Jon Jagger
jon@jaggersoft.com
Table of Contents 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Notes DownloadECMA-334 C# Language Specificationpreviousnextprevious at this levelnext at this level 8 Language Overviewprevious at this levelnext at this level 8.8 StructsThe list of similarities between classes and structs is long-structs can implement interfaces, and can have the same kinds of members as classes. Structs differ from classes in several important ways, however: structs are value types rather than reference types, and inheritance is not supported for structs. Struct values are stored "on the stack" or "in-line". Careful programmers can sometimes enhance performance through judicious use of structs. For example, the use of a struct rather than a class for a Point can make a large difference in the number of memory allocations performed at run time. The program below creates and initializes an array of 100 points. With Point implemented as a class, 101 separate objects are instantiated-one for the array and one each for the 100 elements.
class Point  
{  
   public int x, y;  
   public Point(int x, int y) {  
      this.x = x;  
      this.y = y;  
   }  
}  
class Test  
{  
   static void Main() {  
      Point[] points = new Point[100];  
      for (int i = 0; i < 100; i++)  
      points[i] = new Point(i, i*i);  
   }  
}  
If Point is instead implemented as a struct, as in
struct Point  
{  
   public int x, y;  
   public Point(int x, int y) {  
      this.x = x;  
      this.y = y;  
   }  
}  
only one object is instantiated-the one for the array. The Point instances are allocated in-line within the array. This optimization can be misused. Using structs instead of classes can also make an application run slower or take up more memory, as passing a struct instance by value causes a copy of that struct to be created.
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