C# in Depth

238 CHAPTER 9 Lambda expressions and expression trees{string name = prop.DisplayName;object value = prop.GetValue(e);Console.WriteLine(" {0}={1}", name, value);}}...Button button = new Button();button.Text = "Click me";button.Click += (src, e) => { Log("Click", src, e); };button.KeyPress += (src, e) => { Log("KeyPress", src, e); };button.MouseClick += (src, e) => { Log("MouseClick", src, e); };Form form = new Form();form.AutoSize=true;form.Controls.Add(button);Application.Run(form);Listing 9.5 uses lambda expressions to pass the event name and parameters to the Logmethod, which logs details of the event. We don’t log the details of the source event,beyond whatever its ToString override returns, because there’s an overwhelmingamount of information associated with controls. However, we use reflection over propertydescriptors to show the details of the EventArgs instance passed to us. Here’ssome sample output when you click the button:Event: ClickSender: System.Windows.Forms.Button, Text: Click meArguments: System.Windows.Forms.MouseEventArgsButton=LeftClicks=1X=53Y=17Delta=0Location={X=53,Y=17}Event: MouseClickSender: System.Windows.Forms.Button, Text: Click meArguments: System.Windows.Forms.MouseEventArgsButton=LeftClicks=1X=53Y=17Delta=0Location={X=53,Y=17}All of this is possible without lambda expressions, of course—but it’s a lot neater than itwould have been otherwise. Now that we’ve seen lambdas being converted into delegateinstances, it’s time to look at expression trees, which represent lambda expressionsas data instead of code.9.3 Expression treesThe idea of “code as data” is an old one, but it hasn’t been used much in popular programminglanguages. You could argue that all .NET programs use the concept,because the IL code is treated as data by the JIT, which then converts it into nativeLicensed to Rhona Hadida