I tried changing my above code to :

public IQueryable<ConfigurationEntities> GetServiceByApplicationName(string desigName, string serviceName)
        {
            try
            {
                IQueryable lstEmpByDesig= new IQueryable; //error on this line
 
                using (ConfigurationEntities ce = new ConfigurationEntities())
                {
                    var query = ce.SelEmployeeByDesignationName(desigName);
                    lstEmpByDesig = query;
                }
                return lstEmpByDesig;
            }
            catch
            {
                return null;
            }
        }

But there is an error on IQueryable object declaration where I've mentioned //error on this line.