Join Null Check with Assignment

I recently wrote some code and asked ReSharper to add a null check for me, which it did. Then it suggested that I could simplify the null check by joining it to the assignment.

Intrigued, I let it.

The code went from this:

public void SetMessage(string message)
{
    if (message == null) throw new ArgumentNullException(nameof(message));
    Message = message;
}

To this:

public void SetMessage(string message)
{
    Message = message ?? throw new ArgumentNullException(nameof(message));
}

So, I assign message to the property Message unless it is null in which case I throw the exception. This is a new feature in C# 7 called a “throw expression”.

At first glance, I thought it would still assign null to Message before throwing the exception, but that’s not what the code looks like underneath.

I got out my trusty dotPeek to see what it actually compiled to. (Don’t worry, I’m not going to show you IL, just what the C# looks like without the syntactic sugar). The result was this:

public void SetMessage(string message)
{
  string str = message;
  if (str == null)
    throw new ArgumentNullException("message");
  this.Message = str;
}

Excellent, it is still doing the null check in advance. So the semantics of what I wrote have not changed. That’s great. I learned something new today.

But…

ReShaper also suggested it in an overloaded version of that function that takes two parameters. And the result was not semantically equivalent. So, be careful. Here’s what happened there. I started with this:

public void SetMessage(string message, string transitionMessage)
{
    if (message == null) throw new ArgumentNullException(nameof(message));
    if (transitionMessage == null) throw new ArgumentNullException(nameof(transitionMessage));

    Message = message;
    TransitionMessage = transitionMessage;
}

Let ReSharper refactor to this:

public void SetMessage(string message, string transitionMessage)
{
    Message = message ?? throw new ArgumentNullException(nameof(message));
    TransitionMessage = transitionMessage ?? throw new ArgumentNullException(nameof(transitionMessage));
}

And, I’m beginning to get a little apprehensive at this point because I think I see a problem. In fact, when I look at it in dotPeek, I can see exactly what the issue is. Here’s the same code with the syntactic sugar removed:

public void SetMessage(string message, string transitionMessage)
{
  string str1 = message;
  if (str1 == null)
    throw new ArgumentNullException("message");
  this.Message = str1;
  string str2 = transitionMessage;
  if (str2 == null)
    throw new ArgumentNullException("transitionMessage");
  this.TransitionMessage = str2;
}

It does the first null check, then assigns to the Message property. Then it does the second null check… And that’s not what I want at all. This method should be an all or nothing proposition. Either both properties are set, or neither are changed and this isn’t the case any more.

Caveat Programmator, as they say in Latin.