Html.DropDownList()的用法

古城微笑少年丶 2021-06-26 16:06 332阅读 0赞

## 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：

<p>产品分类：<%=Html.DropDownList("category")%></p>
    
    <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;