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|| Operator (C# Reference)

The conditional-OR operator (||) performs a logical-OR of its bool operands. If the first operand evaluates to true, the second operand isn't evaluated. If the first operand evaluates to false, the second operator determines whether the OR expression as a whole evaluates to true or false.

The operation

x || y

corresponds to the operation

x | y

except that if x is true, y is not evaluated because the OR operation is true regardless of the value of y. This concept is known as "short-circuit" evaluation.

The conditional-OR operator cannot be overloaded, but overloads of the regular logical operators and the true and false operators are, with certain restrictions, also considered to be overloads of the conditional logical operators.

In the following examples, the expression that uses || evaluates only the first operand. The expression that uses | evaluates both operands. In the second example, a run-time exception occurs if both operands are evaluated.

class ConditionalOr
    // Method1 returns true.
    static bool Method1()
        Console.WriteLine("Method1 called.");
        return true;

    // Method2 returns false.
    static bool Method2()
        Console.WriteLine("Method2 called.");
        return false;

    static bool Divisible(int number, int divisor)
        // If the OR expression uses ||, the division is not attempted
        // when the divisor equals 0.
        return !(divisor == 0 || number % divisor != 0);

        // If the OR expression uses |, the division is attempted when
        // the divisor equals 0, and causes a divide-by-zero exception.
        // Replace the return statement with the following line to
        // see the exception.
        //return !(divisor == 0 | number % divisor != 0);

    static void Main()
        // Example #1 uses Method1 and Method2 to demonstrate 
        // short-circuit evaluation.

        Console.WriteLine("Regular OR:");
        // The | operator evaluates both operands, even though after 
        // Method1 returns true, you know that the OR expression is
        // true.
        Console.WriteLine("Result is {0}.\n", Method1() | Method2());

        Console.WriteLine("Short-circuit OR:");
        // Method2 is not called, because Method1 returns true.
        Console.WriteLine("Result is {0}.\n", Method1() || Method2());

        // In Example #2, method Divisible returns True if the
        // first argument is evenly divisible by the second, and False
        // otherwise. Using the | operator instead of the || operator
        // causes a divide-by-zero exception.

        // The following line displays True, because 42 is evenly 
        // divisible by 7.
        Console.WriteLine("Divisible returns {0}.", Divisible(42, 7));

        // The following line displays False, because 42 is not evenly
        // divisible by 5.
        Console.WriteLine("Divisible returns {0}.", Divisible(42, 5));

        // The following line displays False when method Divisible 
        // uses ||, because you cannot divide by 0.
        // If method Divisible uses | instead of ||, this line
        // causes an exception.
        Console.WriteLine("Divisible returns {0}.", Divisible(42, 0));
Regular OR:
Method1 called.
Method2 called.
Result is True.

Short-circuit OR:
Method1 called.
Result is True.

Divisible returns True.
Divisible returns False.
Divisible returns False.

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