C# in Depth

Expression trees241So, what’s the point of doing this? Well, LambdaExpression has a Compile method thatcreates a delegate of the appropriate type. This delegate can now be executed in thenormal manner, as if it had been created using a normal method or any other means.Listing 9.7 shows this in action, with the same expression as before.Listing 9.7Compiling and executing an expression treeExpression firstArg = Expression.Constant(2);Expression secondArg = Expression.Constant(3);Expression add = Expression.Add(firstArg, secondArg);Func compiled = Expression.Lambda(add).Compile();Console.WriteLine(compiled());Arguably listing 9.7 is one of the most convoluted ways of printing out “5” that youcould ask for. At the same time, it’s also rather impressive. We’re programmaticallycreating some logical blocks and representing them as normal objects, and then askingthe framework to compile the whole thing into “real” code that can be executed.You may never need to actually use expression trees this way, or even build them upprogrammatically at all, but it’s useful background information that will help youunderstand how LINQ works.As I said at the beginning of this section, expression trees are not too far removedfrom CodeDOM—Snippy compiles and executes C# code that has been entered asplain text, for instance. However, two significant differences exist between CodeDOMand expression trees.First, expression trees are only able to represent single expressions. They’re notdesigned for whole classes, methods, or even just statements. Second, C# supportsexpression trees directly in the language, through lambda expressions. Let’s take alook at that now.9.3.3 Converting C# lambda expressions to expression treesAs we’ve already seen, lambda expressions can be converted to appropriate delegateinstances, either implicitly or explicitly. That’s not the only conversion that is available,however. You can also ask the compiler to build an expression tree from yourlambda expression, creating an instance of Expression at executiontime. For example, listing 9.8 shows a much shorter way of creating the “return 5”expression, compiling it and then invoking the resulting delegate.Listing 9.8Using lambda expressions to create expression treesExpression return5 = () => 5;Func compiled = return5.Compile();Console.WriteLine(compiled());In the first line of listing 9.8, the () => 5 part is the lambda expression. In this case,putting it in an extra pair of parentheses around the whole thing makes it look worserather than better. Notice that we don’t need any casts because the compiler can verifyLicensed to Rhona Hadida