Explicitly create partitioners for `Parallel.ForEach` calls.

Apparently the default `Parallel.ForEach` logic checks whether the input enumerable is an array or `IList<T>`,
and if it is, creates whole chunks of sequential list/array indices and assigns those to the worker threads.

This is more efficient if the individual items are processed very quickly; but if they take varying amounts of time, a single chunk full of expensive items might keep a single CPU core busy for long after all other CPU cores have gone idle.

ICSharpCode.Decompiler/CSharp/ProjectDecompiler/WholeProjectDecompiler.cs

@@ -33,6 +33,7 @@ using static ICSharpCode.Decompiler.Metadata.MetadataExtensions;
 using ICSharpCode.Decompiler.Metadata;
 using ICSharpCode.Decompiler.Solution;
 using ICSharpCode.Decompiler.DebugInfo;
+using System.Collections.Concurrent;
 
 namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 {
@@ -205,7 +206,7 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 			var progress = ProgressIndicator;
 			DecompilerTypeSystem ts = new DecompilerTypeSystem(module, AssemblyResolver, Settings);
 			Parallel.ForEach(
-				files,
+				Partitioner.Create(files, loadBalance: true),
 				new ParallelOptions {
 					MaxDegreeOfParallelism = this.MaxDegreeOfParallelism,
 					CancellationToken = cancellationToken

ILSpy/Commands/DecompileAllCommand.cs

@@ -19,6 +19,7 @@
 #if DEBUG
 
 using System;
+using System.Collections.Concurrent;
 using System.Diagnostics;
 using System.Linq;
 using System.Threading.Tasks;
@@ -40,7 +41,10 @@ namespace ICSharpCode.ILSpy
 		{
 			Docking.DockWorkspace.Instance.RunWithCancellation(ct => Task<AvalonEditTextOutput>.Factory.StartNew(() => {
 				AvalonEditTextOutput output = new AvalonEditTextOutput();
-				Parallel.ForEach(MainWindow.Instance.CurrentAssemblyList.GetAssemblies(), new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount, CancellationToken = ct }, delegate(LoadedAssembly asm) {
+				Parallel.ForEach(
+					Partitioner.Create( MainWindow.Instance.CurrentAssemblyList.GetAssemblies(), loadBalance: true),
+					new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount, CancellationToken = ct }, 
+					delegate(LoadedAssembly asm) {
 						if (!asm.HasLoadError) {
 							Stopwatch w = Stopwatch.StartNew();
 							Exception exception = null;

ILSpy/Commands/DisassembleAllCommand.cs

@@ -23,6 +23,8 @@ using System.Diagnostics;
 using System.Threading.Tasks;
 using ICSharpCode.ILSpy.TextView;
 using ICSharpCode.ILSpy.Properties;
+using System.Collections.Concurrent;
+
 namespace ICSharpCode.ILSpy
 {
 	[ExportMainMenuCommand(Menu = nameof(Resources._File),  Header = nameof(Resources.DEBUGDisassemble),  MenuCategory = nameof(Resources.Open),  MenuOrder = 2.5)]
@@ -37,7 +39,10 @@ namespace ICSharpCode.ILSpy
 		{
 			Docking.DockWorkspace.Instance.RunWithCancellation(ct => Task<AvalonEditTextOutput>.Factory.StartNew(() => {
 				AvalonEditTextOutput output = new AvalonEditTextOutput();
-				Parallel.ForEach(MainWindow.Instance.CurrentAssemblyList.GetAssemblies(), new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount, CancellationToken = ct }, delegate(LoadedAssembly asm) {
+				Parallel.ForEach(
+					Partitioner.Create(MainWindow.Instance.CurrentAssemblyList.GetAssemblies(), loadBalance: true),
+					new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount, CancellationToken = ct },
+					delegate (LoadedAssembly asm) {
 						if (!asm.HasLoadError) {
 						Stopwatch w = Stopwatch.StartNew();
 						Exception exception = null;

ILSpy/SolutionWriter.cs

@@ -99,7 +99,12 @@ namespace ICSharpCode.ILSpy
 			Stopwatch stopwatch = Stopwatch.StartNew();
 
 			try {
-				await Task.Run(() => Parallel.ForEach(assemblies, n => WriteProject(n, language, solutionDirectory, ct)))
+				// Explicitly create an enumerable partitioner here to avoid Parallel.ForEach's special cases for lists,
+				// as those seem to use static partitioning which is inefficient if assemblies take differently
+				// long to decompile.
+				await Task.Run(() => Parallel.ForEach(Partitioner.Create(assemblies),
+					new ParallelOptions { MaxDegreeOfParallelism = Environment.ProcessorCount, CancellationToken = ct },
+					n => WriteProject(n, language, solutionDirectory, ct)))
 					.ConfigureAwait(false);
 
 				await Task.Run(() => SolutionCreator.WriteSolutionFile(solutionFilePath, projects))