ReSharper: Using [NotNull] to generate better code

My all-time-favorite VS plugin, ReSharper, has amongst all the features one which gets hardly noticed: The Annotation Framework and it’s influence on code generation.

The Annotation Framework is a set of “hints” which can be applied to your code, either directly in form of Attributes or stored as an an external XML file (the ReSharper team was using the later method to enhance the .Net Framework libraries). The attributes, which I usually use, are NotNull and CanBeNull. There are some more but those two give you the greatest benefit.

To use them, add at first a reference to the JetBrains.Annotations.dll to your project. It can be found in the ReSharper installation folder (better add a copy to your project and SCM though).Then you can already start annotating your code, such as:

public class Person
{
  [NotNull]
  public string Name { get; set; }
}

Pretty self-explaing, isn’t it? When trying to assign then null to the person’s name, ReSharper will give you a nice little “Possible null assignment to …” warning. Please keep in mind that this is just a warning – nothing prevents you from compiling and running that code.

However, the coolest thing about it is the impact on code generation. For example, I usually let ReSharper generate my constructor code (Alt-Ins, Generate a constructor which will also initialize my properties). In case of our Name attribute, ReSharper detects the NotNull annotation and automatically adds the required null-parameter check!

public class Person
{
  public Person ([NotNull] string name)
  {
    if (name == null) throw new ArgumentNullException ("name");
    Name = name;
  }

  [NotNull]
  public string Name { get; set; }
}

Now that’s what I call convenience.