Jon Jagger
jon@jaggersoft.com
Table of Contents 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 Notes DownloadECMA-334 C# Language Specificationpreviousnextprevious at this levelnext at this level 18 Structsprevious at this levelnext at this level 18.4 Struct examplesprevious at this levelnext at this level 18.4.2 Database boolean typeThe 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;  
   }  
}  
End of informative text.
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