        /// </summary>
        /// <param name="items">The items used to build each <see cref="SelectListItem"/> of the list.</param>
        /// <param name="dataValueField">The data value field. Used to match the Value property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="dataTextField">The data text field. Used to match the Text property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="dataGroupField">The data group field. Used to match the Group property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="selectedValue">The selected value. Used to match the Selected property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="disabledValues">The disabled values. Used to match the Disabled property of the corresponding
        /// <see cref="SelectListItem"/>.</param>
        /// <param name="disabledGroups">The disabled groups. Used to match the Disabled property of the corresponding
        /// <see cref="SelectListGroup"/>.</param>
        public SelectList(IEnumerable items,
                          string dataValueField,
                          string dataTextField,
                          string dataGroupField,
                          object selectedValue,
                          IEnumerable disabledValues,
                          IEnumerable disabledGroups)
            : base(items,
                   dataValueField,
                   dataTextField,
                   dataGroupField,
                   ToEnumerable(selectedValue),
                   disabledValues,
                   disabledGroups)
        {
            SelectedValue = selectedValue;
        }

        public object SelectedValue { get; private set; }

        private static IEnumerable ToEnumerable(object selectedValue)
        {
            return (selectedValue != null) ? new object[] { selectedValue } : null;
        }
    }
}

SelectListItem.cs

namespace System.Web.Mvc
{
    public class SelectListItem
    {
        /// <summary>
        /// Gets or sets a value that indicates whether this <see cref="SelectListItem"/> is disabled.
        /// </summary>
        public bool Disabled { get; set; }

        /// <summary>
        /// Represents the optgroup HTML element this item is wrapped into.
        /// In a select list, multiple groups with the same name are supported.
        /// They are compared with reference equality.
        /// </summary>
        public SelectListGroup Group { get; set; }

        public bool Selected { get; set; }

        public string Text { get; set; }

        public string Value { get; set; }
    }
}

---

Html.DropDownList()赋默认值：

页面代码如下：

<% 
List<SelectListItem> list = new List<SelectListItem> {
new SelectListItem { Text = "启用", Value = "0",Selected = true},
new SelectListItem { Text = "禁用", Value = "1" } };
%>//list储存dropdownlist的默认值

<%=Html.DropDownList("state",list,Model.state) %>  //state为实体的属性，默认选中"启用"

Html.DropDownList()从数据库读取值：

页面代码如下：

<%= Html.DropDownList("Category", ViewData["Categories"] as SelectList,"--请选择--",new { @class = "my-select-css-class" } )
%>

Controllers代码：

public ActionResult Create() 
{
	List<Category> categories = categoryService.GetAll();
	ViewData["Categories"] = new SelectList(categories, "Id", "Name");
	return View();
}

下面是我自己的代码：
页面代码如下：

<!--数据库提取数据 -->
<p>产品分类：<%=Html.DropDownList("category")%></p>
<!--手动设置数据-->
<p>状态：<%=Html.DropDownList("status")%></p>

Controllers代码：

ViewData["category"] = new SelectList(CategoryManager.GetList(),"id","name");

List<SelectListItem> list = new List<SelectListItem> {
 new SelectListItem { Text = "启用", Value = "0",Selected = true},
 new SelectListItem { Text = "禁用", Value = "1" } 
};

ViewData["status"] = list;