Idiomatic assertions for IEqualityComparer<T>

[Kralizek]

At my company we have ended up creating some implementations of IEqualityComparer<T> so I've worked on an idiomatic assertion to quickly check that they respect some basic requirements:

• Equals(a, a) == true (reflexive property)
• Equals(a, b) == Equals(b, a) (symmetric property)
• Equals(a, b) == Equals(b, c) == Equals(a, c) (transitive property)
• Equals(a, null) == false
• Equals(null, null) == true
• GetHashCode(a) == GetHashCode(a)

I added to this PR all the code produced right now. If there is an interest, I can continue working on this PR.

Here is a sample on how to use it:

Given the following class implementing IEqualityComparer<string>

public class TestEqualityComparer : IEqualityComparer<string>
{
	public bool Equals(string x, string y)
	{
		if (x is null ^ y is null) return false;
		
		return string.Equals(x, y);
	}

	public int GetHashCode(string obj)
	{
		return obj.GetHashCode();
	}
}

Developers can use this assertion as simply as

[Test]
public void IEqualityComparer_interface_is_implemented_correctly()
{
	var fixture = new Fixture();
	
	var assertion = fixture.Create<EqualityComparerAssertion>();
	
	assertion.Verify(typeof(TestEqualityComparer));
}

I haven't gone added the unit tests I have in my own solution and decorated methods and property with documentation comments. Also, I have used few features from C# 8 but I'm not sure if the current build setup supports them.

[aivascu]

@Kralizek thank you for the contribution. I think this will make a nice addition to AutoFixture.

There is a package that implements these same assertions and some more.
I don't know if @baks would like to contribute those features to AutoFixture but at least it could serve as inspiration for this PR.

I think fixing the issues from the appveyor build log and adding the tests would be the first step to moving forward with the PR.

[Kralizek]

@aivascu
Glad you like it! The package you linked doesn't have anything (AFAIK) for implementing IEqualityComparer, hence my PR.

Yep, appveyor went crazy on feedback. I'll work on it when I have some free time.

[Kralizek]

@aivascu I fixed the build errors and added relevant tests so this PR is ready for a review. I looked ad @baks's code as suggested but I couldn't take much as it uses a lot of helper classes and I didn't know if we wanted to import them into AutoFixture.Idioms.

[aivascu]

@Kralizek The PR looks good, I've made only a few code style changes. Let me know if you're OK with them.

[Kralizek]

No worries! Nice changes. I didn't think about putting that ugly pattern match into its own method.

[zvirja]

@Kralizek Nice work, like how thorough we are testing it! Having more assertions of the quality you created could make library insanely useful! Thanks for your contribution! 🤝

[Kralizek]

Glad you like it!