前台：
<asp:TextBox runat="server" onMouseOver="this.focus();this.select()">dsdsds</asp:TextBox>
或
<asp:TextBox runat="server" onMouseOver="this.focus()" onFocus="this.select()">dsdsds</asp:TextBox>
后台：
this.txtbox1.Attributes.Add("onMouseOver", "this.focus();this.select();");
二、得到焦点
后台：
textbox1.Focus();
textbox1.Attributes.Add("onfocus","this.select()");
