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Properties provide a means to manipulate object state but are not stateful mechanisms themselves; something else, often a field member, is the target of the manipulation. The property declaration imposes no requirements on the containing type as to what the stateful item is, or is called. Property declarations have the following syntax: [attributes] [modifiers] type identifier { accessor-declaration }
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5. Data Types
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The property type can be any value or reference type and specifies both the data type that can be assigned to the property and the type returned when the property itself is assigned to something else. Here's a Java implementation of the getter/setter pattern for the Person class discussed previously:
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public class Person { private int thePersonsAge; public void setAge(int p_age) { // Do some validation thePersonsAge = p_age; } public int getAge() { return thePersonsAge; } }
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The equivalent C# implementation using properties is
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public class Person { private int thePersonsAge; public int Age { get { return thePersonsAge; } set { thePersonsAge = value; } }
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The get and set accessors in this example are simplistic and provide no additional benefit compared with making the thePersonsAge field public; however, as function members, properties can contain any executable code. The undeclared variable value used in the set accessor is implicitly provided by the compiler, containing the value the caller has assigned to the property. The type is the same as that of the property declaration, in this example an int. It's possible to implement only the get or the set accessor, thus providing a read-only or writeonly property. However, if the intention is to use the prefix or postfix increment (++) or decrement (--) operator, or any of the compound assignment operators (+=, -=, and so on), both the get and set accessors must be declared. Attempting any action on a property that requires an accessor that has not been implemented will result in a compiler error being raised.
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Modifiers
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The modifiers applicable to properties depend on the context of their declaration. Table 5-17 summarizes the available modifiers for each context.
Table 5-17. Property Declaration Modifier Availability
Property Declaration Context Member of Class Accessibility public protected private internal protected internal Inheritance new abstract sealed virtual override Other N/A readonly N/A volatile static extern When the extern modifier is used, the get and set accessor bodies should be empty statements.
Properties as Members of Interfaces
Member of Member Struct Interface (implicit) N/A N/A N/A N/A
N/A N/A N/A N/A N/A N/A N/A N/A
N/A N/A N/A N/A N/A N/A N/A N/A
A property can be a member of an interface. The structure of the property declaration is the same as discussed earlier in this section; however, the body of the get and set accessors is an empty statement. If only one of the accessors is required, specify this in the interface. For example:
public interface IPerson { int Age {get; set;} string PayrollNumber {get;} }
// No set accessor required.
5. Data Types
Indexers
Sometimes it makes sense to access a class or struct by using index syntax, similar to that used with arrays. This is particularly appropriate if the class contains some collection of related information. C# provides the indexer member type for achieving this functionality. Instead of making field members directly accessible or defining a series of methods for manipulating the underlying data, indexers provide indirect access to state using the familiar array-style index. Like properties, the indexer provides indirect access to the data via definable get or set methods. The index can be any value or reference type. The .NET class libraries make extensive use of indexers to provide access to collections. See 9 for complete details.
Declaration
Indexers are similar to properties in that they provide a means to manipulate object state but are not stateful mechanisms themselves. As with properties, the containing class or struct must implement some other program element as the target of the manipulation. The indexer imposes no requirements as to how this is done. Index declarations have the following syntax: [attributes] [modifiers] type this [ parameters ] { accessor-declarations } Use of an indexer is never performed explicitly through a member name, so indexers do not have identifiers. As a consequence of an unfortunate syntax choice, indexers are declared using the keyword this. The indexer type can be any value or reference type and specifies both the data type that can be assigned to an indexer element and the type of the indexer element returned when assigned to something else. At least one parameter must be specified in the indexer declaration. Multiple parameters result in a multidimensional index, as in the case of a multidimensional array. For example:
public string this [int index1, byte index2, string index3] { }
Parameters can be of any value or reference type, but it is not valid to use the ref or out modifier. Multiple indexers can be defined for a single class. Given that all indexers are declared using the this keyword, additional indexers must be differentiated with different parameter types. The following example demonstrates the declaration of an indexer:
5. Data Types public class TopTenArtists { private string[] artists = new string[10]; public string this [int index] { get { if (index > 0 && index < 11) { return artists[index-1]; } else { return null; } } set { if (index > 0 && index < 11) { artists[index-1] = value; } } } }
The undeclared variable value used in the set accessor is implicitly provided by the compiler. It contains the value the caller is assigning to the property. Its type is the same as that of the property declaration in this instance, an int. The following code demonstrates how the indexer of the TopTenArtists class can be used:
public static void Main() { TopTenArtists artists = new TopTenArtists(); artists[1] = "Rubens"; artists[2] = "Gainsborough"; artists[3] = "Yevseeva"; for (int x = 1; x < 11; x++) { System.Console.WriteLine("Artist {0} is {1}", x, artists[x]); }
This example produces the following output:
Artist Artist Artist Artist Artist Artist Artist Artist Artist Artist 1 is Rubens 2 is Gainsborough 3 is Yevseeva 4 is 5 is 6 is 7 is 8 is 9 is 10 is
As was the case with properties, indexers can implement only the get or set accessor; this provides a read-only or write-only member. As with properties, if the intention is to use the
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