What is the best resource for learning C# expression trees in depth?

Chapter 11 (Inside Expression Trees) and chapter 12 (Extending Linq) of Programming Microsoft Linq (ISBN 13: 978-0-7356-2400-9 or ISBN 10: 0-7356-2400-3) has been invaluable for me. I haven’t read Jons book, but he is a quality guy and explains things well, so I assume that his coverage would also be good.

Another great resource is Bart De Smet’s blog

Also, keep your eye on MSDN, the sample code for building a Simple Linq to Database (by Pedram Rezaei) is about to get about 40 pages of Doco explaining it.

A really, really useful resource for Expression Tree’s in fact I would regard it as a must have is the Expression Tree Visualiser debugging tool.

You should also learn as much as you can about Expression Tree Visitors, there is a pretty good base class inplementation here.

Here is some sample code derived from that Visitor class to do some debugging (I based this on some sample code in the book I mentioned) the prependIndent method call is just an extension method on a string to put a “–” at each indent level.

  internal class DebugDisplayTree : ExpressionVisitor
  {
    private int indentLevel = 0;

    protected override System.Linq.Expressions.Expression Visit(Expression exp)
    {
      if (exp != null)
      {
        Trace.WriteLine(string.Format("{0} : {1} ", exp.NodeType, exp.GetType().ToString()).PrependIndent(indentLevel));
      }
      indentLevel++;
      Expression result = base.Visit(exp);
      indentLevel--;
      return result;
    }
    ...