c# - How do I MOQ the System.IO.FileInfo class... or any other class without an interface?

Question

I am writing a number of unit tests for a logger class I created and I want to simulate the file class. I can't find the interface that I need to use to create the MOQ... so how do you successfully MOQ a class without an interface?

It also isn't clear to me how I can use dependency injection without having an interface available:

private FileInfo _logFile;

public LogEventProcessorTextFile(FileInfo logFile) {
    _logFile = logFile;
}

When I really want to do something like this (note IFileInfo instead of FileInfo):

private IFileInfo _logFile;

public LogEventProcessorTextFile(IFileInfo logFile) {
    _logFile = logFile;
}

Answer 1

Design your code so that instead of accessing the FileInfo class directly, access an interface (named for example IFileInfo) with the same capabilities. In production code you will use a class that just delegates all its functionality to the system FileInfo class, but for unit testing you can mock the interface.

For example, in an application I made that acted differently depending on the current date, I declared the following interface:

interface IDateTimeProvider
{
    DateTime Today();
}

And the production class was just:

class DateTimeProvider : IDateTimeProvider
{
    public DateTime Today()
    {
        return DateTime.Today;
    }
}

You can complement this approach with the usage of a dependency injection engine to decide whether a real class or a mock should be used in each case.