Always use MSBuild for resolving assembly references; do not special-case some .NET BCL assemblies.

This avoids loading the GAC copy of the assembly in said special cases, and loads the reference assembly for the chosen target framework version instead.

src/Main/Base/Project/Src/Gui/Dialogs/ReferenceDialog/GacReferencePanel.cs

@@ -378,7 +378,8 @@ namespace ICSharpCode.SharpDevelop.Gui
 					}
 				});
 			
-			MSBuildInternals.ResolveAssemblyReferences(project, referenceItems.ToArray());
+			string mscorlibPath;
+			MSBuildInternals.ResolveAssemblyReferences(project, referenceItems.ToArray(), out mscorlibPath);
 			
 			WorkbenchSingleton.SafeThreadAsyncCall(
 				delegate {
@@ -422,7 +423,8 @@ namespace ICSharpCode.SharpDevelop.Gui
 						}
 					});
 				
-				MSBuildInternals.ResolveAssemblyReferences(project, referenceItems.ToArray());
+				string mscorlibPath;
+				MSBuildInternals.ResolveAssemblyReferences(project, referenceItems.ToArray(), out mscorlibPath);
 				foreach (ReferenceProjectItem rpi in referenceItems) {
 					if (string.IsNullOrEmpty(rpi.Redist)) continue;
 					if (!redistNameToRequiredFramework.ContainsKey(rpi.Redist)) {

src/Main/Base/Project/Src/Project/CompilableProject.cs

@@ -206,8 +206,6 @@ namespace ICSharpCode.SharpDevelop.Project
 		protected override ParseProjectContent CreateProjectContent()
 		{
 			ParseProjectContent newProjectContent = new ParseProjectContent(this);
-			var mscorlib = AssemblyParserService.GetRegistryForReference(new ReferenceProjectItem(this, "mscorlib")).Mscorlib;
-			newProjectContent.AddReferencedContent(mscorlib);
 			return newProjectContent;
 		}
 		

src/Main/Base/Project/Src/Project/MSBuildBasedProject.cs

@@ -137,9 +137,21 @@ namespace ICSharpCode.SharpDevelop.Project
 			}
 		}
 		
+		volatile string mscorlibPath;
+		
+		/// <summary>
+		/// Gets the path to mscorlib.
+		/// This property is set only after ResolveAssemblyReferences() is called.
+		/// </summary>
+		public string MscorlibPath {
+			get { return mscorlibPath; }
+		}
+		
 		public override void ResolveAssemblyReferences()
 		{
-			MSBuildInternals.ResolveAssemblyReferences(this, null);
+			string mscorlib;
+			MSBuildInternals.ResolveAssemblyReferences(this, null, out mscorlib);
+			this.mscorlibPath = mscorlib;
 		}
 		
 		#region CreateProjectItem

src/Main/Base/Project/Src/Project/MSBuildInternals.cs

@@ -5,8 +5,8 @@ using System;
 using System.Collections.Generic;
 using System.Linq;
 using System.Text.RegularExpressions;
-
 using ICSharpCode.Core;
+using ICSharpCode.SharpDevelop.Dom;
 using Microsoft.Build.Construction;
 using Microsoft.Build.Execution;
 using Microsoft.Build.Framework;
@@ -162,7 +162,7 @@ namespace ICSharpCode.SharpDevelop.Project
 		/// <param name="baseProject">The base project.</param>
 		/// <param name="referenceReplacements">A different set of references to use instead of those in the project.
 		/// Used by the GacReferencePanel.</param>
-		internal static void ResolveAssemblyReferences(MSBuildBasedProject baseProject, ReferenceProjectItem[] referenceReplacements)
+		internal static void ResolveAssemblyReferences(MSBuildBasedProject baseProject, ReferenceProjectItem[] referenceReplacements, out string mscorlibPath)
 		{
 			ProjectInstance project = baseProject.CreateProjectInstance();
 			project.SetProperty("BuildingProject", "false");
@@ -215,18 +215,22 @@ namespace ICSharpCode.SharpDevelop.Project
 				referenceDict[item.Include] = item;
 			}
 			
-			
+			mscorlibPath = null;
 			foreach (ProjectItemInstance item in project.GetItems("_ResolveAssemblyReferenceResolvedFiles")) {
+				DomAssemblyName assemblyName = new DomAssemblyName(item.GetMetadataValue("FusionName"));
+				string fullPath = FileUtility.GetAbsolutePath(baseProject.Directory, item.GetMetadataValue("Identity"));
+				
 				string originalInclude = item.GetMetadataValue("OriginalItemSpec");
 				ReferenceProjectItem reference;
 				if (referenceDict.TryGetValue(originalInclude, out reference)) {
 					reference.AssemblyName = new Dom.DomAssemblyName(item.GetMetadataValue("FusionName"));
-					//string fullPath = item.GetEvaluatedMetadata("FullPath"); is incorrect for relative paths
-					string fullPath = FileUtility.GetAbsolutePath(baseProject.Directory, item.GetMetadataValue("Identity"));
 					reference.FileName = fullPath;
 					reference.Redist = item.GetMetadataValue("Redist");
 					LoggingService.Debug("Got information about " + originalInclude + "; fullpath=" + fullPath);
 					reference.DefaultCopyLocalValue = bool.Parse(item.GetMetadataValue("CopyLocal"));
+				} else if (string.Equals(assemblyName.ShortName, "mscorlib", StringComparison.OrdinalIgnoreCase)) {
+					LoggingService.Debug("Got information for mscorlib: " + fullPath);
+					mscorlibPath = fullPath;
 				} else {
 					LoggingService.Warn("Unknown item " + originalInclude);
 				}

src/Main/Base/Project/Src/Services/ParserService/ParseProjectContent.cs

@@ -58,6 +58,17 @@ namespace ICSharpCode.SharpDevelop
 			// TODO: Translate me
 //			progressMonitor.TaskName = "Resolving references for " + project.Name + "...";
 			project.ResolveAssemblyReferences();
+			MSBuildBasedProject msbuildProject = project as MSBuildBasedProject;
+			if (msbuildProject != null) {
+				string mscorlib = msbuildProject.MscorlibPath;
+				if (string.IsNullOrEmpty(mscorlib)) {
+					AddReferencedContent(AssemblyParserService.DefaultProjectContentRegistry.Mscorlib);
+				} else {
+					AddReferencedContent(AssemblyParserService.DefaultProjectContentRegistry.GetProjectContentForReference("mscorlib", mscorlib));
+				}
+			} else {
+				AddReferencedContent(AssemblyParserService.DefaultProjectContentRegistry.Mscorlib);
+			}
 			foreach (ProjectItem item in items) {
 				if (!initializing) return; // abort initialization
 				progressMonitor.CancellationToken.ThrowIfCancellationRequested();
@@ -76,13 +87,6 @@ namespace ICSharpCode.SharpDevelop
 		
 		internal void ReInitialize1(IProgressMonitor progressMonitor)
 		{
-			var mscorlib = AssemblyParserService.GetRegistryForReference(new ReferenceProjectItem(project, "mscorlib")).Mscorlib;
-			// don't fetch mscorlib within lock - finding the correct registry might access the project, causing
-			// a deadlock between IProject.SyncRoot and the ReferencedContents lock
-			lock (ReferencedContents) {
-				ReferencedContents.Clear();
-				AddReferencedContent(mscorlib);
-			}
 			// prevent adding event handler twice
 			ProjectService.ProjectItemAdded   -= OnProjectItemAdded;
 			ProjectService.ProjectItemRemoved -= OnProjectItemRemoved;

src/Main/ICSharpCode.SharpDevelop.Dom/Project/Src/ProjectContent/ProjectContentRegistry.cs

@@ -300,6 +300,11 @@ namespace ICSharpCode.SharpDevelop.Dom
 		
 		protected virtual Assembly GetDefaultAssembly(string shortName)
 		{
+			return null;
+			/*
+			 * Special cases disabled: using these causes us the load the GAC assemblies,
+			 * but we need to load the reference assemblies instead to support multi-targeting.
+			 * 
 			// These assemblies are already loaded by SharpDevelop, so we
 			// don't need to load them in a separate AppDomain/with Cecil.
 			switch (shortName) {
@@ -332,7 +337,7 @@ namespace ICSharpCode.SharpDevelop.Dom
 					return Assembly.Load("Microsoft.VisualBasic, Version=8.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a");
 				default:
 					return null;
-			}
+			}*/
 		}
 	}
 }