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11.4.2Database boolean type

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”;
}
}

11.4.2Database 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;
}