AttachmentExtensions.cs

public static class AttachmentExtensions
{
	private static readonly ConditionalWeakTable<object, ConcurrentDictionary<string, object>> _attachmentTable =
		new ConditionalWeakTable<object, ConcurrentDictionary<string, object>>();

	/// <summary>
	/// Atomically gets or sets an attached value if it's found.
	/// </summary>
	public static AttachmentResult<T> GetOrSetAttached<T>(this object host, Func<T> factory, string key = null)
	{
		key = key ?? typeof(T).AssemblyQualifiedName;
		var dict = _attachmentTable.GetOrCreateValue(host);
		var found = true;
		var value = dict.GetOrAdd(key, _ => { found = false; return factory(); });
		var castValue = (T)value;
		return new AttachmentResult<T>(found, castValue);
	}

	/// <summary>
	/// Sets an attached value. Any existing value will be overwritten.
	/// If a key isn't provided, the value's assembly qualified type
	/// name will be used.
	/// </summary>
	public static void SetAttached(this object host, object value, string key = null)
	{
		if (value == null && key == null)
			throw new ArgumentNullException(key, $"{nameof(value)} or {nameof(key)} must be provided");

		key = key ?? value.GetType().AssemblyQualifiedName;
		var dict = _attachmentTable.GetOrCreateValue(host);
		dict[key] = value;
	}

	/// <summary>
	/// Gets an attached value using the type's AssemblyQualifiedName as the key.
	/// </summary>
	public static AttachmentResult<object> GetAttached(this object host, Type type) =>
		GetAttached(host, type.AssemblyQualifiedName);

	/// <summary>
	/// Gets an attached value by name.
	/// </summary>
	public static AttachmentResult<object> GetAttached(this object host, string key)
	{
		if (key == null)
			throw new ArgumentNullException(nameof(key));

		var dict = _attachmentTable.GetOrCreateValue(host);
		object value;
		var found = dict.TryGetValue(key, out value);
		return new AttachmentResult<object>(found, value);
	}

	/// <summary>
	/// Gets an attached value by key or by the the type parameter's
	/// AssemblyQualifiedName.
	/// </summary>
	/// <typeparam name="T"></typeparam>
	/// <param name="host"></param>
	/// <param name="key"></param>
	/// <returns></returns>
	public static AttachmentResult<T> GetAttached<T>(this object host, string key = null)
	{
		key = key ?? typeof(T).AssemblyQualifiedName;
		var dict = _attachmentTable.GetOrCreateValue(host);
		object value;
		var found = dict.TryGetValue(key, out value);
		var castValue = found ? (T)value : default(T);
		return new AttachmentResult<T>(found, castValue);
	}

	public static ICollection<string> GetAttachmentKeys(this object host)
	{
		ConcurrentDictionary<string, object> dict;
		if (_attachmentTable.TryGetValue(host, out dict))
			return dict.Keys;

		return new string[0];
	}

	/// <summary>
	/// Removes every attached value from the host object.
	/// </summary>
	public static void ClearAttached(this object host) => _attachmentTable.Remove(host);

	/// <summary>
	/// Removes the specified attached value.
	/// </summary>
	public static AttachmentResult<object> RemoveAttached(this object host, string key)
	{
		var dict = _attachmentTable.GetOrCreateValue(host);
		object value;
		var found = dict.TryRemove(key, out value);
		return new AttachmentResult<object>(found, value);
	}

	/// <summary>
	/// Removes the attached value. If a key isn't supplied
	/// the type parameter's assembly qualified type name will be used.
	/// </summary>
	public static AttachmentResult<T> RemoveAttached<T>(this object host, string key = null)
	{
		key = key ?? typeof(T).AssemblyQualifiedName;
		var dict = _attachmentTable.GetOrCreateValue(host);
		object value;
		var found = dict.TryRemove(key, out value);
		var castValue = found ? (T)value : default(T);
		return new AttachmentResult<T>(found, castValue);
	}

	private static string RefIdKey = Guid.NewGuid().ToString();

	/// <summary>
	/// Unique identifier for the object reference.
	/// </summary>
	public static Guid GetReferenceId(this object obj) =>
		obj.GetOrSetAttached(() => Guid.NewGuid(), RefIdKey).Value;

	public class AttachmentResult<T>
	{
		public bool Found { get; }
		public T Value { get; }

		public AttachmentResult(bool found, T value)
		{
			Found = found;
			Value = value;
		}
	}
}