How far up the hierarchy does a class have to be marked [Serializable]?

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C#

serialization

Very short question:

Why does this class(TCtsPageAdmin) have to be marked [Serializable]?
I'm just trying to serialize a

List<TCtsPage>

TCtsPage class is marked properly.

If I mark TCtsPageAdmin properly with the [Serializable] attribute, then the error moves up the hierarchy -> the object that consumes a TCtsPageadmin object will be the target of the error.

Error:
SaveToFile failed:
Type 'Lrj.Cts.Admin.TCtsPageAdmin' in Assembly 'LrjCtsAdmin, Version=1.0.1.2,
Culture=neutral, PublicKeyToken=null' is not marked as serializable.

[CLSCompliant(true)]
public sealed class TCtsPageAdmin : ILrjObj
{
 ...
    public void SaveToFile(bool aUsePagePath = true)
    {
        String aFileName;
        List<TCtsPage> lst = GetPages("*");

        aFileName = aUsePagePath ? Path.Combine(PagePath, "pages.dat") : 
                                                                    "pages.dat";

        BinaryFormatter binForm = new BinaryFormatter();
        try
        {
            using (Stream fStream = new FileStream(aFileName, FileMode.Create, 
                                              FileAccess.Write, FileShare.None))
            {
                binForm.Serialize(fStream, lst);
                OnSavedToFile(
                  new InfoEventArgs(
                      String.Format("SaveToFile succeeded.\n{0}", aFileName)));
            }
        }
        catch (Exception ex)
        {
            OnSaveToFileFailed(
              new InfoEventArgs(
                    String.Format("SaveToFile failed: \n{0}", ex.Message)));
        }
    }
 ...
}