How to unit-test a class which needs a specific file to be present

Question

I'm currently trying to learn proper unit-test. So now I'm trying to write unit-tests for a class that should map data from an XML-File to the proper objects. Of course all functionality of the class is dependent on the existence of the corresponding XML-file. The XML-file is loaded in the constructor of the class.

I'm using C# with NUnit. So far I've got two tests:

[Test]
public void ShouldAllowInstanceToBeCreatedWhenXMLFileIsPresent()
{
    if (File.Exists(SettingsReader.XML_SETTINGS_PATH))
    {
        SettingsReader settingsReader = new SettingsReader();
        Assert.AreNotEqual(null, settingsReader);
    }
}

[Test]
[ExpectedException("Telekanzlei.Clientmanager.XMLDataLayer.XMLFileNotFoundException")]
public void ShouldThrowExceptionWhenXMLFileIsNotPresent()
{
    if (!File.Exists(SettingsReader.XML_SETTINGS_PATH))
    {
        SettingsReader settingsReader = new SettingsReader();
    }
        else
            throw new XMLFileNotFoundException();
    }

I'm not sure if checking the existence of the file in the test is a proper way to go, so any suggestions on those test are welcome too. But my question is, how to proceed with the following tests. Obviously all following tests are going to fail, if the XML-file is not present.

So do I assume that the XML-file is present, while keeping in mind, that a failing test could just mean that it's not? That wouldn't seem right to me.

Is there a general pattern, to handle a problem like this?

Thx for any help

edit: rewrote the second test, as it was failing if the file was actually present...

edit2: May it is helping to tell you, what the SettingsReader actually does. So far it looks like this:

public class SettingsReader
{
    public static readonly string XML_SETTINGS_PATH = "C:\\Telekanzlei\\Clientmanager_2.0\\Settings.xml";

    public XElement RootXElement { get; private set; }

    public SettingsReader()
    {
        if (!File.Exists(XML_SETTINGS_PATH))
            throw new XMLFileNotFoundException();
        using (var fs = File.OpenRead(XML_SETTINGS_PATH))
        {
            RootXElement = XElement.Load(fs);
        }
    }


}

I'm not sure, but I guess a StreamReader wouldn't be the way to go here, would it?

Answer 1

The problem is not with your unit tests but with the design of the class. I'd suggest refactoring the class so it doesn't open the file but instead operates on a stream. Then your unit tests could simply replace a file stream for a memory stream - simples! :)

public class SettingsReader()
{
    public SettingsReader(System.IO.StreamReader reader)
    {
        // read contents of stream...
    }
}

// In production code:
new SettingsReader(new StreamReader(File.Open("settings.xml")));

// In unit test:
new SettingsReader(new StringReader("<settings>dummy settings</settings>"));

Remember, opening a file and parsing settings data are two very different concerns.