Error: 'The name 'ClientScript' does not exist in the current context' in ASP.Net

i have this error on this line of code

protected void btnSearch_Click(object sender, EventArgs e)
    {
        string Name = Request.Form[txtSearch.UniqueID];
        string username = Request.Form[hfUserName.UniqueID];
        ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Name: " + Name + "\\nID: " + username + "');", true);
    }
}

error

Server Error in '/' Application.
Compilation Error
Description: An error occurred during the compilation of a resource required to service this request. Please review the following specific error details and modify your source code appropriately.

Compiler Error Message: CS0103: The name 'ClientScript' does not exist in the current context

Source Error:


Line 21:         string Name = Request.Form[txtSearch.UniqueID];
Line 22:         string username = Request.Form[hfUserName.UniqueID];
Line 23:         ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Name: " + Name + "\\nID: " + username + "');", true);
Line 24:     }
Line 25: }

Also this page methode is from which section , i dont understand this

url: '<%=ResolveUrl("~/Timeline.aspx/GetNames") %>',

what is the getNames?,

<script type="text/javascript">
         $(function () {
             $("[id$=txtSearch]").autocomplete({
                 source: function (request, response) {
                     $.ajax({
                         url: '<%=ResolveUrl("~/Timeline.aspx/GetNames") %>',
                    data: "{ 'prefix': '" + request.term + "'}",
                    dataType: "json",
                    type: "POST",
                    contentType: "application/json; charset=utf-8",
                    success: function (data) {
                        response($.map(data.d, function (item) {
                            return {
                                label: item.split('-')[0],
                                val: item.split('-')[1]
                            }
                        }))
                    },
                    error: function (response) {
                        alert(response.responseText);
                    },
                    failure: function (response) {
                        alert(response.responseText);
                    }
                });
            },
            select: function (e, i) {
                $("[id$=hfUserName]").val(i.item.val);
            },
            minLength: 1
        });
    });