扩展DropDownList

使用DropDownList,我们习惯在所有待选数据前面加一个空白项,或者"请选择"的提示,微软却没有,这可能就是中西文化的差异.

现在扩展为DropDownListPro

经过代码调试,和看源代码,得知默认选中的行为是在SelectedIndex的Get方法中设置的.所以重载该方法,即可达到目的.

 public class DropDownListPro: DropDownList
    {
        public bool AddBlankItem
        {
            get
            {
                object obj2 = base.ViewState["AddBlankItem"];
                if (obj2 != null)
                {
                    return (bool)obj2;
                }
                return false;
            }
            set
            {
                if (!object.Equals(value, base.ViewState["AddBlankItem"]))
                {
                    base.ViewState["AddBlankItem"] = value;
                }
            }
        }
        private bool m_HasAdded=false;
        public override int SelectedIndex
        {
            get
            {
                if (!m_HasAdded && AddBlankItem)
                {
                    this.Items.Insert(0, new ListItem(" ", "-1"));
                    m_HasAdded = true;
                }               
                return base.SelectedIndex;
            }
            set
            {
                base.SelectedIndex = value;
            }
        }

}

在使用的时候只需要设置 DropDownList1.AddBlankItem=true.