Optimized the AstNode.Descendants property.

Over 3 times faster than the previous implementation - but still slower than a visitor.
Fastest is a recursive function based on a for-loop ("for (AstNode child = node.FirstChild; child != null; child = child.NextSibling)").

ICSharpCode.NRefactory.CSharp/Analysis/DefiniteAssignmentAnalysis.cs

@@ -443,7 +443,7 @@ namespace ICSharpCode.NRefactory.CSharp.Analysis
 				// the special values are valid as output only, not as input
 				Debug.Assert(data == CleanSpecialValues(data));
 				DefiniteAssignmentStatus status = data;
-				foreach (AstNode child in node.Children) {
+				for (AstNode child = node.FirstChild; child != null; child = child.NextSibling) {
 					analysis.analysisCancellationToken.ThrowIfCancellationRequested();
 					
 					Debug.Assert(!(child is Statement)); // statements are visited with the CFG, not with the visitor pattern

ICSharpCode.NRefactory.CSharp/Ast/AstNode.cs

@@ -23,6 +23,7 @@
 // LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 // THE SOFTWARE.
+
 using System;
 using System.Collections;
 using System.Collections.Generic;
@@ -288,17 +289,31 @@ namespace ICSharpCode.NRefactory.CSharp
 		/// Gets all descendants of this node (excluding this node itself).
 		/// </summary>
 		public IEnumerable<AstNode> Descendants {
-			get {
-				return Utils.TreeTraversal.PreOrder (this.Children, n => n.Children);
-			}
+			get { return GetDescendants(false); }
 		}
 		
 		/// <summary>
 		/// Gets all descendants of this node (including this node itself).
 		/// </summary>
 		public IEnumerable<AstNode> DescendantsAndSelf {
-			get {
-				return Utils.TreeTraversal.PreOrder (this, n => n.Children);
+			get { return GetDescendants(true); }
+		}
+		
+		IEnumerable<AstNode> GetDescendants(bool includeSelf)
+		{
+			if (includeSelf)
+				yield return this;
+			Stack<AstNode> nextStack = new Stack<AstNode>();
+			nextStack.Push(null);
+			AstNode pos = firstChild;
+			while (pos != null) {
+				if (pos.nextSibling != null)
+					nextStack.Push(pos.nextSibling);
+				yield return pos;
+				if (pos.firstChild != null)
+					pos = pos.firstChild;
+				else
+					pos = nextStack.Pop();
 			}
 		}
 		
@@ -322,7 +337,7 @@ namespace ICSharpCode.NRefactory.CSharp
 		{
 			return Ancestors.OfType<T>().FirstOrDefault();
 		}
-				
+		
 		public AstNodeCollection<T> GetChildrenByRole<T> (Role<T> role) where T : AstNode
 		{
 			return new AstNodeCollection<T> (this, role);

ICSharpCode.NRefactory.CSharp/Parser/CSharpParser.cs

@@ -3809,7 +3809,7 @@ namespace ICSharpCode.NRefactory.CSharp
 			var cu = Parse (new StringReader (code), "parsed.cs", lineModifier - 1);
 			if (cu == null)
 				return Enumerable.Empty<EntityDeclaration> ();
-			var td = cu.Children.FirstOrDefault () as TypeDeclaration;
+			var td = cu.FirstChild as TypeDeclaration;
 			if (td != null) {
 				var members = td.Members.ToArray();
 				// detach members from parent

ICSharpCode.NRefactory.ConsistencyCheck/VisitorBenchmark.cs

@@ -24,31 +24,122 @@ using ICSharpCode.NRefactory.CSharp;
 
 namespace ICSharpCode.NRefactory.ConsistencyCheck
 {
+	/// <summary>
+	/// Determines the fastest way to retrieve a List{IdentifierExpression} of all identifiers
+	/// in a compilation unit.
+	/// </summary>
 	public class VisitorBenchmark
 	{
 		public static void Run(IEnumerable<CompilationUnit> files)
 		{
 			files = files.ToList();
-			RunTest("DepthFirstAstVisitor", files, cu => cu.AcceptVisitor(new DepthFirst()));
-			RunTest("DepthFirstAstVisitor<object>", files, cu => cu.AcceptVisitor(new DepthFirst<object>()));
-			RunTest("DepthFirstAstVisitor<int>", files, cu => cu.AcceptVisitor(new DepthFirst<int>()));
-			RunTest("DepthFirstAstVisitor<object, object>", files, cu => cu.AcceptVisitor(new DepthFirst<object, object>(), null));
-			RunTest("DepthFirstAstVisitor<int, int>", files, cu => cu.AcceptVisitor(new DepthFirst<int, int>(), 0));
-			RunTest("ObservableAstVisitor", files, cu => cu.AcceptVisitor(new ObservableAstVisitor()));
-			RunTest("ObservableAstVisitor<object, object>", files, cu => cu.AcceptVisitor(new ObservableAstVisitor<object, object>(), null));
-			RunTest("ObservableAstVisitor<int, int>", files, cu => cu.AcceptVisitor(new ObservableAstVisitor<int, int>(), 0));
+			
+			RunTest("recursive method using for", files, (cu, list) => WalkTreeFor(cu, list));
+			RunTest("recursive method using foreach", files, (cu, list) => WalkTreeForEach(cu, list));
+			RunTest("non-recursive loop", files, (cu, list) => WalkTreeNonRecursive(cu, list));
+			RunTest("foreach over Descendants.OfType()", files, (cu, list) => {
+			        	foreach (var node in cu.Descendants.OfType<IdentifierExpression>()) {
+			        		list.Add(node);
+			        	}
+			        });
+			RunTest("DepthFirstAstVisitor", files, (cu, list) => cu.AcceptVisitor(new DepthFirst(list)));
+			RunTest("DepthFirstAstVisitor<object>", files, (cu, list) => cu.AcceptVisitor(new DepthFirst<object>(list)));
+			RunTest("DepthFirstAstVisitor<object, object>", files, (cu, list) => cu.AcceptVisitor(new DepthFirst<object, object>(list), null));
+			RunTest("ObservableAstVisitor", files, (cu, list) => {
+			        	var visitor = new ObservableAstVisitor();
+			        	visitor.EnterIdentifierExpression += list.Add;
+			        	cu.AcceptVisitor(visitor);
+			        });
+			RunTest("ObservableAstVisitor<object, object>", files, (cu, list) => {
+			        	var visitor = new ObservableAstVisitor<object, object>();
+			        	visitor.IdentifierExpressionVisited += (id, data) => list.Add(id);
+			        	cu.AcceptVisitor(visitor, null);
+			        });
 		}
 		
-		class DepthFirst : DepthFirstAstVisitor {}
-		class DepthFirst<T> : DepthFirstAstVisitor<T> {}
-		class DepthFirst<T, S> : DepthFirstAstVisitor<T, S> {}
+		static void WalkTreeForEach(AstNode node, List<IdentifierExpression> list)
+		{
+			foreach (AstNode child in node.Children) {
+				IdentifierExpression id = child as IdentifierExpression;
+				if (id != null) {
+					list.Add(id);
+				}
+				WalkTreeForEach(child, list);
+			}
+		}
+		
+		static void WalkTreeFor(AstNode node, List<IdentifierExpression> list)
+		{
+			for (AstNode child = node.FirstChild; child != null; child = child.NextSibling) {
+				IdentifierExpression id = child as IdentifierExpression;
+				if (id != null) {
+					list.Add(id);
+				}
+				WalkTreeFor(child, list);
+			}
+		}
+		
+		static void WalkTreeNonRecursive(AstNode root, List<IdentifierExpression> list)
+		{
+			AstNode pos = root;
+			while (pos != null) {
+				{
+					IdentifierExpression id = pos as IdentifierExpression;
+					if (id != null) {
+						list.Add(id);
+					}
+				}
+				if (pos.FirstChild != null) {
+					pos = pos.FirstChild;
+				} else {
+					pos = pos.GetNextNode();
+				}
+			}
+		}
+		
+		class DepthFirst : DepthFirstAstVisitor {
+			readonly List<IdentifierExpression> list;
+			public DepthFirst(List<IdentifierExpression> list) { this.list = list; }
+			public override void VisitIdentifierExpression(IdentifierExpression identifierExpression)
+			{
+				list.Add(identifierExpression);
+				base.VisitIdentifierExpression(identifierExpression);
+			}
+		}
+		class DepthFirst<T> : DepthFirstAstVisitor<T> {
+			readonly List<IdentifierExpression> list;
+			public DepthFirst(List<IdentifierExpression> list) { this.list = list; }
+			public override T VisitIdentifierExpression(IdentifierExpression identifierExpression)
+			{
+				list.Add(identifierExpression);
+				return base.VisitIdentifierExpression(identifierExpression);
+			}
+		}
+		class DepthFirst<T, S> : DepthFirstAstVisitor<T, S> {
+			readonly List<IdentifierExpression> list;
+			public DepthFirst(List<IdentifierExpression> list) { this.list = list; }
+			public override S VisitIdentifierExpression(IdentifierExpression identifierExpression, T data)
+			{
+				list.Add(identifierExpression);
+				return base.VisitIdentifierExpression(identifierExpression, data);
+			}
+		}
 		
-		static void RunTest(string text, IEnumerable<CompilationUnit> files, Action<CompilationUnit> action)
+		static void RunTest(string text, IEnumerable<CompilationUnit> files, Action<CompilationUnit, List<IdentifierExpression>> action)
 		{
+			// validation:
+			var list = new List<IdentifierExpression>();
+			foreach (var file in files) {
+				list.Clear();
+				action(file, list);
+				if (!list.SequenceEqual(file.Descendants.OfType<IdentifierExpression>()))
+					throw new InvalidOperationException();
+			}
 			Stopwatch w = Stopwatch.StartNew();
 			foreach (var file in files) {
 				for (int i = 0; i < 20; i++) {
-					action(file);
+					list.Clear();
+					action(file, list);
 				}
 			}
 			w.Stop();