82 lines
3.4 KiB
XML
82 lines
3.4 KiB
XML
<?xml version="1.0"?>
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<clause number="18.4.2" title="Database boolean type" informative="true">
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<paragraph>The DBBool struct below implements a three-valued logical type. The possible values of this type are DBBool.True, DBBool.False, and DBBool.Null, where the Null member indicates an unknown value. Such three-valued logical types are commonly used in databases. <code_example><![CDATA[
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using System;
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public struct DBBool
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{
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// The three possible DBBool values.
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public static readonly DBBool Null = new DBBool(0);
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public static readonly DBBool False = new DBBool(-1);
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public static readonly DBBool True = new DBBool(1);
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// Private field that stores -1, 0, 1 for False, Null, True.
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sbyte value;
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// Private instance constructor. The value parameter must be -1, 0, or 1.
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DBBool(int value) {
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this.value = (sbyte)value;
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}
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// Properties to examine the value of a DBBool. Return true if this
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// DBBool has the given value, false otherwise.
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public bool IsNull { get { return value == 0; } }
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public bool IsFalse { get { return value < 0; } }
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public bool IsTrue { get { return value > 0; } }
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// Implicit conversion from bool to DBBool. Maps true to DBBool.True
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// and false to DBBool.False.
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public static implicit operator DBBool(bool x) {
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return x? True: False;
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}
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// Explicit conversion from DBBool to bool. Throws an exception if the
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// given DBBool is Null, otherwise returns true or false.
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public static explicit operator bool(DBBool x) {
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if (x.value == 0) throw new InvalidOperationException();
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return x.value > 0;
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}
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// Equality operator. Returns Null if either operand is Null,
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// otherwise returns True or False.
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public static DBBool operator ==(DBBool x, DBBool y) {
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if (x.value == 0 || y.value == 0) return Null;
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return x.value == y.value? True: False;
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}
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// Inequality operator. Returns Null if either operand is Null,
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// otherwise returns True or False.
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public static DBBool operator !=(DBBool x, DBBool y) {
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if (x.value == 0 || y.value == 0) return Null;
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return x.value != y.value? True: False;
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}
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// Logical negation operator. Returns True if the operand is False,
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// Null if the operand is Null, or False if the operand is True.
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public static DBBool operator !(DBBool x) {
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return new DBBool(-x.value);
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}
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// Logical AND operator. Returns False if either operand is False,
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// otherwise Null if either operand is Null, otherwise True.
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public static DBBool operator &(DBBool x, DBBool y) {
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return new DBBool(x.value < y.value? x.value: y.value);
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}
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// Logical OR operator. Returns True if either operand is True,
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// otherwise Null if either operand is Null, otherwise False.
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public static DBBool operator |(DBBool x, DBBool y) {
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return new DBBool(x.value > y.value? x.value: y.value);
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}
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// Definitely true operator. Returns true if the operand is True,
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// false otherwise.
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public static bool operator true(DBBool x) {
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return x.value > 0;
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}
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// Definitely false operator. Returns true if the operand is False,
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// false otherwise.
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public static bool operator false(DBBool x) {
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return x.value < 0;
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}
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}
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]]></code_example></paragraph>
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<paragraph>End of informative text. </paragraph>
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</clause>
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