C# Language Specification

Database boolean type

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.

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";
}
}

12.