Доработано Demo

BotPages.Core/Pages/PageStateAttribute.cs

@@ -3,7 +3,7 @@
 /// <summary>
 /// Атрибут для свойств страницы, которые должны сохраняться в StateStorage.
 /// </summary>
-[AttributeUsage(AttributeTargets.Property)]
+[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field)]
 public sealed class StatefullAttribute : Attribute
 {
     /// <summary>

BotPages.Core/Pages/StatefullPage.cs

@@ -21,7 +21,7 @@ public abstract class StatefullPage : Page
     /// <summary>
     /// Загружает значения свойств из StateStorage.
     /// </summary>
-    internal async Task LoadState(PageContext ctx, CancellationToken ct)
+    protected async Task LoadState(PageContext ctx, CancellationToken ct)
     {
         foreach (var prop in GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance))
         {
@@ -40,7 +40,7 @@ public abstract class StatefullPage : Page
     /// <summary>
     /// Сохраняет значения свойств в StateStorage.
     /// </summary>
-    internal async Task SaveState(PageContext ctx, CancellationToken ct)
+    protected async Task SaveState(PageContext ctx, CancellationToken ct)
     {
         foreach (var prop in GetType().GetProperties(BindingFlags.Public | BindingFlags.Instance))
         {