In AutoComplete.asmx write following code
<%@ WebService Language="C#" CodeBehind="~/AutoComplete.cs" Class="AutoComplete" %>
And in AutoComplete.cs write according to your search value
[WebMethod]
public string[] studentName(string prefixText)
{
select Select_Object = new select();
string[] student_Name = Select_Object.StudentName(prefixText);
return student_Name;
}