Properties in C#

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{ // Validate LastName assignment if(value == null) { // Report error throw new ArgumentNullException (); } else { // Remove any whitespace around // the new last name value = valueTrim(); if(value == "") { throw new ArgumentException ( "LastName cannot be blank");4 } else _LastName = value; } } } private string _LastName; // }
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With this new implementation, the code throws an exception if LastName is assigned an invalid value, either from another member of the same class or via a direct assignment to LastName from inside ProgramMain() The ability to intercept an assignment and validate the parameters by providing a field-like API is one of the advantages of properties4 It is a good practice to only access a property-backing field from inside the property implementation In other words, always use the property, rather than calling the field directly In many cases, this is true even from code within the same class as the property If following this practice, when code such as validation code is added, the entire class immediately takes advantage of it (As described later in the chapter, one exception to this occurs when the field is marked as read-only because then the value cannot be set once class instantiation completes, even in a property setter) Although rare, it is possible to assign a value inside the setter, as Listing 519 does In this case, the call to valueTrim() removes any whitespace surrounding the new last name value
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4 Apologies to Teller, Cher, Sting, Madonna, Bono, Prince, and Liberace, and so on
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5: Classes
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Read-Only and Write-Only Properties By removing either the getter or the setter portion of a property, you can change a property s accessibility Properties with only a setter are writeonly, which is a relatively rare occurrence Similarly, providing only a getter will cause the property to be read-only; any attempts to assign a value will cause a compile error To make Id read-only, for example, you would code it as shown in Listing 520
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Listing 520: Defining a Read-Only Property
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class Program { static void Main() { Employee employee1 = new Employee(); employee1Initialize(42); // ERROR: Property or indexer 'EmployeeId' // cannot be assigned to -- it is read-only employee1Id = "490"; } } class Employee { public void Initialize(int id) { // Use field because Id property has no setter, // it is read-only _Id = idToString(); } // // Id property declaration public string Id { get { return _Id; } // No setter provided } private string _Id; }
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Properties
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Listing 520 assigns the field from within the Employee constructor rather than the property (_Id = id) Assigning via the property causes a compile error, as it does in ProgramMain()
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Access Modifiers on Getters and Setters As previously mentioned, it is a good practice not to access fields from outside their properties because doing so circumvents any validation or additional logic that may be inserted Unfortunately, C# 10 did not allow different levels of encapsulation between the getter and setter portions of a property It was not possible, therefore, to create a public getter and a private setter so that external classes would have read-only access to the property while code within the class could write to the property In C# 20, support was added for placing an access modifier on either the get or the set portion of the property implementation (not on both), thereby overriding the access modifier specified on the property declaration Listing 521 demonstrates how to do this
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Listing 521: Placing Access Modifiers on the Setter
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class Program { static void Main() { Employee employee1 = new Employee(); employee1Initialize(42); // ERROR: The property or indexer 'EmployeeId' // cannot be used in this context because the set // accessor is inaccessible employee1Id = "490"; } }
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class Employee { public void Initialize(int id) { // Set Id property Id = idToString(); } //
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