Elegant check for null and exit in C#

Question

What is an elegant way of writing this?

if (lastSelection != null)
{
    lastSelection.changeColor();
}
else
{
    MessageBox.Show("No Selection Made");
    return;
}

changeColor() is a void function and the function that is running the above code is a void function as well.

Answer 1

Although I personally really dislike having multiple return statements in a function, according to the defensive programming I've seen, you'd trap the error condition and exit, and let anything else through:

if(lastSelection == null)
{
    MessageBox.Show("No Selection Made");
    return;
}

lastSelection.changeColor();

It's hard to say how I'd do this in my own work without seeing the entire function this belongs in.

Answer 2

You can reduce clutter by reversing the condition:

if (lastSelection == null)
{
    MessageBox.Show("No Selection Made");
    return;
}

lastSelection.changeColor();

Answer 3

I'd be tempted to make sure that lastSelection was never null, and have it point to an "empty" object in cases where it would be null in the current design. Then, there's no need for a null check. Engineering possible errors out of the system is always good practice.

IOW, I'd ask if the inelegance in the code is how you do the null check, or whether it's the fact that a null check is required at all.

Answer 4

I think your code is good: you checked for the normal case (lastSelection != null) first then for the abnormal case.

I may just add a single modification:

if (lastSelection != null)
{
    lastSelection.changeColor();
    //your other normal code
}
else
{
    MessageBox.Show("No Selection Made");
    //return; //no need for this now
}

Answer 5

Seeing everybody else got the good answers, DON'T do this:

if (lastSelection == null) goto ERR;
lastSelection.changeColor();
//... possibly more stuff...
return;
ERR:
MessageBox.Show("No Selection Made");

Answer 6

A practice catching up fast is to write null first as below :

if (null == lastSelection)
{
    MessageBox.Show("No Selection Made");
    return;
}

lastSelection.changeColor();

Answer 7

This is yet another way of implementing this.

        Action _action = (lastSelection != null
                              ?
                                  new Action(lastSelection.changeColor)
                              :
                                  () => Console.WriteLine("No Selection Made"));
        _action.Invoke();

Perhaps a bit overkill, but you will certainly have the people maintaining your code scratching their heads :-)