List<int> agents = taskdal.GetOfficeAgents(Branches.aarhusBranch);
    if (lastAgentIDAarhus != -1)
    {
        int index = agents.IndexOf(lastAgentIDAarhus);
        if (agents.Count > index + 1)
        {
            lastAgentIDAarhus = agents[index + 1];
        }
        else
        {
            lastAgentIDAarhus = agents[0];
        }
    }
    else
    {
        lastAgentIDAarhus = agents[0];
    }

lastAgentIDAarhus = agents[index == -1 ? 0 : index % agents.Count];

public static T NextOf<T>(this IList<T> list, T item)
{
    var indexOf = list.IndexOf(item);
    return list[indexOf == list.Count - 1 ? 0 : indexOf + 1];
}

List<string> names = new List<string>();

names.Add("jonh");
names.Add("mary");

string name = String.Empty;

name = names.NextOf(null);   //name == jonh

name = names.NextOf("jonh"); //name == mary

name = names.NextOf("mary"); //name == jonh

public static IEnumerable<T> AsCircularEnumerable<T>(this IEnumerable<T> enumerable)
  {
    var enumerator = enumerable.GetEnumerator();
    if(!enumerator.MoveNext())
      yield break;

    while (true)
    {
      yield return enumerator.Current;
      if(!enumerator.MoveNext())
        enumerator = enumerable.GetEnumerator();
    }
  }

var agents = new List<int> {1, 2, 3, 4, 123, 234, 345, 546};

  foreach(var i in agents.AsCircularEnumerable())
  {
    Console.WriteLine(i);
  }

public static T Next<T>(this IList<T> list, T item)
{
    var nextIndex = list.IndexOf(item) + 1;

    if (nextIndex == list.Count)
    {
        return list[0];
    }

    return list[nextIndex];
}

List<int> agents = taskdal.GetOfficeAgents(Branches.aarhusBranch);
if (lastAgentIDAarhus != -1)
{
    int index = agents.IndexOf(lastAgentIDAarhus);
    lastAgentIDAarhus = (agents.Count > index + 1 ? agents[index + 1] : agents[0]);
}
else
{
    lastAgentIDAarhus = agents[0];
}

public static class ListExtensions
{
    public static TType Next<TType>(this IList<TType> list, TType item)
    {
        if (list == null) return default(TType);

        var itemIndex = list.IndexOf(item);
        if (itemIndex < 0) return list.FirstOrDefault();

        var nextIndex = itemIndex + 1;

        return nextIndex >= list.Count
            ? list.FirstOrDefault()
            : list[nextIndex];
    }
}