How to harness "File.Open"? What are good strategies?

[Tormod]

Hi.

My test projects have data files as part of the solution. To avoid dependencies on the file system on the CI server, I embed them in the test assembly as manifest resources. The problem is when I intercept File.Open calls using JustMock. What do I do? What do I return? Do I pass it a Mock<FileInfo>? Since "File.Open()" is mentioned about as often as "DateTime.Now" to show the strengths of TypeMock and JustMock, what are good practices/strategies to employ when dealing with it?

http://stackoverflow.com/questions/3466355/how-can-i-pass-a-solution-resource-as-a-filestream-object-to-an-intercepted-file

[Ricky]

Hi  Tormod,
I prepared a sample based on your requirement and added that to the stackoverlow question that you have asked.

Just to let you know that  I found a issue with Arg.IsAny<byte[]> and the example is done with a fixed  internal version. If you are interested I can provide you the build  to the email you have specified in your Telerik account along with the Mock.Partial(..).ForAll fix  for the other System.Environment related issue.

Regards,
Mehfuz

[Tormod]

Thanks a lot! This was definitely useful.
There was a trick there that I didn't expect. I guess I need to tune in to the power of JustMock.

After creating this object like this:
FileStream fs = (FileStream)FormatterServices.GetSafeUninitializedObject(typeof(FileStream));  

I had to read this line at least three times.
Mock.Arrange(() => fs.Write(Arg.IsAny<byte[]>(), Arg.AnyInt, Arg.AnyInt)

Can you take any single method on any running instance of any type and just mock it?
My thought was that I would create a <FileInfo> mock. And return it from statically mocked File.Open().

I still need to work out how to be able to create a FileInfo object which in actuality reads from a test project resource, but this helped me a long way. And I can fiddle around with it myself.
I don't have an immediate need for it, but I hope you get the argument Arg.Any<> clause object figured out.

For clarity and posterity in this thread I've included your solution. 

        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]); 
        } 

[Ricky]

Hello Tormod,

It's nice that things helped you a bit. About the Arg.IsAny<T>(), it is also possible to write it in the following way to ignore any arguments specified:

Mock.Arrange(() => fs.Write(null, 0, 0).IgnoreArgument();

Secondly, it is also possible to create the instance of FileStream using Mock.Create<T>() but since here we are using specifically fs.Write method, other ways like creating  the instance using FormatterServices can significantly reduce the test execution time.



Hope that helps,
the Telerik team