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diff --git a/src/Editor/Text/Def/TextUI/Tags/IStructureTag.cs b/src/Editor/Text/Def/TextUI/Tags/IStructureTag.cs
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+++ b/src/Editor/Text/Def/TextUI/Tags/IStructureTag.cs
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+//
+//  Copyright (c) Microsoft Corporation. All rights reserved.
+//  Licensed under the MIT License. See License.txt in the project root for license information.
+//
+namespace Microsoft.VisualStudio.Text.Tagging
+{
+    using Microsoft.VisualStudio.Text.Adornments;
+
+    /// <summary>
+    /// Represents a structural code block, which is used for vertical structural line adornments
+    /// and outlining collapse regions.
+    /// </summary>
+    /// <remarks>
+    /// IStructureTag is the replacement for the <see cref="IBlockTag"/> which should not be used.
+    /// </remarks>
+    public interface IStructureTag : ITag
+    {
+        /// <summary>
+        /// The Snapshot from which this IStructureTag was generated.
+        /// </summary>
+        ITextSnapshot Snapshot { get; }
+
+        /// <summary>
+        /// Gets the span containing the entire contents of the block (minus the block header).
+        /// This span will be collapsed or expanded when the block outlining adornment is invoked.
+        /// </summary>
+        /// <remarks>
+        /// If this parameter is null, block structure adornments will still be drawn as long as
+        /// the <see cref="GuideLineHorizontalAnchorPoint"/> and the <see cref="GuideLineSpan"/> are both provided,
+        /// however, no outlining adornment will be drawn.
+        /// </remarks>
+        Span? OutliningSpan { get; }
+
+        /// <summary>
+        /// Gets the span of the statement that controls the structural block.
+        /// </summary>
+        /// <remarks>
+        /// <para>
+        /// For example, in the following snippet of code,
+        /// <code>
+        /// if (condition1 &amp;&amp;
+        ///     condition2) // comment
+        /// {
+        ///     something;
+        /// }
+        /// </code>
+        /// this.HeaderSpan would extend from the start of the "if" to the end of comment.
+        /// this.OutliningSpan would extend from the end of "// comment" to the end of the "}".
+        /// </para>
+        /// <para>
+        /// If this parameter is null, block structure adornments will still be drawn as long as
+        /// the <see cref="OutliningSpan"/> is provided, or the <see cref="GuideLineHorizontalAnchorPoint"/>
+        /// and the <see cref="GuideLineSpan"/> are both provided.
+        /// </para>
+        /// </remarks>
+        Span? HeaderSpan { get; }
+
+        /// <summary>
+        /// Gets the vertical span within which the block structure adornment will be drawn.
+        /// </summary>
+        /// <remarks>
+        /// For a block to have an adornment, it must not be of type <see cref="PredefinedStructureTagTypes.Nonstructural"/>,
+        /// and the implementer must also provide the GuideLineHorizontalAnchor. The adornment is drawn from the top of the
+        /// line containing the start of the span to the bottom of the line containing the bottom of the span. If null,
+        /// the GuideLineSpan is inferred from the OutliningSpan and the HeaderSpan.
+        /// </remarks>
+        Span? GuideLineSpan { get; }
+
+        /// <summary>
+        /// Gets the point with which the block structure adornment will be horizontally aligned.
+        /// </summary>
+        /// <remarks>
+        /// This point can be on any line and is used solely for determining horizontal position. If null,
+        /// or if <see cref="GuideLineSpan"/> is null, this point is computed from the HeaderSpan and
+        /// OutliningSpan via heuristics.
+        /// </remarks>
+        int? GuideLineHorizontalAnchorPoint { get; }
+
+        /// <summary>
+        /// Determines the semantic type of the structural block.
+        /// </summary>
+        /// <remarks>
+        /// See <see cref="PredefinedStructureTagTypes"/> for the canonical types.
+        /// Use <see cref="PredefinedStructureTagTypes.Nonstructural"/> for blocks that will not have any visible affordance
+        /// (but will be used for outlining).
+        /// </remarks>
+        string Type { get; }
+
+        /// <summary>
+        /// Determines whether a block can be collapsed.
+        /// </summary>
+        bool IsCollapsible { get; }
+
+        /// <summary>
+        /// Determines whether a block is collapsed by default.
+        /// </summary>
+        bool IsDefaultCollapsed { get; }
+
+        /// <summary>
+        /// Determines whether a block is an implementation block.
+        /// </summary>
+        /// <remarks>
+        /// Implementation blocks are the blocks of code following a method definition.
+        /// They are used for commands such as the Visual Studio Collapse to Definition command,
+        /// which hides the implementation block and leaves only the method definition exposed.
+        /// </remarks>
+        bool IsImplementation { get; }
+
+        /// <summary>
+        /// Gets the data object for the collapsed UI. If the default is set, returns null.
+        /// </summary>
+        object GetCollapsedForm();
+
+        /// <summary>
+        /// Gets the data object for the collapsed UI tooltip. If the default is set, returns null.
+        /// </summary>
+        object GetCollapsedHintForm();
+    }
+}