Nullable in Storage classes (neo-project#3670)

* Nullable

* Optimize

* Fix nullables

* Fix ut

* More

* Update src/Neo/Persistence/DataCache.cs

Co-authored-by: Christopher Schuchardt <cschuchardt88@gmail.com>

* Update src/Neo/Persistence/MemorySnapshot.cs

Co-authored-by: Christopher Schuchardt <cschuchardt88@gmail.com>

* Update src/Neo/Persistence/MemoryStore.cs

Co-authored-by: Christopher Schuchardt <cschuchardt88@gmail.com>

* Remove pragma

* Clean

* SnapshotCache

* Change exception

* Fix conflicts

* Update src/Neo/Persistence/DataCache.cs

Co-authored-by: Christopher Schuchardt <cschuchardt88@gmail.com>

* Update src/Neo/Persistence/DataCache.cs

Co-authored-by: nan01ab <yjcc201374@outlook.com>

* Avoid logic change

---------

Co-authored-by: Christopher Schuchardt <cschuchardt88@gmail.com>
Co-authored-by: nan01ab <yjcc201374@outlook.com>

Author: 3 people

src/Neo/Persistence/ClonedCache.cs

@@ -9,55 +9,60 @@
 // Redistribution and use in source and binary forms with or without
 // modifications are permitted.
 
+#nullable enable
+
 using Neo.SmartContract;
 using System.Collections.Generic;
 
 namespace Neo.Persistence
 {
     class ClonedCache : DataCache
     {
-        private readonly DataCache innerCache;
+        private readonly DataCache _innerCache;
 
         public ClonedCache(DataCache innerCache)
         {
-            this.innerCache = innerCache;
+            _innerCache = innerCache;
         }
 
         protected override void AddInternal(StorageKey key, StorageItem value)
         {
-            innerCache.Add(key, value.Clone());
+            _innerCache.Add(key, value.Clone());
         }
 
         protected override void DeleteInternal(StorageKey key)
         {
-            innerCache.Delete(key);
+            _innerCache.Delete(key);
         }
 
         protected override bool ContainsInternal(StorageKey key)
         {
-            return innerCache.Contains(key);
+            return _innerCache.Contains(key);
         }
 
         /// <inheritdoc/>
         protected override StorageItem GetInternal(StorageKey key)
         {
-            return innerCache[key].Clone();
+            return _innerCache[key].Clone();
         }
 
         protected override IEnumerable<(StorageKey, StorageItem)> SeekInternal(byte[] keyOrPreifx, SeekDirection direction)
         {
-            foreach (var (key, value) in innerCache.Seek(keyOrPreifx, direction))
+            foreach (var (key, value) in _innerCache.Seek(keyOrPreifx, direction))
                 yield return (key, value.Clone());
         }
 
-        protected override StorageItem TryGetInternal(StorageKey key)
+        protected override StorageItem? TryGetInternal(StorageKey key)
         {
-            return innerCache.TryGet(key)?.Clone();
+            return _innerCache.TryGet(key)?.Clone();
         }
 
         protected override void UpdateInternal(StorageKey key, StorageItem value)
         {
-            innerCache.GetAndChange(key).FromReplica(value);
+            var entry = _innerCache.GetAndChange(key)
+                ?? throw new KeyNotFoundException();
+
+            entry.FromReplica(value);
         }
     }
 }