Value Types in C#.NET

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Value types directly contain their values, as shown in Figure 81 In other words, the variable refers to the same location in memory that the value is stored Because of this, when a different variable is assigned the same value, a memory copy of the original variable's value is made to the location of the new variable A second variable of the same value type cannot refer to the same location in memory as the first variable So changing the value of the first variable will not affect the value in the second
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Figure 81 Value Types Contain the Data Directly
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Similarly, passing a value type to a method such as ConsoleWriteLine() will also result in a memory copy, and any changes to the parameter value inside the method will not affect the original value within the calling function Since value types require a memory copy, they generally should be defined to consume a small amount of memory (less than 16 bytes approximately) The amount of memory that is required for the value type is fixed at compile time and will not change at runtime This fixed size allows value types to be stored in the area of memory known as the stack
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Reference Types
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In contrast, reference types and the variables that refer to them point to the data storage location (see Figure 82) Reference types store the reference (memory address) where the data is located, instead of the data directly Therefore, to access the data, the runtime will read the memory location out of the variable and then jump to the location in memory that contains the data The memory area of the data a reference type points to is the heap
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Figure 82 Reference Types Point to the Heap
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Dereferencing a reference type to access its value involves an extra hop However, a reference type does not require the same memory copy of the data that a value type does, resulting in circumstances when reference types are more efficient When assigning one reference type variable to another reference type variable, only a memory copy of the address occurs, and as such, the memory copy required by a reference type is always the size of the address itself (A 32-bit processor will copy 32 bits and a 64-bit processor will copy 64 bits, and so on) Obviously, not copying the data would be faster than a value type's behavior if the data size is large Since reference types copy only the address of the data, two different variables can point to the same data, and changing the data through one variable will change the data for the other variable as well This happens both for assignment and for method calls Therefore, a method can affect the data of a reference type back at the caller
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Besides string and object, all the C# primitive types are value types Furthermore, numerous additional value types are provided within the framework It also is possible for developers to define their own value types that behave like user-defined primitives To define a custom value type, you use the same type of structure as you would to define classes and interfaces The key difference in syntax is simply that value types use the keyword struct, as shown in Listing 81
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Listing 81 Defining struct
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// Use keyword struct to declare a value type struct Angle { public Angle(int hours, int minutes, int seconds) { _Hours = hours; _Minutes = minutes; _Seconds = seconds; } public int Hours { get { return _Hours; } } private int _Hours; public int Minutes { get { return _Minutes; } } private int _Minutes; public int Seconds { get { return _Seconds; } } private int _Seconds; public Angle Move(int hours, int minutes, int seconds) { return new Angle( Hours + hours, Minutes + minutes Seconds + seconds) } } // Declaring a class - a reference type // (declaring it as a struct would create a value type // larger than 16 bytes) class Coordinate { public Angle Longitude { get { return _Longitude; }
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set { _Longitude = value; } } private Angle _Longitude; public Angle Latitude { get { return _Latitude; } set { _Latitude = value; } } private Angle _Latitude; }
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This listing defines Angle as a value type that stores the hours, minutes, and seconds of an angle, either longitude or latitude The resulting C# type is a struct
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