IFile.Open(Read) cannot be mocked with NSubstitute anymore due to System.IO.Abstractions.FileSystemStream

[rklec]

Describe the bug
I am upgrading from System.IO.Abstractions v17.0.18 to v19.2.29.

To Reproduce

using System.IO;
using System.IO.Abstractions;
using System.IO.Abstractions.TestingHelpers;

// ...
 [Test]
 public async Task FileSystemStreamBrokenTest()
 {
 var fileSystem = Substitute.For<IFileSystem>();
 var fileStream = System.IO.Abstractions.FileSystemStream.Null;
 fileSystem.File.OpenRead(default!).ReturnsForAnyArgs(fileStream);

 fileSystem.File.OpenRead("somePath");
 }

This fails with an NSubsitute error though:

Also, Resharper suggests to simplify it in a way that it is essentially the same as this:

 [Test]
 public async Task FileSystemStreamBrokenTest()
 {
 var fileSystem = Substitute.For<IFileSystem>();
 var fileStream = System.IO.Stream.Null;
 fileSystem.File.OpenRead(default!).ReturnsForAnyArgs(fileStream);

 fileSystem.File.OpenRead("somePath");
 }

...which may explain why it fails.

Before all that I had a custom class for fileStream, which just also had the most common stuff implemented, so it was like your mock, except of the special type. Now the types don't match (I guess since v18.0.2) and as such I need to adapt it...

I tried this, too:

 [Test]
 public async Task FileSystemStreamBrokenTest()
 {
 var fileSystem = Substitute.For<IFileSystem>();

 var fileStream = new MockFileStreamFactory(new MockFileSystem()).New("anything.example", FileMode.Open);
 fileSystem.File.OpenRead(default!).ReturnsForAnyArgs(fileStream);

 fileSystem.File.OpenRead("anything.example");
 }

but that (kinda correctly) fails with:

System.IO.FileNotFoundException : Could not find file 'anything.example'.

....but I just wanna mock "any file", I don't care about what content etc.

Expected behavior
System.IO.Abstractions.FileSystemStream.Null should be useful as a wrapper, when you really don't care about your file handle/anything, but just want to mock it. I also don't wanr to mock a whole file system and provide any/all files in it, like I likely could with MockFileSystem.

Additional context
Finally found a workaround as . always exists:

 [Test]
 public async Task FileSystemStreamBrokenTest()
 {
 var fileSystem = Substitute.For<IFileSystem>();

 var fileStream = new MockFileStreamFactory(new MockFileSystem()).New(".", FileMode.Open);
 fileSystem.File.OpenRead(default!).ReturnsForAnyArgs(fileStream);

 fileSystem.File.OpenRead(".");
 }

IMHO there should be a more convenient way though.

System

		<PackageReference Include="System.IO.Abstractions" Version="19.2.29" />
		<PackageReference Include="System.IO.Abstractions.TestingHelpers" Version="19.2.29" />

and NSubstitute 4.4.0

[fgreinacher]

Thanks for the detailed report!

I think we should shadow the Stream.Null field in our base class.

Contributions welcome :)

[sheila-stewart]

This is how I'm working around the issue, in case it can help anyone else:

/// <summary>
/// Wrapper around a <see cref="Stream"/> with no backing store, which
/// is used as a replacement for a <see cref="FileSystemStream"/>. As
/// such it implements the same properties and methods as a <see cref="FileSystemStream"/>.
/// </summary>
public class NullFileSystemStream : FileSystemStream
{
 /// <inheritdoc cref="Stream.Null" />
 public static new NullFileSystemStream Null { get; } = new NullFileSystemStream();

 /// <summary>
 /// Initializes a new instance of <see cref="NullFileSystemStream" />.
 /// </summary>
 private NullFileSystemStream() : base(Stream.Null, ".", true)
 { }
}

[vbreuss]

I think we should shadow the Stream.Null field in our base class.

@fgreinacher: I don't think it would be a good idea, to add a test helper in the interface project. But we could add an option to simply create a "Null-Stream" similar to the solution from @sheila-stewart to the TestingHelpers project.

[fgreinacher]

I think we should shadow the Stream.Null field in our base class.

@fgreinacher: I don't think it would be a good idea, to add a test helper in the interface project. But we could add an option to simply create a "Null-Stream" similar to the solution from @sheila-stewart to the TestingHelpers project.

@vbreuss Why do you consider this a test helper? The Null stream is exposed by the base class and we should provide a working wrapper I think?

[vbreuss]

@vbreuss Why do you consider this a test helper? The Null stream is exposed by the base class and we should provide a working wrapper I think?

@fgreinacher:
The interface wraps a FileStream which does not have a static Null property itself, but only the base type, which returns a Stream and not a FileStream.

If we include this in the interface, we add something that is not there in the real file system.

I think it a good idea to simplify creating a Null-FileSystemStream, but I would add this to the TestingHelper project and not to the interface project itself.

[sheila-stewart]

Maybe add the ability to mock a stream with no backing store like this:

var nullStream = new MockFileStreamFactory(new MockFileSystem()).Null;