Question

I extend upon a legacy library which accesses files on the harddrive. I have such files as embedded resources in my unit test project.

I have ported parts of the library to accept streams, which allows me to use GetManifestResourceStream to pass my embedded resource to the legacy library. This works fine, but it is a slight hassle. And people maintaining those libraries don’t appreciate the “clutter” or having to publish new releases.

JustMock and TypeMock allows me to intercept the File.Open commmand, and I wish to pass the library a FileStream object, but how do I construct a FileStream Object from an Embedded Manifest Resource?
I could of course create a physical file, but I don’t wish to touch the file system while running tests.

Answer 1

I have cooked a sample based on the requirments you mentioned here. I used in memory stream for it but can done with embedded resource as well.

           byte[] actual = new byte[255];

        // writing locally, can be done from resource manifest as well.

        using (StreamWriter writer = new StreamWriter(new MemoryStream(actual)))
        {
            writer.WriteLine("Hello world");
            writer.Flush();
        }

        // arrange the file system.

        FileStream fs = (FileStream)FormatterServices
            .GetSafeUninitializedObject(typeof(FileStream));

        // mocking the specific call and setting up expectations.
        Mock.Arrange(() => fs.Write(Arg.IsAny<byte[]>(), Arg.AnyInt, Arg.AnyInt))
            .DoInstead((byte[] content, int offset, int len) =>
        {
            actual.CopyTo(content, offset);
        });

        // return custom filestream for File.Open.
        Mock.Arrange(() => File.Open(Arg.AnyString, Arg.IsAny<FileMode>()))
             .Returns(fs);


        // act
        var fileStream =  File.Open("hello.txt", FileMode.Open);
        byte[] fakeContent = new byte[actual.Length];

        // original task
        fileStream.Write(fakeContent, 0, actual.Length);

        // assert
        Assert.Equal(fakeContent.Length, actual.Length);

        for (var i = 0; i < fakeContent.Length; i++)
        {
            Assert.Equal(fakeContent[i], actual[i]);
        }

Since i am moking a mscorlib member and FileStream.Write is a instance call / not contains in the default set File, DateTime, FileInfo. I also added the following line during TestInitailization.

           Mock.Partial<FileStream>()
              .For<byte[], int, int>((x, content, offset, len) => 
            x.Write(content, offset, len));

[Disclaimer i work for telerik]

Hope that helps,

Mehfuz