11.4.2Логический тип базы данных

Представленная ниже структура DBBool реализует трехзначный логический тип. Возможными значениями этого типа являются значения DBBool.True, DBBool.False и DBBool.Null, где член Null указывает неизвестное значение. Подобные трехзначные логические типы часто используются в базах данных.

using System;

public struct DBBool { // The three possible DBBool values.

public static readonly DBBool Null = new DBBool(0); public static readonly DBBool False = new DBBool(-1); public static readonly DBBool True = new DBBool(1);

// Private field that stores –1, 0, 1 for False, Null, True.

sbyte value;

// Private instance constructor. The value parameter must be –1, 0, or 1.

DBBool(int value) { this.value = (sbyte)value; }

// Properties to examine the value of a DBBool. Return true if this // DBBool has the given value, false otherwise.

public bool IsNull { get { return value == 0; } }

public bool IsFalse { get { return value < 0; } }

public bool IsTrue { get { return value > 0; } }

// Implicit conversion from bool to DBBool. Maps true to DBBool.True and // false to DBBool.False.

public static implicit operator DBBool(bool x) { return x? True: False; }

// Explicit conversion from DBBool to bool. Throws an exception if the // given DBBool is Null, otherwise returns true or false.

public static explicit operator bool(DBBool x) { if (x.value == 0) throw new InvalidOperationException(); return x.value > 0; }

// Equality operator. Returns Null if either operand is Null, otherwise // returns True or False.

public static DBBool operator ==(DBBool x, DBBool y) { if (x.value == 0 || y.value == 0) return Null; return x.value == y.value? True: False; }

// Inequality operator. Returns Null if either operand is Null, otherwise // returns True or False.

public static DBBool operator !=(DBBool x, DBBool y) { if (x.value == 0 || y.value == 0) return Null; return x.value != y.value? True: False; }

// Logical negation operator. Returns True if the operand is False, Null // if the operand is Null, or False if the operand is True.

public static DBBool operator !(DBBool x) { return new DBBool(-x.value); }

// Logical AND operator. Returns False if either operand is False, // otherwise Null if either operand is Null, otherwise True.

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

// Logical OR operator. Returns True if either operand is True, otherwise // Null if either operand is Null, otherwise False.

public static DBBool operator |(DBBool x, DBBool y) { return new DBBool(x.value > y.value? x.value: y.value); }

// Definitely true operator. Returns true if the operand is True, false // otherwise.

public static bool operator true(DBBool x) { return x.value > 0; }

// Definitely false operator. Returns true if the operand is False, false // otherwise.

public static bool operator false(DBBool x) { return x.value < 0; }

public override bool Equals(object obj) { if (!(obj is DBBool)) return false; return value == ((DBBool)obj).value; }

public override int GetHashCode() { return value; }

public override string ToString() { if (value > 0) return "DBBool.True"; if (value < 0) return "DBBool.False"; return "DBBool.Null"; } }