c# - 如何自定义asp.net DropDownList 以支持自定义数据绑定(bind)字段

Question

有什么方法可以自定义 asp.net 下拉列表以支持自定义数据绑定(bind)字段。它不应该是控件的自定义属性。它应该能够通过 DataBind(); 方法与相同的数据源绑定(bind)数据。在此自定义服务器控件中，我尝试访问新的自定义字段，并根据该值对数据源的特定行进行一些计算。

标准控制代码

<asp:DropDownList runat="server" DataTextField="TextField" DataValueField="ValueField"/> 

新的自定义控件应该是这样的，

<asp:DropDownList runat="server" DataTextField="TextField" DataValueField="ValueField" DataCustomField="CustomField"/>

Answer 1

您可以将 DropDownList 扩展为如下形式:

public class MyDropDownList : DropDownList
{
    public MyDropDownList()
    {   
    }

    public string CustomProperty { get; set; }

    protected override void OnPreRender(EventArgs e)
    {
        base.OnPreRender(e);
        int i = 0;
        foreach (var item in this.DataSource as IEnumerable)
        {
            PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(item);
            PropertyDescriptor pd = properties.Find(CustomProperty, true);
            this.Items[i].Attributes.Add(CustomProperty, pd.GetValue(item).ToString());
            i++;
        }          
    }
}

并按如下方式在您的标记中使用它:

<cc1:MyDropDownList ID="MyDropDownList1" DataTextField="Name" DataValueField="Department" CustomProperty="ImageUrl" runat="server">
</cc1:MyDropDownList>

就我而言，我绑定(bind)到 List<Employee>具有一些属性，例如 Name , Department和 ImageUrl .呈现下拉列表后如下所示:

<select name="ctl00$MainContent$MyDropDownList1" id="MainContent_MyDropDownList1">
    <option selected="selected" value="Human resources" ImageUrl="http://www.freedigitalphotos.net/images/gal_images/av-_146.jpg">Employee 0</option>
    <option value="Information Technology" ImageUrl="http://www.freedigitalphotos.net/images/gal_images/av-_314.jpg">Employee 1</option>
    <option value="Human resources" ImageUrl="http://www.freedigitalphotos.net/images/gal_images/av-_146.jpg">Employee 2</option>

    <option value="Information Technology" ImageUrl="http://www.freedigitalphotos.net/images/gal_images/av-_314.jpg">Employee 3</option>
    <option value="Human resources" ImageUrl="http://www.freedigitalphotos.net/images/gal_images/av-_146.jpg">Employee 4</option>
</select>

更新:(@emrahozguner 通过 Twitter 向我发送了一个关于如何在 CustomProperty 事件上检索 SelectedIndexChanged 的问题)

public class MyDropDownList : DropDownList
    {
        public MyDropDownList()
        {
        }

        public string CustomProperty
        {
            get;
            set;
        }

        public string SelectedCustomProperty
        {
            get
            {
                //Use the SelectedIndex to retrieve the right element from ViewState
                return ViewState["CustomProperty" + this.SelectedIndex] as string;
            }
        }

        protected override void OnPreRender(EventArgs e)
        {
            base.OnPreRender(e);
            int i = 0;
            if (this.DataSource != null)
            {
                foreach (var item in this.DataSource as IEnumerable)
                {
                    PropertyDescriptorCollection properties = TypeDescriptor.GetProperties(item);
                    PropertyDescriptor pd = properties.Find(CustomProperty, true);
                    this.Items[i].Attributes.Add(CustomProperty, pd.GetValue(item).ToString());
                    //We need to save the CustomProperty value on ViewState if we want to be able to retrieve it...
                    ViewState["CustomProperty" + i] = pd.GetValue(item).ToString();
                    i++;
                }
            }
        }
    }

然后您访问 CustomProperty在 SelectedIndexChanged这样:

protected void MyDropDownList1_SelectedIndexChanged(object sender, EventArgs e)
    {
        MyDropDownList l = (sender as MyDropDownList);
        if (l != null)
        {

            string selectedCustomProperty = l.SelectedCustomProperty;
            //Do something cool with this selectedCustomProperty 
        }
    }

免责声明:这不是唯一的方法，但它是我能想到的最简单的方法，无需覆盖 LoadViewState和 SaveViewState等