Add TryAdd method.

src/ZoneTree/Core/ZoneTree.ReadWrite.cs

@@ -61,6 +61,14 @@ public bool TryGet(in TKey key, out TValue value)
         return TryGetFromReadonlySegments(in key, out value);
     }
 
+    public bool TryAdd(in TKey key, in TValue value)
+    {
+        if (ContainsKey(key))
+            return false;
+        Upsert(in key, in value);
+        return true;
+    }
+
     public bool TryGetAndUpdate(in TKey key, out TValue value, ValueUpdaterDelegate<TValue> valueUpdater)
     {
         if (IsReadOnly)

src/ZoneTree/IZoneTree.cs

@@ -34,13 +34,21 @@ public interface IZoneTree<TKey, TValue> : IDisposable
     bool ContainsKey(in TKey key);
 
     /// <summary>
-    /// Tries to get the value of the given key.
+    /// Tries to retrieve the value associated with the specified key.
     /// </summary>
-    /// <param name="key">The key of the element.</param>
+    /// <param name="key">The key of the element to retrieve.</param>
     /// <param name="value">The value of the element associated with the key.</param>
     /// <returns>true if the key is found; otherwise, false</returns>
     bool TryGet(in TKey key, out TValue value);
 
+    /// <summary>
+    /// Tries to add the specified key and value to the collection.
+    /// </summary>
+    /// <param name="key">The key of the element to add.</param>
+    /// <param name="value">The value of the element to add.</param>
+    /// <returns>true if the key and value were added successfully; otherwise, false if the key already exists.</returns>
+    bool TryAdd(in TKey key, in TValue value);
+
     /// <summary>
     /// Tries to get the value of the given key and
     /// updates the value using value updater if found any.