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Wednesday 27 August 2014

Simplifying method parameter checking

Next version of C# that will be coming with Visual Studio (supposedly version 2014) will have a new operator called the null-propagating operator (?.) allowing us to write null conditional statement expressions. This will help us simplify our code tremendously by collapsing several lines of conditional blocks to a single statement. Combining it with null-coalescing operator makes it even more powerful as seen in this example: string username = GetUser()?.name ?? "anonymous"; Object will be null checked before trying to access its members so we'll avoid the infamous null reference exception. It will therefore automatically continue to null coalescing part of the statement. You can read more about it on this link.

Parameter null value checking code is something we frequently write at the beginning of method body to avoid null reference runtime exceptions. You know these if statements right at the beginning of your methods:

   1:  public string DoSomething(Model data)
   2:  {
   3:      if (data == null)
   4:      {
   5:          throw new ArgumentNullException("data");
   6:      }
   8:      // actual processing
   9:  }

Even static code analysis tools complain if you don't do these checks and to be honest this is a very good practice to avoid many future bugs in your code. The problem isn't this code per se, but rather that we're writing these seemingly same lines over and over again. Many developers tried simplifying this process using different techniques but the one I'm going to show here is a bit different.

What actually seems to be wrong

I'm not going to delve too much into upper code but there are immediately two problems that pop into my eyes:

  • repetitive code writing and
  • magic strings
First one is problematic because it takes some time type all this code and second one is problematic because you may easily mistype these strings. Not too much of a problem but your exceptions would refer to invalid parameter names.

Avoiding magic strings - the simple way

There are other elegant ways of null auto-checking techniques of which I like the one written by who else than Jon Skeet but mine isn't so sophisticated and also avoids mistyping magic strings by automating their generation.

I'm using development tool's capabilities to do this. Namely Visual Studio code snippets. So instead of manually typing all those if statements you can just type in checknull press TAB and Visual Studio will replace that with a complete if statement and put your editing caret in place of parameter name in the if condition. Intellisense will then assist you to get the name right and by pressing the ENTER key Visual Studio will also automatically populate magic string in exception constructor. Voila. As simple as that. So instead of writing all that code, you can just write two things:

  1. checknull snippet word and
  2. method parameter name
And that's it. Nothing else.

Do we always check parameters for null?

Looking at my own code there are two most frequent parameter checks:

  • null checking
  • string null and empty string checking
Therefore I'm providing here two code snippets for each of them. One being activated by checknull and the other by checkstring.

Get snippets

You have to save snippets in the Visual Studio snippets folder. Following folder path is the default. Adjust version accordingly to yours. %HomePath%\Documents\Visual Studio {version}\Code Snippets

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