Suppressing CA1062 with a Helper Function

I think Visual Studio’s static analysis warning for checking parameters for null before they are used is generally a good idea, but the code to check the parameters can get really tedious.

Before addressing the tedium, though, why does it even matter whether you check for null arguments and throw a ArgumentNullException? You’ll just get a NullReferenceException if you don’t anyway. Is the ArgumentNullException somehow better?

The ArgumentNullException has a couple of benefits: It clearly indicates that the caller is to blame and by including the parameter name quickly narrows down the problem instead of requiring you to figure out which reference within the body of the method is null.

Although an ArgumentNullException is preferable, the null checking can overwhelm a short method. Here’s a contrived example:

public static void PrintThings(string[] things)
{
  if (things == null)
  {
    throw new ArgumentNullException("things");
  }

  foreach (var thing in things)
  {
    Console.WriteLine(thing);
  }
}

Half the method is dedicated to checking the parameter for null. Now imagine if a relatively short method had three parameters to check. It would be all null checks and hardly any code.

In the spirit of conciseness, I’ve frequently had some kind of helper function that does the null check:

public static void PrintThings(string[] things)
{
  Arg.NotNull("things", things);

  foreach (var thing in things)
  {
    Console.WriteLine(thing);
  }
}

The trouble is, Visual Studio’s static analysis doesn’t recognize this as a parameter validation of “things”, so you’ll get the CA1062 warning.

The work-around is to create a ValidatedNotNullAttribute and include it in the helper method, which signals to static analysis that, trust me, I really am checking the parameter. Here’s a complete example:

using System;

namespace CA1062Test
{
    public static class Program
    {
        static void Main()
        {
            PrintThings(new string[] { "one", "two" });
        }

        public static void PrintThings(string[] things)
        {
            Arg.NotNull("things", things);

            foreach (var thing in things)
            {
                Console.WriteLine(thing);
            }
        }

    }

    static class Arg
    {
        public static void NotNull(string name, [ValidatedNotNull] object value)
        {
            if (value == null)
            {
                throw new ArgumentNullException(name);
            }
        }
    }

    sealed class ValidatedNotNullAttribute : Attribute
    {
    }
}

Notice the trivial ValidatedNotNullAttribute implementation — it doesn’t matter what it does so long as it is named “ValidatedNotNullAttribute”. The attribute is applied to the parameter being checked in the NotNull function, so it only has to be specified once in the helper declaration, and not on each use of the helper.

4 thoughts on “Suppressing CA1062 with a Helper Function

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