Conflicting out/ref argument set

[dtchepak]

It is possible to set argument values from .Returns and .AndDoes. At present the one set in .Returns wins (not sure we should always rely on this for future versions), but this can result in confusing results.

        public interface ILookup { bool Lookup(string key, out int i); }

        [Fact]
        public void Test() {
            var sub = Substitute.For<ILookup>();

            sub.Lookup("blah", out Arg.Any<int>())
               .Returns(x => {
                   x[1] = 42;
                   return true;
                })
               .AndDoes(x => x[1] = 45);

            sub.Lookup("blah", out var value);

            Assert.Equal(42, value); // The final values is 42 from the Returns, not 45 from AndDoes
        }

Draft documentation:

NS3006

CheckId	NS3006
Category	Argument specification

Cause

Conflicting assignments to out/ref arguments.

Rule description

A violation of this rule occurs when an out or ref argument is assigned via CallInfo in both a Returns and an AndDoes call.

How to fix violations

To fix a violation of this rule remove one of the assignments, so the argument is only assigned in either the Returns or the AndDoes block (not in both).

// Incorrect, argument assigned in `Returns` and `AndDoes`:
dictionary.TryGetValue("sample", out Arg.Any<int>())
   .Returns(x => {
       x[1] = 42;
       return true;
    })
   .AndDoes(x => x[1] = 45);

// Correct, only assigned in `AndDoes`:
dictionary.TryGetValue("sample", out Arg.Any<int>())
   .Returns(true)
   .AndDoes(x => x[1] = 45);

// Correct, only assign in `Returns`:
dictionary.TryGetValue("sample", out Arg.Any<int>())
   .Returns(x => {
       x[1] = 42;
       return true;
    });

How to suppress violations

This warning can be suppressed by disabling the warning in the ruleset file for the project.
The warning can also be suppressed programmatically for an assembly:

[assembly: System.Diagnostics.CodeAnalysis.SuppressMessage("Argument specification", "NS3006:Conflicting argument assignments.", Justification = "Reviewed")]

Or for a specific code block:

#pragma warning disable NS3006 // Conflicting argument assignments.
// the code which produces warning
#pragma warning disable NS3006 // Conflicting argument assignments.