Alexandra Rusina explains on her blog the new VS 2010 Expression Tree visualization capabilities at debug time. Basically, a new meta language is used to make expression tree complex hierarchy more concrete. As far as I understand, this meta language is based on some new .NET v4 framework feature. As a consequence, when debugging a .NET 3.5 assembly in VS 2010, you don’t have access to this new meta language visualization feature.
Moreover while this meta language can express things more concisely, I guess they’ll be a range of scenarios where I’ll prefer to keep using C# 2008 Samples Expression Tree Visualizer (Expression Tree visualization through a Windows Form Tree) or Manuel Abadia Linq Expression Debugger Visualizer (Expression tree visualization through a graph). I noticed that both these addin doesn’t work as-is on VS 2010 and the trick I wanted to share here is that, to use them under VS 2010, you just need to recompile them by referencing the VS 2010 assembly Microsoft.VisualStudio.DebuggerVisualizers.dll (found in C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE\ReferenceAssemblies\v2.0\Microsoft.VisualStudio.DebuggerVisualizers.dll) instead of the VS 2008 assembly with the same name. This works like a charm with the C# 2008 Samples Expression Tree Visualize used in VS 2010 on .NET 4 and .NET 3.5 projects. I expect the same good result with the Manuel Abadia piece of code (but source code is not available here so I couldn’t recompile, Manuel can you do that?).