true Operator (C# Reference)
Returns the bool value true to indicate that an operand is true and returns false otherwise.
Prior to C# 2.0, the true and false operators were used to create user-defined nullable value types that were compatible with types such as SqlBool. However, the language now provides built-in support for nullable value types, and whenever possible you should use those instead of overloading the true and false operators. For more information, see Nullable Types (C# Programming Guide).
With nullable Booleans, the expression a != b is not necessarily equal to !(a == b) because one or both of the values might be null. You need to overload both the true and false operators separately to correctly identify the null values in the expression. The following example shows how to overload and use the true and false operators.
// For example purposes only. Use the built-in nullable bool
// type (bool?) whenever possible.
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,
// 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,
// 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";
}
}
A type that overloads the true and false operators can be used for the controlling expression in if, do, while, and for statements and in conditional expressions.
If a type defines operator true, it must also define operator false.
A type cannot directly overload the conditional logical operators (&& and ||), but an equivalent effect can be achieved by overloading the regular logical operators and operators true and false.
C# Language Specification
For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.