C#: How to Implement and use a NotNull and CanBeNull attribute

Question

I want to let programmers and myself know that a method does not want null and if you do send null to it anyways, the result will not be pretty.

There is a NotNullAttribute and a CanBeNullAttribute in Lokad Shared Libraries, in the Lokad.Quality namespace.

But how does that work? I looked at the source-code of those two attributes, and it looks like this:

[AttributeUsage(
 AttributeTargets.Method | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.Delegate |
  AttributeTargets.Field, AllowMultiple = false, Inherited = true)]
[NoCodeCoverage]
public sealed class NotNullAttribute : Attribute
{
}

[AttributeUsage(
 AttributeTargets.Method | AttributeTargets.Parameter | AttributeTargets.Property | AttributeTargets.Delegate |
  AttributeTargets.Field, AllowMultiple = false, Inherited = true)]
[NoCodeCoverage]
public sealed class CanBeNullAttribute : Attribute
{
}

Two empty classes inheriting from Attribute. How are they used? Do you have to look up xml-documentation and know that it is there? Cause I tried to both make my own copy of the attribute and to use the Lokad version, but when I tried to send a null directly in, I got no message. Neither from ReSharper nor from VS. Which I kind of expected actually. But how are they used? Can I somehow make VS generate warnings for me if I try to send something that is null in there? Or is it just used in some kind of testing framework? Or?

Answer 1

In the mid-term, "code contracts" (in 4.0) will be a better answer to this. They are available now (with academic or commercial licences), but will be more integrated in VS2010. This can provide both static analysis and runtime support.

(edit) example:

Contract.RequiresAlways( x != null );

Simple as that... the code contracts engine works at the IL level, so it can analyse that and throw warnings/errors from calling code during build, or at runtime. For backwards compatibility, if you have existing validation code, you can just tell it where the sanity checking ends, and it'll do the rest:

if ( x == null ) throw new ArgumentNullException("x");
Contract.EndContractBlock();

Answer 2

This can be done either with AOP, whereby an Advice verifies at run-time whether a method parameter is null and whether nulls are allowed. See PostSharp and Spring.NET for AOP.

As for ReSharper, see Annotated Framework:

We have analyzed a great share of .NET Framework Class Library, as well as NUnit Framework, and annotated it through external XML files, using a set of custom attributes from the JetBrains.Annotations namespace, specifically:

• StringFormatMethodAttribute (for methods that take format strings as parameters)
• InvokerParameterNameAttribute (for methods with string literal arguments that should match one of caller parameters)
• AssertionMethodAttribute (for assertion methods)
• AssertionConditionAttribute (for condition parameters of assertion methods)
• TerminatesProgramAttribute (for methods that terminate control flow)
• CanBeNullAttribute (for values that can be null)
• NotNullAttribute (for values that can not be null)