src/log4net/Util/GlobalContextProperties.cs

#region Apache License
//
// Licensed to the Apache Software Foundation (ASF) under one or more 
// contributor license agreements. See the NOTICE file distributed with
// this work for additional information regarding copyright ownership. 
// The ASF licenses this file to you under the Apache License, Version 2.0
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// the License. You may obtain a copy of the License at
//
// http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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#endregion

namespace log4net.Util;

/// <summary>
/// Implementation of Properties collection for the <see cref="log4net.GlobalContext"/>
/// </summary>
/// <remarks>
/// <para>
/// This class implements a properties collection that is thread safe and supports both
/// storing properties and capturing a read only copy of the current propertied.
/// </para>
/// <para>
/// This class is optimized to the scenario where the properties are read frequently
/// and are modified infrequently.
/// </para>
/// </remarks>
/// <author>Nicko Cadell</author>
public sealed class GlobalContextProperties : ContextPropertiesBase
{
  /// <summary>
  /// The read only copy of the properties.
  /// </summary>
  /// <remarks>
  /// <para>
  /// This variable is declared <c>volatile</c> to prevent the compiler and JIT from
  /// reordering reads and writes of this thread performed on different threads.
  /// </para>
  /// </remarks>
  private volatile ReadOnlyPropertiesDictionary _readOnlyProperties = [];

  /// <summary>
  /// Lock object used to synchronize updates within this instance
  /// </summary>
  private readonly object _syncRoot = new();

  /// <summary>
  /// Constructor
  /// </summary>
  /// <remarks>
  /// <para>
  /// Initializes a new instance of the <see cref="GlobalContextProperties" /> class.
  /// </para>
  /// </remarks>
  internal GlobalContextProperties()
  {
  }

  /// <summary>
  /// Gets or sets the value of a property
  /// </summary>
  /// <value>
  /// The value for the property with the specified key
  /// </value>
  /// <remarks>
  /// <para>
  /// Reading the value for a key is faster than setting the value.
  /// When the value is written a new read only copy of 
  /// the properties is created.
  /// </para>
  /// </remarks>
  public override object? this[string key]
  {
    get => _readOnlyProperties[key];
    set
    {
      lock (_syncRoot)
      {
        var mutableProps = new PropertiesDictionary(_readOnlyProperties)
        {
          [key] = value
        };
        _readOnlyProperties = new ReadOnlyPropertiesDictionary(mutableProps);
      }
    }
  }

  /// <summary>
  /// Remove a property from the global context
  /// </summary>
  /// <param name="key">the key for the entry to remove</param>
  /// <remarks>
  /// <para>
  /// Removing an entry from the global context properties is relatively expensive compared
  /// with reading a value. 
  /// </para>
  /// </remarks>
  public void Remove(string key)
  {
    lock (_syncRoot)
    {
      if (_readOnlyProperties.Contains(key))
      {
        var mutableProps = new PropertiesDictionary(_readOnlyProperties);
        mutableProps.Remove(key);
        _readOnlyProperties = new ReadOnlyPropertiesDictionary(mutableProps);
      }
    }
  }

  /// <summary>
  /// Clear the global context properties
  /// </summary>
  public void Clear()
  {
    lock (_syncRoot)
    {
      _readOnlyProperties = [];
    }
  }

  /// <summary>
  /// Get a readonly immutable copy of the properties
  /// </summary>
  /// <returns>the current global context properties</returns>
  /// <remarks>
  /// <para>
  /// This implementation is fast because the GlobalContextProperties class
  /// stores a readonly copy of the properties.
  /// </para>
  /// </remarks>
  internal ReadOnlyPropertiesDictionary GetReadOnlyProperties() => _readOnlyProperties;
}