Preventing ReSharper's 'implicitly captured closure' warning in FakeItEasy unit tests

I noticed that some of my unit tests have a ReSharper warning on the A.CallTo() methods of FakeItEasy, suggesting that some local variables are captured implicitly by the lambda expression.

This warning appears since ReSharper doesn’t know that the lambda expression is executed immediately when the A.CallTo method is invoked. Fortunately, with the help of External Annotations, in particular, the InstantHandle annotation, we can teach ReSharper about it!

The InstantHandle annotation tells ReSharper that the method parameter is completely handled when the invoked method is on the stack. If the parameter is a delegate (which in our case, it is), it indicates that the delegate is executed when the method is executed.

To add this information about FakeItEasy’s A.CallTo() overloads that accept delegates, we can create an external XML file, which looks like this:

<?xml version="1.0" encoding="utf-8"?>
<assembly name="FakeItEasy">
  <member name="M:FakeItEasy.A.CallTo(System.Linq.Expressions.Expression{System.Action})">
    <parameter name="callSpecification">
      <attribute ctor="M:JetBrains.Annotations.InstantHandleAttribute.#ctor" />
    </parameter>
  </member>
  <member name="M:FakeItEasy.A.CallTo1(System.Linq.Expressions.Expression{System.Func{0}})">
    <parameter name="callSpecification">
      <attribute ctor="M:JetBrains.Annotations.InstantHandleAttribute.#ctor" />
    </parameter>
  </member>
</assembly>

Download the XML via GitHub Gist,

This instructs ReSharper to add an instance of the InstantHandleAttribute to a parameter called callSpecification of two CallTo method overloads, once which takes an Expression<Action>, the other, which takes an Expression<Func<T>>. Save this file as FakeItEasy.ExternalAnnotations.xml and place it **alongside **FakeItEasy.dll (inside libNET40 of your NuGet package, for example). Reload your solution, and the warnings will now disappear!

Stay tuned, as I explain in the next blog post how to create those XML annotations easily with a little known ReSharper trick!