C# in Depth

Simplified initialization217The part in braces at the end of each line is the object initializer. Again, it’s just compilertrickery. The IL used to initialize tom3 and tom4 is identical, and indeed it’s verynearly 2 the same as we used for tom1. Predictably, the code for tom5 is nearly the sameas for tom2. Note how for tom4 we omitted the parentheses for the constructor. Youcan use this shorthand for types with a parameterless constructor, which is what getscalled in the compiled code.After the constructor has been called, the specified properties are set in the obviousway. They’re set in the order specified in the object initializer, and you can onlyspecify any particular property at most once—you can’t set the Name property twice,for example. (You could, however, call the constructor taking the name as a parameter,and then set the Name property. It would be pointless, but the compiler wouldn’tstop you from doing it.) The expression used as the value for a property can be anyexpression that isn’t itself an assignment—you can call methods, create new objects(potentially using another object initializer), pretty much anything.You may well be wondering just how useful this is—we’ve saved one orImportant!Oneexpression toinitialize anobjecttwo lines of code, but surely that’s not a good enough reason to make thelanguage more complicated, is it? There’s a subtle point here, though:we’ve not just created an object in one line—we’ve created it in oneexpression. That difference can be very important. Suppose you want tocreate an array of type Person[] with some predefined data in it. Evenwithout using the implicit array typing we’ll see later, the code is neat andreadable:Person[] family = new Person[]{new Person { Name="Holly", Age=31 },new Person { Name="Jon", Age=31 },new Person { Name="Tom", Age=4 },new Person { Name="William", Age=1 },new Person { Name="Robin", Age=1 }};Now, in a simple example like this we could have written a constructor taking both thename and age as parameters, and initialized the array in a similar way in C# 1 or 2.However, appropriate constructors aren’t always available—and if there are severalconstructor parameters, it’s often not clear which one means what just from the position.By the time a constructor needs to take five or six parameters, I often find myselfrelying on IntelliSense more than I want to. Using the property names is a great boonto readability in such cases.This form of object initializer is the one you’ll probably use most often. However,there are two other forms—one for setting subproperties, and one for adding to collections.Let’s look at subproperties—properties of properties—first.2In fact, the variable’s new value isn’t assigned until all the properties have been set. A temporary local variableis used until then. This is very rarely noticeable, though, and where it is the code should probably be morestraightforward anyway.Licensed to Rhona Hadida