Fixed NullReferenceException in MSBuildBasedProject.get_MinimumSolutionVersion() when removing a project from the solution while the build is running. (http://community.sharpdevelop.net/forums/t/12316.aspx)

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

@@ -4,6 +4,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.IO;
 using System.Linq;
 using System.Text;
 using System.Threading;
@@ -274,7 +275,15 @@ namespace ICSharpCode.SharpDevelop.Project
 			
 			public void DoStartBuild(object state)
 			{
-				project.StartBuild(options, this);
+				string name = string.Empty;
+				try {
+					name = project.Name;
+					project.StartBuild(options, this);
+				} catch (ObjectDisposedException) {
+					// Handle ObjectDisposedException that occurs when trying to build a project that was unloaded.
+					ReportError(new BuildError(null, "The project '" + name + "' was unloaded."));
+					Done(false);
+				}
 			}
 			
 			public void ReportError(BuildError error)
@@ -630,6 +639,9 @@ namespace ICSharpCode.SharpDevelop.Project
 				}
 			}
 			if (messagesToReport != null) {
+				// we can report these messages outside the lock:
+				// while they swap order with messages currently coming in (ReportMessage),
+				// this shouldn't be a problem as nodes should not report messages after they finish building.
 				messagesToReport.ForEach(ReportMessageInternal);
 			}
 			if (newNodeWithOutputLockAlreadyFinishedBuilding) {

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

@@ -98,6 +98,11 @@ namespace ICSharpCode.SharpDevelop.Project
 		public override int MinimumSolutionVersion {
 			get {
 				lock (SyncRoot) {
+					// This property is called by CSharpProject.StartBuild (and other derived StartBuild methods),
+					// so it's important that we throw an ObjectDisposedException for disposed projects.
+					// The build engine will handle this exception (occurs when unloading a project while a build is running)
+					if (projectFile == null)
+						throw new ObjectDisposedException("MSBuildBasedProject");
 					if (string.IsNullOrEmpty(projectFile.ToolsVersion) || projectFile.ToolsVersion == "2.0") {
 						return Solution.SolutionVersionVS2005;
 					} else if (projectFile.ToolsVersion == "3.0" || projectFile.ToolsVersion == "3.5") {