#if NETFRAMEWORK && !NET472_OR_GREATER
// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.

using System.Collections.Generic;

namespace System.Runtime.CompilerServices
{
	/// <summary>
	/// Indicates that the use of <see cref="ValueTuple"/> on a member is meant to be treated as a tuple with element names.
	/// </summary>
	//[CLSCompliant(false)]
	[AttributeUsage(AttributeTargets.Field | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.ReturnValue | AttributeTargets.Class | AttributeTargets.Struct | AttributeTargets.Event)]
	internal sealed class TupleElementNamesAttribute : Attribute
	{
		private readonly string?[] _transformNames;

		/// <summary>
		/// Initializes a new instance of the <see
		/// cref="TupleElementNamesAttribute"/> class.
		/// </summary>
		/// <param name="transformNames">
		/// Specifies, in a pre-order depth-first traversal of a type's
		/// construction, which <see cref="ValueType"/> occurrences are
		/// meant to carry element names.
		/// </param>
		/// <remarks>
		/// This constructor is meant to be used on types that contain an
		/// instantiation of <see cref="ValueType"/> that contains
		/// element names.  For instance, if <c>C</c> is a generic type with
		/// two type parameters, then a use of the constructed type <c>C{<see
		/// cref="ValueTuple{T1, T2}"/>, <see
		/// cref="ValueTuple{T1, T2, T3}"/></c> might be intended to
		/// treat the first type argument as a tuple with element names and the
		/// second as a tuple without element names. In which case, the
		/// appropriate attribute specification should use a
		/// <c>transformNames</c> value of <c>{ "name1", "name2", null, null,
		/// null }</c>.
		/// </remarks>
		public TupleElementNamesAttribute(string?[] transformNames)
		{
			if (transformNames == null)
			{
				throw new ArgumentNullException(nameof(transformNames));
			}

			_transformNames = transformNames;
		}

		/// <summary>
		/// Specifies, in a pre-order depth-first traversal of a type's
		/// construction, which <see cref="ValueTuple"/> elements are
		/// meant to carry element names.
		/// </summary>
		public IList<string?> TransformNames => _transformNames;
	}
}
#endif
