Question

A lot of times when reading source code I see something like this:

public void Foo(Bar bar)
{
    if (bar == null) return;

    bar.DoSomething();
}

I do not like this, but I appear to be in the wrong as this form of defensive programming is considered good. Is it though? For example, why is bar null to begin with? Isn’t doing checks like this akin to applying a bandage to a problem rather than solving the real solution? Not only does it complicate functions with additional lines of code but it also prevents the programmer from seeing potential bugs.

Here’s another example:

public void Foo(int x)
{
    int clientX = Math.Max(x, 0); // Ensures x is never negative
}

Others look at that and see defensive programming but I see future bugs when a programmer accidentally passes a negative value and the program suddenly breaks and no one knows why because this little bit of logic swallowed the potentially revealing exception.

Now, please do not confuse checking if user input is valid versus what I am asking here. Obviously user input should be checked. What I am asking only pertains to code that does not interact with the user or his or her input.

Answer 1

this int clientX = Math.Max(x, 0); is NOT defensive programming – it is masquerading potential problems!

Defensive programming would be

if ( x < 0 )
     throw new Exception ( "whatever" ); // or return false or...

and defensive programming is absolutely recommended… you never know how this code will be called in the future so you make sure that it handles anything appropriately i.e. things it is unable to handle must be filtered out as early as possible and the caller must be “notified” (for example by a meaningful exception)…