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diff --git a/src/mscorlib/src/System/Runtime/CompilerServices/TupleElementNamesAttribute.cs b/src/mscorlib/src/System/Runtime/CompilerServices/TupleElementNamesAttribute.cs
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+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System.Collections.Generic;
+
+namespace System.Runtime.CompilerServices
+{
+ /// <summary>
+ /// Indicates that the use of <see cref="System.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)]
+ public 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="System.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="System.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="System.ValueTuple{T1, T2}"/>, <see
+ /// cref="System.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="System.ValueTuple"/> elements are
+ /// meant to carry element names.
+ /// </summary>
+ public IList<string> TransformNames => _transformNames;
+ }
+} \ No newline at end of file