What does it mean when there are two pipes (||) next to each other?

Question

An example of it in use would be...

if (temp < 30 || temp > 90) { MessageBox.Show("Error"); }

What do two pipes (||) together mean?

Answer 1

One pipe is a logical OR operator which always evaluates both operands.

Two pipes is short-circuiting logical OR operator, which only evaluates the second operand if the first one is false. This is especially useful if the second operand is a heavy function that you don't want to evaluate unnecessarily or it is something that can throw an exception, e.g.:

if(myList == null || myList.Count == 0){
    //do something
}

In this example, if myList is null, the second operand is never evaluated. If we use one pipe instead, the second operand will be evaluated and will throw an exception because myList is null.

Answer 2

A pipe | in C# (and many other languages [except when a single pipe is used as a bitwise logical operator, such as in Java]) is the logical operator OR.

The double pipe || is shortcut OR. It means that if the first is true, then the operation will automatically quit, because one condition is already true. Therefore, OR must be true. (A single |, therefore, means that it will check all conditions first before evaluating, which is slower and usually not useful).

In your example:

if (temp < 30 || temp > 90) { MessageBox.Show("Error"); }

• temp < 30: first condition
• ||: logical OR
• temp > 90: second condition

This means, if (the first condition) OR (the second condition) is true, then show "Error".