Just like System.Web.Abstractions, but for System.IO. Yay for testable IO access!

NuGet only:

Install-Package System.IO.Abstractions

and/or:

Install-Package System.IO.Abstractions.TestingHelpers

At the core of the library is IFileSystem and FileSystem. Instead of calling methods like File.ReadAllText directly, use IFileSystem.File.ReadAllText. We have exactly the same API, except that ours is injectable and testable.

public class MyComponent
{
    readonly IFileSystem fileSystem;

    // <summary>Create MyComponent with the given fileSystem implementation</summary>
    public MyComponent(IFileSystem fileSystem)
    {
        this.fileSystem = fileSystem;
    }
    /// <summary>Create MyComponent</summary>
    public MyComponent() : this( 
        fileSystem: new FileSystem() //use default implementation which calls System.IO
    ) 
    {
    }

    public void Validate()
    {
        foreach (var textFile in fileSystem.Directory.GetFiles(@"c:\", "*.txt", SearchOption.TopDirectoryOnly))
        {
            var text = fileSystem.File.ReadAllText(textFile);
            if (text != "Testing is awesome.")
                throw new NotSupportedException("We can't go on together. It's not me, it's you.");
        }
    }
}

The library also ships with a series of test helpers to save you from having to mock out every call, for basic scenarios. They are not a complete copy of a real-life file system, but they'll get you most of the way there.

[Test]
public void MyComponent_Validate_ShouldThrowNotSupportedExceptionIfTestingIsNotAwesome()
{
    // Arrange
    var fileSystem = new MockFileSystem(new Dictionary<string, MockFileData>
    {
        { @"c:\myfile.txt", new MockFileData("Testing is meh.") },
        { @"c:\demo\jQuery.js", new MockFileData("some js") },
        { @"c:\demo\image.gif", new MockFileData(new byte[] { 0x12, 0x34, 0x56, 0xd2 }) }
    });
    var component = new MyComponent(fileSystem);

    try
    {
        // Act
        component.Validate();
    }
    catch (NotSupportedException ex)
    {
        // Assert
        Assert.AreEqual("We can't go on together. It's not me, it's you.", ex.Message);
        return;
    }

    Assert.Fail("The expected exception was not thrown.");
}

We even support casting from the .NET Framework's untestable types to our testable wrappers:

FileInfo SomeBadApiMethodThatReturnsFileInfo()
{
    return new FileInfo("a");
}

void MyFancyMethod()
{
    var testableFileInfo = (FileInfoBase)SomeBadApiMethodThatReturnsFileInfo();
    ...
}