C# 3.0 Pocket Reference, 2nd Edition

Structs

A struct is similar to a class, with the following key differences:

• A struct is a value type, whereas a class is a reference type.

• A struct does not support inheritance (other than implicitly deriving from object).

A struct can have all the members a class can, except:

• A parameterless constructor

• A finalizer

• Virtual members

A struct is used instead of a class when value type semantics are desirable. Good examples of structs are numeric types, where it is more natural for assignment to copy a value rather than a reference. Because a struct is a value type, each instance does not require instantiation of an object on the heap. This can be important when creating many instances of a type, for example, with an array.

Struct Construction Semantics

The construction semantics of a struct are as follows:

• A parameterless constructor implicitly exists, which you can’t override. This performs a bitwise-zeroing of its fields.

• When you define a struct constructor, you must explicitly assign every field.

• You can’t have field initializers in a struct.

Here is an example of declaring and calling struct constructors:

	public struct Point
	{
	  int x, y;
	  public Point (int x, int y) {this.x = x; this.y = y;}
	}

	...
	Point p1 = new Point ();     // p1.x and p1.y will be 0
	Point p2 = new Point (1, 1); // p1.x and p1.y will be 1

The next example generates three compile-time errors:

public struct Point { int x = 1; // Illegal, cannot initialize field int y; public Point() {} // Illegal, cannot have // parameterless ...