C# in Depth

Expression trees239code to run on your CPU. That’s quite deeply hidden, though, and while libraries existto manipulate IL programmatically, they’re not widely used.Expression trees in .NET 3.5 provide an abstract way of representing some code as atree of objects. It’s like CodeDOM but operating at a slightly higher level, and only forexpressions. The primary use of expression trees is in LINQ, and later in this sectionwe’ll see how crucial expression trees are to the whole LINQ story.C# 3 provides built-in support for converting lambda expressions to expressiontrees, but before we cover that let’s explore how they fit into the .NET Frameworkwithout using any compiler tricks.9.3.1 Building expression trees programmaticallyExpression trees aren’t as mystical as they sound, although some of the uses they’reput to look like magic. As the name suggests, they’re trees of objects, where each nodein the tree is an expression in itself. Different types of expressions represent the differentoperations that can be performed in code: binary operations, such as addition;unary operations, such as taking the length of an array; method calls; constructorcalls; and so forth.The System.Linq.Expressions namespace contains the various classes that representexpressions. All of them derive from the Expression class, which is abstract andmostly consists of static factory methods to create instances of other expressionclasses. It exposes two properties, however:■■The Type property represents the .NET type of the evaluated expression—youcan think of it like a return type. The type of an expression that fetches theLength property of a string would be int, for example.The NodeType property returns the kind of expression represented, as a memberof the ExpressionType enumeration, with values such as LessThan, Multiply,and Invoke. To use the same example, in myString.Length the property accesspart would have a node type of MemberAccess.There are many classes derived from Expression, and some of them can have manydifferent node types: BinaryExpression, for instance, represents any operation withtwo operands: arithmetic, logic, comparisons, array indexing, and the like. This iswhere the NodeType property is important, as it distinguishes between different kindsof expressions that are represented by the same class.I don’t intend to cover every expression class or node type—there are far toomany, and MSDN does a perfectly good job of explaining them. Instead, we’ll try to geta general feel for what you can do with expression trees.Let’s start off by creating one of the simplest possible expression trees, adding twoconstant integers together. Listing 9.6 creates an expression tree to represent 2+3.Listing 9.6 A very simple expression tree, adding 2 and 3Expression firstArg = Expression.Constant(2);Expression secondArg = Expression.Constant(3);Expression add = Expression.Add(firstArg, secondArg);Console.WriteLine(add);Licensed to Rhona Hadida