Fix config.Root calculation (#199)

* Fix config internal Fallback mutation

* Use fallback values of fallback configs

* Made config fully build on creation, and less mutable

* Config root calculation fix - use Value instead of Root from children

* Refactoring

* Added a comment

src/Hocon.Configuration/Config.cs

@@ -18,29 +18,41 @@ namespace Hocon
  /// </summary>
  public class Config : HoconRoot
  {
+ [Obsolete("For json serialization/deserialization only", true)]
+ private Config()
+ {
+ }
+
  /// <inheritdoc />
  /// <summary>
  /// Initializes a new instance of the <see cref="Config" /> class.
  /// </summary>
- private Config()
+ private Config(HoconValue value, Config fallback)
  {
+ Fallback = fallback;
+ Value = value;
+
+ Root = GetRootValue();
  }
 
- /// <inheritdoc cref="Config()" />
+ /// <inheritdoc cref="Config(HoconValue, Config)" />
  /// <param name="root">The root node to base this configuration.</param>
  /// <exception cref="T:System.ArgumentNullException">"The root value cannot be null."</exception>
  public Config(HoconRoot root) : base(root?.Value, root?.Substitutions ?? Enumerable.Empty<HoconSubstitution>())
  {
+ Root = GetRootValue();
  }
 
- /// <inheritdoc cref="Config()" />
+ /// <inheritdoc cref="Config(HoconValue, Config)" />
  /// <param name="source">The configuration to use as the primary source.</param>
  /// <param name="fallback">The configuration to use as a secondary source.</param>
  /// <exception cref="ArgumentNullException">The source configuration cannot be null.</exception>
  public Config(HoconRoot source, Config fallback) : base(source?.Value,
  source?.Substitutions ?? Enumerable.Empty<HoconSubstitution>())
  {
  Fallback = fallback;
+
+ Root = GetRootValue();
  }
 
  /// <summary>
@@ -54,27 +66,12 @@ public Config(HoconRoot source, Config fallback) : base(source?.Value,
  /// <summary>
  /// The configuration used as a secondary source.
  /// </summary>
- public Config Fallback { get; private set; }
+ public Config Fallback { get; }
 
  /// <summary>
  /// The root node of this configuration section
  /// </summary>
- public virtual HoconValue Root
- {
- get
- {
- var elements = new List<IHoconElement>();
- for (var config = this; config != null; config = config.Fallback?.Copy())
- {
- elements.AddRange(config.Value);
- }
-
- var aggregated = new HoconValue(null);
- aggregated.AddRange(elements.AsEnumerable().Reverse());
-
- return aggregated;
- }
- }
+ public virtual HoconValue Root { get; }
 
  /// <summary>
  /// Returns string representation of <see cref="Config" />, allowing to include fallback values
@@ -95,11 +92,7 @@ public string ToString(bool useFallbackValues)
  protected Config Copy()
  {
  //deep clone
- return new Config
- {
- Fallback = Fallback?.Copy(),
- Value = (HoconValue) Value.Clone(null)
- };
+ return new Config((HoconValue) Value.Clone(null), Fallback?.Copy());
  }
 
  protected override HoconValue GetNode(HoconPath path, bool throwIfNotFound = false)
@@ -144,14 +137,10 @@ public virtual Config WithFallback(Config fallback)
  {
  if (fallback == this)
  throw new ArgumentException("Config can not have itself as fallback", nameof(fallback));
-
- var clone = Copy();
-
- var current = clone;
- while (current.Fallback != null) current = current.Fallback;
- current.Fallback = fallback.Copy();
-
- return clone;
+
+ // If Fallback is not set - we will set it in new copy
+ // If Fallback was set - just use it, but with adding new fallback values
+ return new Config((HoconValue) Value.Clone(null), Fallback?.WithFallback(fallback) ?? fallback);
  }
 
  /// <summary>
@@ -199,6 +188,24 @@ public override IEnumerable<KeyValuePair<string, HoconField>> AsEnumerable()
  used.Add(kvp.Key);
  }
  }
+
+ /// <summary>
+ /// Performs aggregation of Value and all Fallbacks into single <see cref="HoconValue"/> object
+ /// </summary>
+ private HoconValue GetRootValue()
+ {
+ var elements = new List<IHoconElement>();
+
+ for (var config = this; config != null; config = config.Fallback?.Copy())
+ {
+ elements.AddRange(config.Value);
+ }
+
+ var aggregated = new HoconValue(null);
+ aggregated.AddRange(elements.AsEnumerable().Reverse());
+
+ return aggregated;
+ }
  }
 
  /// <summary>