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Initializing with a Constructor
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A constructor is a special method which allows you to perform some setup when you create an instance of a class. Just like any other method, you can provide it with parameters, but it doesn t have an explicit return value. Constructors always have the same name as their containing class. Example 3-6 adds a constructor that takes the plane s identifier. Because the constructor is a member of the class, it s allowed to use the Identifier property s private setter.
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class Plane { public Plane(string newIdentifier) { Identifier = newIdentifier; } public string Identifier { get; private set; }
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Notice how the constructor looks like a standard method declaration, except that since there s no need for a return type specifier, we leave that out. We don t even write void, like we would for a normal method that returns nothing. And it would be weird if we did; in a sense this does return something the newly created Plane it just does so implicitly. What sort of work should you do in a constructor Opinion is divided on the subject should you do everything required to make the object ready to use, or the minimum necessary to make it safe The truth is that it is a judgment call there are no hard and fast rules. Developers tend to think of a constructor as being a relatively low-cost operation, so enormous amounts of heavy lifting (opening files, reading data) might be a bad idea. Getting the object into a fit state for use is a good objective, though, because requiring other functions to be called before the object is fully operational tends to lead to bugs. We need to update our Main function to use this new constructor and to get rid of the line of code that was setting the property, as Example 3-7 shows.
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static void Main(string[] args) { Plane someBoeing777 = new Plane("BA0049"); Console.WriteLine( "Your plane has identifier {0}", someBoeing777.Identifier); } Console.ReadKey();
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Notice how we pass the argument to the constructor inside the parentheses, in much the same way that we pass arguments in a normal method call. If you compile and run that, you ll see the same output as before but now we have an identifier that can t be changed by users of the object.
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Be very careful when you talk about properties that can t be changed because they have a private setter. Even if you can t set a property, you may still be able to modify the state of the object referred to by that property. The built-in string type happens to be immune to that because it is immutable (i.e., it can t be changed once it has been created), so making the setter on a string property private does actually prevent clients from changing the property, but most types aren t like that.
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Speaking of properties that might need to change, our specification requires us to know the speed at which each plane is traveling. Sadly, our specification didn t mention the units in which we were expected to express that speed. Let s assume it is miles per hour,
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and add a suitable property. We ll use the floating-point double data type for this. Example 3-8 shows the code to add to Plane.
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public double SpeedInMilesPerHour { get; set; }
If we were to review this design with the customer, they might point out that while they have some systems that do indeed want the speed in miles per hour the people they liaise with in European air traffic control want the speed in kilometers per hour. To avoid confusion, we will add another property so that they can get or set the speed in the units with which they are familiar. Example 3-9 shows a suitable property.
public double SpeedInKilometersPerHour { get { return SpeedInMilesPerHour * 1.609344; } set { SpeedInMilesPerHour = value / 1.609344; } }
We ve done something different here rather than just writing get; and set; we ve provided code for these accessors. This is another reason we have to declare the accessors explicitly the C# compiler needs to know whether we want to write a custom property implementation. We don t want to use an ordinary property in Example 3-9, because our SpeedInKilo metersPerHour is not really a property in its own right it s an alternative representation for the information stored in the SpeedInMilesPerHour property. If we used the normal property syntax for both, it would be possible to set the speed as being both 100 mph and 400 km/h, which would clearly be inconsistent. So instead we ve chosen to implement SpeedInKilometersPerHour as a wrapper around the SpeedInMilesPerHour property. If you look at the getter, you ll see that it returns a value of type double. It is equivalent to a function with this signature:
public double get_SpeedInKilometersPerHour()
The setter seems to provide an invisible parameter called value, which is also of type double. So it is equivalent to a method with this signature:
public void set_SpeedInKilometersPerHour(double value)
This value parameter is a contextual keyword C# only considers it to be a keyword in property or event accessors. (Events are described in 5.) This means you re allowed to use value as an identifier in other contexts for example, you can write a method that takes a parameter called value. You can t do that with other keywords you can t have a parameter called class, for example.
This is a very flexible system indeed. You can provide properties that provide real storage in the class to store their data, or calculated properties that use any mechanism you like to get and/or set the values concerned. This choice is an implementation detail hidden from users of our class we can switch between one and the other without changing our class s public face. For example, we could switch the implementation of these speed properties around so that we stored the value in kilometers per hour, and calculated the miles per hour Example 3-10 shows how these two properties would look if the real value was in km/h.
public double SpeedInMilesPerHour { get { return SpeedInKilometersPerHour / 1.609344; } set { SpeedInKilometersPerHour = value * 1.609344; } } public double SpeedInKilometersPerHour { get; set; }
As far as users of the Plane class are concerned, there s no discernible difference between the two approaches the way in which properties work is an encapsulated implementation detail. Example 3-11 shows an updated Main function that uses the new properties. It neither knows nor cares which one is the real one.
static void Main(string[] args) { Plane someBoeing777 = new Plane("BA0049"); someBoeing777.SpeedInMilesPerHour = 150.0; Console.WriteLine( "Your plane has identifier {0}, " + "and is traveling at {1:0.00}mph [{2:0.00}kph]", someBoeing777.Identifier, someBoeing777.SpeedInMilesPerHour, someBoeing777.SpeedInKilometersPerHour); someBoeing777.SpeedInKilometersPerHour = 140.0; Console.WriteLine( "Your plane has identifier {0}, " + "and is traveling at {1:0.00}mph [{2:0.00}kph]", someBoeing777.Identifier, someBoeing777.SpeedInMilesPerHour, someBoeing777.SpeedInKilometersPerHour); } Console.ReadKey();
Although our public API supports two different units for speed while successfully keeping the implementation for that private, there s something unsatisfactory about that implementation. Our conversion relies on a magic number (1.609344) that appears repeatedly. Repetition impedes readability, and is prone to typos (I know that for a fact. I ve typed it incorrectly once already this morning while preparing the example!) There s an important principle in programming: don t repeat yourself (or dry, as it s sometimes abbreviated). Your code should aim to express any single fact or concept no more than once, because that way, you only need to get it right once. It would be much better to put this conversion factor in one place, give it a name, and refer to it by that instead. We can do that by declaring a field.
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