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Using a Property
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Using a property is just like using a field. To get the value, you use the property name, and to assign a new value, you use the property name and the assignment operator (=). The following code demonstrates using the property and public field defined in Listing 8-2: class Listing 02 Test { static void Main(string[] args) { // create a new instance of the Product class Product prod = new Product(); // set the value of the fields prod.ItemsInStock = -20; prod.PricePerItem = 5.23; // get the total value of the products in stock Console.WriteLine("Total stock value: {0}", prod.GetTotalValueOfStock()); // wait for input before exiting Console.WriteLine("Press enter to finish"); Console.ReadLine(); } }
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CHAPTER 8 PROPERTIES, INDEXERS, AND OPERATORS
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If we compile and run the previous statements and the class in Listing 8-2, we get the following results: Unhandled Exception: System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values. at Product.set ItemsInStock(Int32 value) ... Press any key to continue . . .
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Creating an Automatically Implemented Property
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The previous example included code statements that validated the value in the set accessor. Often, though you will just want your property to mediate access to a field because it is good practice. It means you can add features such as validation or transformation later without having to update the classes that call your properties. The property in Listing 8-3 demonstrates simply exposing a private field. Listing 8-3. A Field Mediation Property class Product { private int itemsInStock; public int ItemsInStock { get { return itemsInStock; } set { itemsInStock = value; } } } You can see what s happening here; the private field itemsInStock is mediated by the public property ItemsInStock. The get accessor returns the field value, and the set access assigns the provided value to the field. C# has a nice simplification of this pattern, which is called an automatically implemented property; Listing 8-4 contains an example. Listing 8-4. Using an Automatically Implemented Property class Product { public int ItemsInStock { get; set; } } There are no bodies for the accessors in an automatically implemented property. You just use the get and set keywords, followed by a semicolon. Most importantly, there is no field either. Automatically implemented properties allow you to reduce the clutter in your code if you are just using a property to get and set the value of a field. When you compile your class, the C# system creates a variable that is of the same type as the property and generates accessors just like the ones in Listing 8-3. We don t have any access to the variable or even know its name. We can only use the property to get and set its value.
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CHAPTER 8 PROPERTIES, INDEXERS, AND OPERATORS
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You don t have to implement both accessors in a property. If you omit the set accessor, you create a read-only property, and if you omit the get accessor, you create a write-only property. Listing 8-5 contains examples of both kinds. Listing 8-5. Read-Only and Write-Only Properties class Product { private int unitsInStock; private double pricePerItem; public int ItemsInStock { get { return unitsInStock; } } public double PricePerItem { set { pricePerItem = value; } } } The ItemsInStock property is read-only, meaning that the value of the private field cannot be set outside of the class. The PricePerItem property is write-only, meaning that the value of the pricePerItem field can be set, but not read, from outside the class; write-only properties are not that frequently used.
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Creating a Computed Property
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Properties are very flexible and need not be used just to mediate access to fields. They can also be used to access values that are computed on the fly. Listing 8-6 contains an example. Listing 8-6. Using a Computed Property class Product { public int ItemsInStock { get; set; } public double PricePerItem { get; set; } public double TotalValueOfStock { get { return ItemsInStock * PricePerItem; } } } In this listing, there are two automatically implemented properties, ItemsInStock and PricePerItem, and a read-only computed property called TotalValueOfStock, which uses the other two properties to return a result. When the get accessor for this property is used, the values of the other two properties are multiplied together. There is no field backing this property. The result is generated dynamically each time the get accessor is used. Using a computed property blurs the distinction between a method and property; where you draw the line is a matter of personal style. I tend to use computed properties only if
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