Using AutoFixture to create MockFileSystem and its corresponding types

Question

I'm using AutoFixture to test some code that uses System.IO.Abstractions.IFileSystem.

I also have some code that uses IFileInfo and IDirectoryInfo. When I create my AutoFixture Fixture object, I need to accomplish a couple of things:

1. I need all IFileSystem parameters to be automatically given a frozen (singleton) instance of MockFileSystem.
2. All IFileInfo instances need to originate from MockFileSystem via MockFileSystem.FileInfo.FromFileName() somehow. If there's a way to take an existing IFileInfo that AutoFixture created and replace it, I can use the FullName property to pass to MockFileSystem so that it's connected to that instance.
3. Similar to the previous point, I need IDirectoryInfo instances to originate from MockFileSystem.

I personally find the AutoFixture customization system very confusing and lacks proper documentation. There's pieces of information out there but I'm not sure how to accomplish my goals.

Answer 1

As I struggled through solving my own problem, I eventually came up with this class:

public class MockFileSystemSpecimenBuilder : ICustomization
{
    public void Customize(IFixture fixture)
    {
        var fs = new MockFileSystem();
        fixture.Inject(fs);

        fixture.Customize<IFileInfo>(x => x.FromFactory(() =>
        {
            var name = $"MockFile-{fixture.Create<string>()}";
            return fs.CurrentDirectory().File(name);
        }));

        fixture.Customize<IDirectoryInfo>(x => x.FromFactory(() =>
        {
            var name = $"MockDirectory-{fixture.Create<string>()}";
            return fs.CurrentDirectory().SubDirectory(name);
        }));
    }
}

I add it to my Fixture class after its created in my factory method:

public static Fixture Create()
{
    var fixture = new Fixture
    {
        OmitAutoProperties = true
    };

    fixture
        .Customize(new AutoNSubstituteCustomization {ConfigureMembers = true})
        .Customize(new MockFileSystemSpecimenBuilder());

    return fixture;
}

One thing I really enjoy about this solution is that I do not need to explicitly Freeze() the IFileSystem type anymore. The mock filesystem is intrinsic to this fixture so I always get correct file system behavior. This ends up being less error prone and more convenient.