That's because AjaxControlToolkit expects the method to be in the following format:
[System.Web.Services.WebMethod]
[System.Web.Script.Services.ScriptMethod]
public string[] GetCompletionList(string prefixText, int count);
Change your method this way, and do something w/ "count" if you need it
public string[] GetCompletionList(string prefixText, int count)
{
List<string> objListSuburb = new List<string>();
Initialize(StoredProcedure.GETBYSTATE);
DataBase.AddInParameter(DbCommand, Parameters.BUILDSUBURB, DbType.String, prefixText);
using (IDataReader objDataReader = DataBase.ExecuteReader(DbCommand))
{
while (objDataReader.Read())
{
objListSuburb.Add(objDataReader[Parameters.BUILDSUBURB].ToString());
}
}
return objListSuburb.ToArray();
}