无法将类型“System.Web.UI.WebControls.DropDownList”隐式转换为“DropDownList”

[ai小虎]

protected void Button4_Click(object sender, EventArgs e)
{
DropDownList obj_ddl;
obj_ddl = DropDownList1;

System.Web.UI.WebControls.DropDownList ddl;
ddl = DropDownList1;



}
都是一样的声明，为什么第一种（ DropDownList obj_ddl; obj_ddl = DropDownList1;）写法出现如下错误：
无法将类型“System.Web.UI.WebControls.DropDownList”隐式转换为“DropDownList”
而第二种写法（ System.Web.UI.WebControls.DropDownList ddl; ddl = DropDownList1; ）
却可以？？？求指教

[csdn_aspnet]

类型强转一下