Question

I wrote a simple class to manage business objects.

class Manager
{ 
 string[] GetNames();
 BObject GetObject(string name);
 void Saveobject(BObject obj);
}

It serializes /deserializes the objects as files on a local disk. I wrote Unit tests for the class and run them. That was fine so far. The problem happens when my test were run on build server because of file access permission I was not allowed to write files on the server. It’s obvious I cannot test that way.

I think how to unit test this. One approach I can see is to extract an interface and creat a mock object for testing. But I want to test the class itself. How can I do it?

Answer 1

The class presumably calls file system operations File.OpenRead(), File.OpenWrite() etc. (I assume that this is C# due to the camel casing.) Then, you could create an interface for those operations, e.g.:

public interface IFileSystem {
    StreamReader OpenRead(string fileName);
    StreamWriter OpenWrite(string fileName);
}

and make the constructor of Manager take an instance of IFileSystem. Then, write a (non-mock) class that implements IFileSystem by calling the actual File.OpenRead() and File.OpenWrite() methods and use this one in the production code. In the tests, you use a mock framework, as mentioned by @Digger (my personal preference is Moq, but I haven’t tried Rhino Mocks, so I have nothing negative to say about it) to mock out IFileSystem and use the mock to verify that the methods were called with the correct serialized data.

EDIT: Per request, an example in NUnit with Moq (I don’t have an IDE here, so it’s untested; feel free to correct it):

[Test]
public void BObjectShouldBeSerializedToFile() {
    var fileSystemMock = new Mock<IFileSystem>();
    var stream = new MemoryStream();
    fileSystemMock.Setup(f => f.OpenWrite("theFileNameYouExpect.txt")).Returns(new StreamWriter(stream)).Verifiable();
    var manager = new Manager(fileSystemMock.Object);

    manager.SaveObject(new BObject(...));

    stream.Seek(0, SeekOrigin.Begin);
    Assert.That(...); // Perform asserts on the stream contents here
    fileSystemMock.Verify(); // Not really necessary, but verify that `OpenWrite` was called
}