Order of execution in an if statement

Question

Does the C# language definition guarantee that in the line

if ( func_one() && func_two() )

1) func_one() will be executed first
2) func_two() will only be executed if and only if func_one() returns true

My C# book evades the issue.

Answer 1

Yes C# does short circuit evaluation of boolean expressions.  Therefore in any boolean expression of type x op y, x is evaluated first and y is evaluated only if it is meaningful to do so.  In the case of an AND operation if x is false then y is not evaluated.  For OR if x is true then y is not evaluated. 

 

This applies to all boolean expressions and not just those in an IF statement.  This is covered under section 14.11.1 of the C# Specification available online at MSDN.

 

Michael Taylor - 12/21/07

http://p3net.mvps.org

 

 

Answer 2

yes that's rigth this brief example can prove it.

Code Snippet

 

class Program

{

public static bool method1(bool valor)

{

Console.WriteLine("Method value {0}", valor.ToString());

return valor;

}

static void Main(string[] args)

{

if(method1(false) && method1(true))

Console.WriteLine("OK");

 

Console.WriteLine("---------");

 

if (method1(true) && method1(false))

Console.WriteLine("OK");

 

Console.Read();

}

}

 

 

 

try to compile in visual studio, csc, or mono compiler and the result should be exactly the same.

Answer 3

Thank you both.

I realise that it does evaluate them in order, but I wanted this future-proofed and portable. I had a nasty experience with a language called Powerbuilder that did not guarantee the order - at least not then.

The expression I needed was 'short-circuit'. I have now found the section in Hejlsberg, Wiltamuth, Gold - the ultimate bathroom reader.

The forums today are sufficiently slow that I can get a decent amount of work donw while waiting for something to post. (Or is it just me?)

Answer 4

If you key VS2005 help Index for && symbol the explanation will be explicitly as to what you outlined in the first post.