Add serialization support for BKTrees.

src/Workspaces/Core/Portable/Utilities/BKTree.Edge.cs

@@ -21,6 +21,17 @@ public Edge(int editDistance, int childNodeIndex)
                 EditDistance = editDistance;
                 ChildNodeIndex = childNodeIndex;
             }
+
+            internal void WriteTo(ObjectWriter writer)
+            {
+                writer.WriteInt32(EditDistance);
+                writer.WriteInt32(ChildNodeIndex);
+            }
+
+            internal static Edge ReadFrom(ObjectReader reader)
+            {
+                return new Edge(reader.ReadInt32(), reader.ReadInt32());
+            }
         }
     }
 }
\ No newline at end of file

src/Workspaces/Core/Portable/Utilities/BKTree.Node.cs

@@ -27,6 +27,18 @@ public Node(char[] lowerCaseCharacters, int edgeCount, int firstEdgeIndex)
                 EdgeCount = edgeCount;
                 FirstEdgeIndex = firstEdgeIndex;
             }
+
+            internal void WriteTo(ObjectWriter writer)
+            {
+                writer.WriteValue(LowerCaseCharacters);
+                writer.WriteInt32(EdgeCount);
+                writer.WriteInt32(FirstEdgeIndex);
+            }
+
+            internal static Node ReadFrom(ObjectReader reader)
+            {
+                return new Node((char[])reader.ReadValue(), reader.ReadInt32(), reader.ReadInt32());
+            }
         }
     }
 }

src/Workspaces/Core/Portable/Utilities/BKTree.Serialization.cs

+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Roslyn.Utilities
+{
+    internal partial class BKTree
+    {
+        internal void WriteTo(ObjectWriter writer)
+        {
+            writer.WriteInt32(this._nodes.Length);
+            foreach (var node in _nodes)
+            {
+                node.WriteTo(writer);
+            }
+
+            writer.WriteInt32(this._edges.Length);
+            foreach (var edge in _edges)
+            {
+                edge.WriteTo(writer);
+            }
+        }
+
+        internal static BKTree ReadFrom(ObjectReader reader)
+        {
+            var nodes = new Node[reader.ReadInt32()];
+            for (var i = 0; i < nodes.Length; i++)
+            {
+                nodes[i] = Node.ReadFrom(reader);
+            }
+
+            var edges = new Edge[reader.ReadInt32()];
+            for (var i = 0; i < edges.Length; i++)
+            {
+                edges[i] = Edge.ReadFrom(reader);
+            }
+
+            return new BKTree(nodes, edges);
+        }
+    }
+}

src/Workspaces/Core/Portable/Utilities/BKTree.cs

@@ -12,6 +12,10 @@ namespace Roslyn.Utilities
 {
     internal partial class BKTree
     {
+        public static readonly BKTree Empty = new BKTree(
+            SpecializedCollections.EmptyArray<Node>(),
+            SpecializedCollections.EmptyArray<Edge>());
+
         // We have two completely flat arrays of structs (except for the char[] values the nodes
         // point to).  These arrays fully represent the BK tree.  The structure is as follows:
         //

src/Workspaces/Core/Portable/Workspaces.csproj

@@ -418,6 +418,7 @@
     <Compile Include="Simplification\SimplifyTypeNameCodeAction.cs" />
     <Compile Include="Utilities\BKTree.Edge.cs" />
     <Compile Include="Utilities\BKTree.Node.cs" />
+    <Compile Include="Utilities\BKTree.Serialization.cs" />
     <Compile Include="Utilities\ForegroundThreadDataKind.cs" />
     <Compile Include="Utilities\IReadOnlyDictionaryExtensions.cs" />
     <Compile Include="Utilities\ValuesSources\CachedWeakValueSource.cs" />