11.3.9Destructors

A struct is not permitted to declare a destructor.

11.3.10Static constructors

Static constructors for structs follow most of the same rules as for classes. The execution of a static constructor for a struct type is triggered by the first of the following events to occur within an application domain:

• A static member of the struct type is referenced.

• An explicitly declared constructor of the struct type is called.

The creation of default values (§11.3.4) of struct types does not trigger the static constructor. (An example of this is the initial value of elements in an array.)

11.4Struct examples

The following shows two significant examples of using struct types to create types that can be used similarly to the built-in types of the language, but with modified semantics.

11.4.1Database integer type

The DBInt struct below implements an integer type that can represent the complete set of values of the int type, plus an additional state that indicates an unknown value. A type with these characteristics is commonly used in databases.

using System;

public struct DBInt { // The Null member represents an unknown DBInt value.

public static readonly DBInt Null = new DBInt();

// When the defined field is true, this DBInt represents a known value // which is stored in the value field. When the defined field is false, // this DBInt represents an unknown value, and the value field is 0.

int value; bool defined;

// Private instance constructor. Creates a DBInt with a known value.

DBInt(int value) { this.value = value; this.defined = true; }

// The IsNull property is true if this DBInt represents an unknown value.

public bool IsNull { get { return !defined; } }

// The Value property is the known value of this DBInt, or 0 if this // DBInt represents an unknown value.

public int Value { get { return value; } }

// Implicit conversion from int to DBInt.

public static implicit operator DBInt(int x) { return new DBInt(x); }

// Explicit conversion from DBInt to int. Throws an exception if the // given DBInt represents an unknown value.

public static explicit operator int(DBInt x) { if (!x.defined) throw new InvalidOperationException(); return x.value; }

public static DBInt operator +(DBInt x) { return x; }

public static DBInt operator -(DBInt x) { return x.defined ? -x.value : Null; }

public static DBInt operator +(DBInt x, DBInt y) { return x.defined && y.defined? x.value + y.value: Null; }

public static DBInt operator -(DBInt x, DBInt y) { return x.defined && y.defined? x.value - y.value: Null; }

public static DBInt operator *(DBInt x, DBInt y) { return x.defined && y.defined? x.value * y.value: Null; }

public static DBInt operator /(DBInt x, DBInt y) { return x.defined && y.defined? x.value / y.value: Null; }

public static DBInt operator %(DBInt x, DBInt y) { return x.defined && y.defined? x.value % y.value: Null; }

public static DBBool operator ==(DBInt x, DBInt y) { return x.defined && y.defined? x.value == y.value: DBBool.Null; }

public static DBBool operator !=(DBInt x, DBInt y) { return x.defined && y.defined? x.value != y.value: DBBool.Null; }

public static DBBool operator >(DBInt x, DBInt y) { return x.defined && y.defined? x.value > y.value: DBBool.Null; }

public static DBBool operator <(DBInt x, DBInt y) { return x.defined && y.defined? x.value < y.value: DBBool.Null; }

public static DBBool operator >=(DBInt x, DBInt y) { return x.defined && y.defined? x.value >= y.value: DBBool.Null; }

public static DBBool operator <=(DBInt x, DBInt y) { return x.defined && y.defined? x.value <= y.value: DBBool.Null; }

public override bool Equals(object obj) { if (!(obj is DBInt)) return false; DBInt x = (DBInt)obj; return value == x.value && defined == x.defined; }

public override int GetHashCode() { return value; }

public override string ToString() { return defined? value.ToString(): “DBInt.Null”; } }