Mock File methods in .NET (like File.Copy("1.txt", "2.txt"))

Question

We have some methods that call File.Copy, File.Delete, File.Exists, etc. How can we test these methods without actually hitting the file system?

I consider myself a unit testing n00b, so any advice is appreciated.

Answer 1

public interface IFile {
    void Copy(string source, string dest);
    void Delete(string fn);
    bool Exists(string fn);
}

public class FileImpl : IFile {
    public virtual void Copy(string source, string dest) { File.Copy(source, dest); }
    public virtual void Delete(string fn) { File.Delete(fn); }
    public virtual bool Exists(string fn) { return File.Exists(fn); }
}

[Test]
public void TestMySystemCalls() {
    var filesystem = new Moq.Mock<IFile>();
    var obj = new ClassUnderTest(filesystem);
    filesystem.Expect(fs => fs.Exists("MyFile.txt")).Return(true);
    obj.CheckIfFileExists(); // doesn't hit the underlying filesystem!!!
}

Answer 2

If you absolutely have to do this, Typemock Isolator is your friend.

I can't say I've used it myself, and I would try to design my way around it instead, but it'll do the job as far as I'm aware.