using FakeItEasy;

using System.IO;

using NUnit.Framework;

​

public interface IFileProxy

{

    Stream Open(string path, FileMode mode);

}

​

public class MyFileProxy : IFileProxy

{

    public Stream Open(string path, FileMode mode)

    {

        return File.Open(path, mode);

    }

}

​

public class ClassThatUsesFileStuff

{

    private readonly IFileProxy fileProxy;

    public ClassThatUsesFileStuff(IFileProxy fileProxy)

    {

        this.fileProxy = fileProxy;

    }

    public void DoSomething()

    {

        var stream = fileProxy.Open("my path here", FileMode.OpenOrCreate);

        // maybe more logic here

    }

}

​

public class WhenDoingSomething

{

    [Test]

    public void ItShouldOpenAFile()

    {

        var fileProxy = A.Fake<IFileProxy>();

        var sut = new ClassThatUsesFileStuff(fileProxy);

        sut.DoSomething();

        A.CallTo(() => fileProxy.Open("my path here", FileMode.OpenOrCreate)).MustHaveHappened();

    }

    [Test]

    public void ItUsesAStream()

    {

        // if you want to have the business logic use the stream use a memorystream

        var fileProxy = A.Fake<IFileProxy>();

        A.CallTo(() => fileProxy.Open("my path here", FileMode.OpenOrCreate))

            .Returns(new MemoryStream());

    }

}