82: Initializing a struct Field within a Declaration, Resulting in an Error in C#.NET

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Listing 82: Initializing a struct Field within a Declaration, Resulting in an Error
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struct Angle { // // ERROR: Fields cannot be initialized at declaration time // int _Hours = 42; // }
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This does not eliminate the need to initialize the field In fact, just as with classes, the compiler will issue a warning on struct fields that remain uninitialized after instantiation if they also cannot be accessed from outside the assembly (because they are decorated with internal or private modifiers, for example) Fortunately, C# supports constructors with parameters and they come with an interesting initialization requirement They must initialize all fields within the struct Failure to do so causes a compile error The constructor in Listing 83 that initializes the property rather than the field, for example, produces a compile error
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Structs
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Listing 83: Accessing Properties before Initializing All Fields
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// // // // // // // // The 'this' object cannot be used before all of its fields are assigned to public Angle(int hours, int minutes, int seconds) { Hours = hours; // Shorthand for thisHours = hours; Minutes = minutes // Shorthand for thisMinutes = ; Seconds = seconds // Shorthand for thisSeconds = ; } ERROR:
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The error reports that methods and properties (Hours implies thisHours) are accessed prior to the initialization of all fields To resolve the issue, you need to initialize the fields directly, as demonstrated in Listing 81
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ADVANCED TOPIC Using new with Value Types Invoking the new operator on a reference type compiles to the CIL instruction newobj new is available to value types as well, but in contrast, the underlying CIL instruction is initobj This instruction initializes the memory with default values (the equivalent of assigning default(<type>) in C# 20)
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Unlike classes, structs do not support finalizers For local variable value types, memory is allocated on the stack, so there is no need for the garbage collector to handle the value type s cleanup and no finalizer is called before the stack is unwound For value types that are part of a reference type, the data is stored on the heap and memory is cleaned up as part of the reference object s garbage collection
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Language Contrast: C++ struct Defines Type with Public Members
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In C++, the difference between structs and classes is simply that by default, a struct s members are public C# doesn t include this subtle distinction The contrast is far greater in C#, where struct significantly changes the memory behavior from that of a class
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8: Value Types
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Using the default Operator To provide a constructor that didn t require _Seconds would not avoid the requirement that _Seconds still required initialization You can assign the default value of _Seconds using 0 explicitly or, in C# 20, using the default operator Listing 84 passes the default value into the Angle constructor that includes _Seconds However, the default operator can be used outside the this constructor call (_Seconds = default(int), for example) It is a way to specify the value for the default of a particular type
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Listing 84: Using the default Operator to Retrieve the Default Value of a Type
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// Use keyword struct to declare a value type struct Angle { public Angle(int hours, int minutes) : this( hours, minutes, default(int) ) { } // }
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Inheritance and Interfaces with Value Types All value types are sealed In addition, all value types derive from SystemValueType This means that the inheritance chain for structs is always from object to ValueType to the struct Value types can implement interfaces, too Many of those built into the framework implement interfaces such as IComparable and IFormattable ValueType brings with it the behavior of value types, but it does not include any additional members (all of its members override object s virtual members) However, as with classes, you can override the virtual members of SystemObject The rules for overriding are virtually the same as with reference types (see 9) However, one difference is that with value types, the default implementation for GetHashCode() is to forward the call to the first non-null field within the struct Also, Equals()
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