using Uniconta.API.System;
using Uniconta.ClientTools.DataModel;
using Uniconta.DataModel;

public class NumberSerieHelper 
{
    public static async Task<string> GetNext(CrudAPI api, string serieName)
    {
        var numberSerie = ClientHelper.GetRefClient<NumberSerieClient>(api.CompanyId, typeof(NumberSerie), serieName);
        if (numberSerie == null)
            return null;

        if (numberSerie.Blocked)
            return null;
        if (!numberSerie._GenerateKey) // geen is nummerreeks? dan niet gebruiken...
            return null;

        var numberSerieAPI = new NumberSerieAPI(api);
        var number = await numberSerieAPI.GetNewNumber(serieName);
        var length = numberSerie.Length;
        var fullNumber = $"{numberSerie.Prefix}{PadZeros(number, length)}";
        return fullNumber;
    }

    private static string PadZeros(long number, int length) => $"{number}".PadLeft(length, '0');
}

using Uniconta.DataModel;
using Uniconta.API.System;

var crudApi = new CrudAPI(baseApi);
var numberSeries = await crudApi.Query<Uniconta.DataModel.NumberSerie>().ToList();
var serieName = numberSeries.First().KeyStr;

                  
var numberSerieAPI = new NumberSerieAPI(baseApi);
var numberSerie = ClientHelper.GetRefClient<NumberSerieClient>(this.api.CompanyId, typeof(NumberSerie), "Serie");

var number = numberSerieAPI.GetNewNumber("Serie");
var fullNumber = numberSerie.Prefix + numberSerieAPI.GetNewNumber("Serie");

var number = numberSerieAPI.ViewNextNumber("Serie");
numberSerieAPI.SetNumber("Serie", 100);
numberSerieAPI.CancelNumber("Serie", 100);

467800cookie-checkUniconta NumberSerie GetNewNumber