<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>

<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>

<asp:TextBox id="txtProject_Name_C" runat="server" Width="250px" MaxLength="35" CssClass="txt"></asp:TextBox> 
<cc1:TextBoxWatermarkExtender ID="TextBoxWatermarkExtender1" runat="server" TargetControlID="txtProject_Name_C" WatermarkText="请在此处输入工程名称(支持模糊查询)" WatermarkCssClass="watermarked">
</cc1:TextBoxWatermarkExtender>

.watermarked
{
    color:Gray;
    font-size:small;   
    }