If target assembly is not in ClassBrowser's workspace, now "Show in Class Browser" automatically loads the assembly temporarily (as unpinned) into the workspace. Added a separate icon for unpinned assemblies.

src/Main/Base/Project/Dom/ClassBrowser/AssemblyTreeNode.cs

@@ -56,7 +56,11 @@ namespace ICSharpCode.SharpDevelop.Dom.ClassBrowser
 		public override object Icon {
 			get {
 				if (model.Context.IsValid) {
+					if (model.IsUnpinned()) {
+						return SD.ResourceService.GetImageSource("Icons.16x16.AssemblyUnpinned");
+					} else {
 						return SD.ResourceService.GetImageSource("Icons.16x16.Assembly");
+					}
 				} else {
 					return SD.ResourceService.GetImageSource("Icons.16x16.AssemblyError");
 				}

src/Main/Base/Project/Dom/ClassBrowser/ClassBrowserTreeView.cs

@@ -8,6 +8,7 @@ using System.Windows.Controls;
 using ICSharpCode.Core;
 using ICSharpCode.NRefactory.TypeSystem;
 using ICSharpCode.TreeView;
+using ICSharpCode.SharpDevelop.Parser;
 using ICSharpCode.SharpDevelop.Project;
 
 namespace ICSharpCode.SharpDevelop.Dom.ClassBrowser
@@ -61,6 +62,24 @@ namespace ICSharpCode.SharpDevelop.Dom.ClassBrowser
 			}
 		}
 		
+		private SharpTreeNode FindAssemblyTreeNode(string fullAssemblyName)
+		{
+			var assemblyTreeNode = this.Root.Children.FirstOrDefault(
+				node => {
+					if (node is AssemblyTreeNode) {
+						var asmTreeNode = (AssemblyTreeNode) node;
+						if (node.Model is IAssemblyModel) {
+							var asmModel = (IAssemblyModel) node.Model;
+							return asmModel.FullAssemblyName == fullAssemblyName;
+						}
+					}
+					
+					return false;
+				});
+			
+			return assemblyTreeNode;
+		}
+		
 		public bool GoToEntity(IEntity entity)
 		{
 			// Try to find assembly in workspace
@@ -102,25 +121,28 @@ namespace ICSharpCode.SharpDevelop.Dom.ClassBrowser
 				
 				if (namespaceChildren == null) {
 					// Try to find assembly of passed entity among additional assemblies
-					var assemblyTreeNode = this.Root.Children.FirstOrDefault(
-						node => {
-							if (node is AssemblyTreeNode) {
-								var asmTreeNode = (AssemblyTreeNode) node;
-								if (node.Model is IAssemblyModel) {
-									var asmModel = (IAssemblyModel) node.Model;
-									return asmModel.FullAssemblyName == entityAssembly.FullAssemblyName;
+					var assemblyTreeNode = FindAssemblyTreeNode(entityAssembly.FullAssemblyName);
+					if (assemblyTreeNode != null) {
+						assemblyTreeNode.EnsureLazyChildren();
+						namespaceChildren = assemblyTreeNode.Children;
 					}
 				}
 				
-							return false;
-						});
+				if (namespaceChildren == null) {
+					// Add assembly to workspace (unpinned), if not available in ClassBrowser
+					IAssemblyParserService assemblyParser = SD.GetService<IAssemblyParserService>();
+					if (assemblyParser != null) {
+						IAssemblyModel unpinnedAssemblyModel = assemblyParser.GetAssemblyModel(new FileName(entityAssembly.UnresolvedAssembly.Location));
+						if (unpinnedAssemblyModel != null) {
+							this.UnpinnedAssemblies.Assemblies.Add(unpinnedAssemblyModel);
+							var assemblyTreeNode = FindAssemblyTreeNode(entityAssembly.FullAssemblyName);
 							if (assemblyTreeNode != null) {
 								assemblyTreeNode.EnsureLazyChildren();
 								namespaceChildren = assemblyTreeNode.Children;
 							}
 						}
-				
-				// TODO Add assembly to workspace, if not available in ClassBrowser
+					}
+				}
 				
 				if (namespaceChildren != null) {
 					var nsTreeNode = namespaceChildren.FirstOrDefault(

src/Main/SharpDevelop/Dom/ClassBrowser/Commands.cs

@@ -101,10 +101,14 @@ namespace ICSharpCode.SharpDevelop.Dom.ClassBrowser
 			var classBrowser = SD.GetService<IClassBrowser>();
 			if (classBrowser != null) {
 				IAssemblyModel assemblyModel = (IAssemblyModel) parameter;
+				if (assemblyModel.IsUnpinned()) {
+					classBrowser.UnpinnedAssemblies.Assemblies.Remove(assemblyModel);
+				} else {
 					classBrowser.MainAssemblyList.Assemblies.Remove(assemblyModel);
 				}
 			}
 		}
+	}
 	
 	/// <summary>
 	/// RunAssemblyWithDebuggerCommand.