Ensure that method return value is not discarded for library

Question

My library has some methods whose return value should never be discarded. Leaking them is a very popular mistake even for me, the author. So I want the compiler to alert programmer when it does so.

Such value may be either stored or used as an argument for another method. It's not strictly to use the stored value but if it's simply discarded it's 100% error.

Is there any easy to setup way to enforce this for my library users?

var x = instance.Method(); // ok
field = instance.Method(); // ok
instance.OtherMethod(instance.Method()); // ok
MyMethod(instance.Method()); // ok, no need to check inside MyMethod
instance.Method(); // callvirt and pop - error!

I thought about making IL analyzer for post-build event but it feels like so overcomplicated...

Answer 1

The Nicolai's answer enables ruleset for any types but I needed this check for only my library types (I don't want to force my library users to apply rule set on all their code).

Using out everywhere as suggested in the comments makes the library usage to hard.

Therefore I've chosen another approach.

1. In finalizer I check whether any method was called (it's enough for me to confirm usage). If not - InvalidOperationException. Object creation StackTrace is optionally recorded and appended to the error message.
2. User may call SetNotLeaked() to disable the check for particular object and all internal objects recursively.

This is not a compile-time check but it will surely be noticed.

This is not a very elegant solution and it breaks some guidelines but it does what I need, doesn't make user to view through unnecessary warnings (RuleSet solution) and doesn't affect code cleanliness (out).

For tests I had to make a base class where I setup Appdomain.UnhandledException handler in SetUp method and check (after GC.Collect) whether any exception was thrown in TearDown because finalizer is called from another thread and NUnit otherwise shows the test as passed.