为ListControl存储title属性到ViewState

      前几天遇到个问题，有一个DropDownList需要对某些Item对标记。我们大家知道Dropdownlist最终呈现是<select>，最后我们想到html是:

  <select>
        <option value="volvo" title="share">Volvo</option>
        <option value="saab"  title="share">Saab</option>
        <option value="mercedes">Mercedes</option>
        <option value="audi">Audi</option>
    </select>

     在Server端我们使用Attributes给他们加上title属性, 如以下代码片段：

       return ddlresultset.Select(t =>
            {
                var newitem = new ListItem(t.UserId.HasValue ? t.Name : "*" + t.Name, t.Id.ToString());
                if (!t.UserId.HasValue)
                    newitem.Attributes.Add("title", "share");
                return newitem;
            }).OrderBy(d => d.Text).ToArray();

现在我们绑定上去，当我们做POSTBACK时，发现title属性丢掉了，原因是DropDownList控制没有存储title属性到ViewState。我们看下DropDownList类继承关系:

System.Web.UI.Control
System.Web.UI.WebControls.WebControl
System.Web.UI.WebControls.BaseDataBoundControl
System.Web.UI.WebControls.DataBoundControl
System.Web.UI.WebControls.ListControl
System.Web.UI.WebControls.BulletedList
System.Web.UI.WebControls.CheckBoxList
System.Web.UI.WebControls.DropDownList
System.Web.UI.WebControls.ListBox
System.Web.UI.WebControls.RadioButtonList

相同的问题将出现派生ListControl的控件。解决方法，是重写SaveViewState 与 LoadViewState方法，看如下代码：

        /// <summary>
        /// Saves the current view state of the <see cref="T:System.Web.UI.WebControls.ListControl"/> -derived control and the items it contains.
        /// </summary>
        /// <returns>
        /// An <see cref="T:System.Object"/> that contains the saved state of the <see cref="T:System.Web.UI.WebControls.ListControl"/> control.
        /// </returns>
        protected override object SaveViewState()
        {
            // Create an object array with one element for the CheckBoxList's
            // ViewState contents, and one element for each ListItem in skmCheckBoxList
            object[] state = new object[this.Items.Count + 1];

            object baseState = base.SaveViewState();
            state[0] = baseState;

            // Now, see if we even need to save the view state
            bool itemHasAttributes = false;
            for (int i = 0; i < this.Items.Count; i++)
            {
                if (this.Items[i].Attributes.Count > 0)
                {
                    itemHasAttributes = true;

                    // Create an array of the item's Attribute's keys and values
                    object[] attribKV = new object[this.Items[i].Attributes.Count * 2];
                    int k = 0;
                    foreach (string key in this.Items[i].Attributes.Keys)
                    {
                        attribKV[k++] = key;
                        attribKV[k++] = this.Items[i].Attributes[key];
                    }

                    state[i + 1] = attribKV;
                }
            }

            // return either baseState or state, depending on whether or not
            // any ListItems had attributes
            if (itemHasAttributes)
                return state;
            else
                return baseState;
        }


        /// <summary>
        /// Loads the previously saved view state of the <see cref="T:System.Web.UI.WebControls.DetailsView"/> control.
        /// </summary>
        /// <param name="savedState">An <see cref="T:System.Object"/> that represents the state of the <see cref="T:System.Web.UI.WebControls.ListControl"/> -derived control.</param>
        protected override void LoadViewState(object savedState)
        {
            if (savedState == null) return;

            // see if savedState is an object or object array
            if (savedState is object[])
            {
                // we have an array of items with attributes
                object[] state = (object[])savedState;
                base.LoadViewState(state[0]);   // load the base state

                for (int i = 1; i < state.Length; i++)
                {
                    if (state[i] != null)
                    {
                        // Load back in the attributes
                        object[] attribKV = (object[])state[i];
                        for (int k = 0; k < attribKV.Length; k += 2)
                            this.Items[i - 1].Attributes.Add(attribKV[k].ToString(),
                                                           attribKV[k + 1].ToString());
                    }
                }
            }
            else
                // we have just the base state
                base.LoadViewState(savedState);
        }
    }

这样也可实现，我们看下呈现后ViewState中有我们那个title属性了,通过对ViewState的Decode 清晰可见：

很简单，希望对您Develop有帮助!

作者：Petter Liu
出处：http://www.cnblogs.com/wintersun/
本文版权归作者和博客园共有，欢迎转载，但未经作者同意必须保留此段声明，且在文章页面明显位置给出原文连接，否则保留追究法律责任的权利。
该文章也同时发布在我的独立博客中-Petter Liu Blog。