[fix]修正普通可空字段遇到并行冲突问题，改为并行字段

NewLife.CubeNC/Session/SessionProvider.cs

@@ -1,5 +1,5 @@
-using NewLife.Caching;
+using System.Collections.Concurrent;
-using NewLife.Collections;
+using NewLife.Caching;
 
 namespace NewLife.Cube;
 
@@ -24,12 +24,52 @@ public class SessionProvider
         Cache.SetExpire(sessionKey, Expire);
 
         //!! 临时修正可空字典的BUG
-        if (Cache is MemoryCache mc && dic is not NullableDictionary<String, Object>)
+        if (Cache is MemoryCache mc && dic is not NullableDictionary2<String, Object>)
         {
-            dic = new NullableDictionary<String, Object>(dic, StringComparer.Ordinal);
+            dic = new NullableDictionary2<String, Object>(dic, StringComparer.Ordinal);
             mc.Set(sessionKey, dic);
         }
 
         return dic;
     }
+}
+
+
+/// <summary>可空字典。获取数据时如果指定键不存在可返回空而不是抛出异常</summary>
+/// <typeparam name="TKey"></typeparam>
+/// <typeparam name="TValue"></typeparam>
+class NullableDictionary2<TKey, TValue> : ConcurrentDictionary<TKey, TValue>, IDictionary<TKey, TValue> where TKey : notnull
+{
+    /// <summary>实例化一个可空字典</summary>
+    public NullableDictionary2() { }
+
+    /// <summary>指定比较器实例化一个可空字典</summary>
+    /// <param name="comparer"></param>
+    public NullableDictionary2(IEqualityComparer<TKey> comparer) : base(comparer) { }
+
+    /// <summary>实例化一个可空字典</summary>
+    /// <param name="dic"></param>
+    public NullableDictionary2(IDictionary<TKey, TValue> dic) : base(dic) { }
+
+    /// <summary>实例化一个可空字典</summary>
+    /// <param name="dic"></param>
+    /// <param name="comparer"></param>
+    public NullableDictionary2(IDictionary<TKey, TValue> dic, IEqualityComparer<TKey> comparer) : base(dic, comparer) { }
+
+    /// <summary>获取 或 设置 数据</summary>
+    /// <param name="item"></param>
+    /// <returns></returns>
+    public new TValue this[TKey item]
+    {
+        get
+        {
+            if (TryGetValue(item, out var v)) return v;
+
+            return default!;
+        }
+        set
+        {
+            base[item] = value;
+        }
+    }
 }