How do you mock out the file system in C# for unit testing?

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Edit: Install the NuGet package System.IO.Abstractions.

This package did not exist when this answer was originally accepted. The original answer is provided for historical context below:

You could do it by creating an interface:

interface IFileSystem {
    bool FileExists(string fileName);
    DateTime GetCreationDate(string fileName);
}

and creating a ‘real’ implementation which uses
System.IO.File.Exists() etc. You can then mock this interface using a
mocking framework; I recommend Moq.

Edit: somebody’s done this and kindly posted it online here.

I’ve used this approach to mock out DateTime.UtcNow in an IClock
interface (really really useful for our testing to be able to control
the flow of time!), and more traditionally, an ISqlDataAccess
interface.

Another approach might be to use TypeMock, this allows you to
intercept calls to classes and stub them out. This does however cost
money, and would need to be installed on your whole team’s PCs and
your build server in order to run, also, it apparently won’t work for
the System.IO.File, as it can’t stub mscorlib.

You could also just accept that certain methods are not unit testable
and test them in a separate slow-running integration/system tests
suite.

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