Apply suggestions from code review

Co-authored-by: kasperk81 <[email protected]>

Author: edvilme

src/Cli/dotnet/commands/dotnet-sln/remove/Program.cs

@@ -19,21 +19,28 @@ internal class RemoveProjectFromSolutionCommand : CommandBase
         private readonly string _fileOrDirectory;
         private readonly IReadOnlyCollection<string> _projects;
 
-        private int CountNonFolderDescendants(SolutionModel solution, SolutionFolderModel item, Dictionary<SolutionFolderModel, int> cached)
+        private int CountNonFolderDescendants(SolutionFolderModel item, Dictionary<SolutionFolderModel, int> cached, 
+            Dictionary<SolutionFolderModel, List<SolutionItem>> childrenByParent)
         {
             if (cached.ContainsKey(item))
             {
                 return cached[item];
             }
+
             int count = item.Files?.Count ?? 0;
-            var children = solution.SolutionItems.Where(i => i.Parent == item);
-            foreach (var child in children)
+
+            // Retrieve children directly from the precomputed index
+            if (childrenByParent.TryGetValue(item, out var children))
             {
-                count += child is SolutionFolderModel folderModel
-                    ? CountNonFolderDescendants(solution, folderModel, cached)
-                    : 1;
+                foreach (var child in children)
+                {
+                    count += child is SolutionFolderModel folderModel
+                        ? CountNonFolderDescendants(folderModel, cached, childrenByParent)
+                        : 1;
+                }
             }
-            cached.Add(item, count);
+
+            cached[item] = count;  // Using 'cached[item] = count;' instead of 'Add' for better clarity
             return count;
         }
 
@@ -111,10 +118,14 @@ private async Task RemoveProjectsAsync(string solutionFileFullPath, IEnumerable<
                 }
             }
 
+            var childrenByParent = solution.SolutionItems
+                .GroupBy(item => item.Parent)
+                .ToDictionary(g => g.Key, g => g.ToList());
+
             Dictionary<SolutionFolderModel, int> nonFolderDescendantsCount = new();
             foreach (var item in solution.SolutionFolders)
             {
-                CountNonFolderDescendants(solution, item, nonFolderDescendantsCount);
+                CountNonFolderDescendants(item, nonFolderDescendantsCount, childrenByParent);
             }
 
             var emptyFolders = nonFolderDescendantsCount.Where(i => i.Value == 0).Select(i => i.Key);