submodule对gitea不管用，所以直接拉了一份拉格兰

Lagrange.Core/Lagrange.OneBot/Utility/LiteDbUtility.cs

@@ -0,0 +1,49 @@
+using Lagrange.Core.Message;
+using Lagrange.Core.Message.Entity;
+using LiteDB;
+
+namespace Lagrange.OneBot.Utility;
+
+public static class LiteDbUtility
+{
+    public static BsonValue IMessageEntitySerialize(IMessageEntity entity)
+    {
+        var type = entity.GetType();
+        var result = BsonMapper.Global.Serialize(type, entity);
+        result["_type"] = new BsonValue(DefaultTypeNameBinder.Instance.GetName(type));
+        return result;
+    }
+
+    public static IMessageEntity IMessageEntityDeserialize(BsonValue bson)
+    {
+        if (!bson.IsDocument) throw new Exception("bson not BsonDocument");
+
+        var doc = bson.AsDocument;
+        if (!doc.TryGetValue("_type", out var typeBson) || !typeBson.IsString)
+        {
+            throw new Exception("no `_type` or `_type` not string");
+        }
+
+        var type = DefaultTypeNameBinder.Instance.GetType(typeBson.AsString);
+        
+        if (type == typeof(MarkdownEntity)) return MarkdownEntityDeserialize(doc);
+
+        return (IMessageEntity)BsonMapper.Global.Deserialize(type, bson);
+    }
+
+    private static MarkdownEntity MarkdownEntityDeserialize(BsonDocument doc)
+    {
+        if (!doc.TryGetValue("Data", out var dataBson) || !dataBson.IsDocument)
+        {
+            throw new Exception("no `Data` or `Data` not document");
+        }
+
+        var dataDocument = dataBson.AsDocument;
+        if (!dataDocument.TryGetValue("Content", out var contentBson) || !contentBson.IsString)
+        {
+            throw new InvalidCastException("no `Data.Content` or `Data.Content` not string");
+        }
+
+        return new(new MarkdownData() { Content = contentBson.AsString });
+    }
+}