Should all classes be testable?

Question

I am trying to write unit tests for a new project I have created and I've run into an issue where I can't work out how a class that I am intending to write is actually testable. Below I've simplified the class that I am trying to write to give you an idea of what I am trying to achieve.

So I have an XML parser that will simply access an XML file from a given URL, extract the data that I need and return it as an object. So my code will look something like this (Validation and population not completed yet but you get the idea):

public UserDetails ParseUserDetails(string request, string username, string password)
{
    var response = new XmlDocument();
    response.Load(string.Format(request + "?user={0}&password={1}", username, password));

    // Validation checks

    return new UserDetails { // Populate object with XML nodes };
}

Currently my class isn't testable. I can't mock the load to throw a WebException to see how my class handles errors and until I pass through a valid URL it will always throw an exception when I run tests against this class. I also cannot test the data coming back from the class since I can't mock the XML document since it's loaded from another URL.

I could split this out into a mockable object that retrieves the XML from the URL and name it something like IXmlDocumentLoader but later I run into the same problem where I have a class like this:

public class XmlDocumentLoader : IXmlDocumentLoader
{
    public XmlDocument LoadXmlDocument(string request, string username, string password)
    {
        var response = new XmlDocument();
        response.Load(string.Format(request + "?user={0}&password={1}", username, password));

        return response;
    }
}

This would make the ParseUserDetails method more testable but now the class XmlDocumentLoader is not testable. So have I just moved the problem elsewhere? My question is really do all classes have to be testable or am I misunderstanding unit testing?

Answer 1

This for sure is an "opinion" question, and as so, probably will be closed.

But I'll give you a suggestion.

Split everything. Use the principle of "an object should do only one thing". Donwload a file is one thing, validate it is another one. If you separete both, you can test both.

You can test your download system on a generic file (do not have to be a production site), and test if the system works. And you can provide a "fake file" to test the validation, that also do not have to be a production file.

Both tests will give you an idea of how much your code is working as expected.

Answer 2

The XmlDocument.Load(string filename) documentation in MSDN defines the filename parameter thusly:

filename: URL for the file containing the XML document to load. The URL can be either a local file or an HTTP URL (a Web address).

(emphasis mine)

So your code is entirely testable if your ParseUserDetails tests pass something like file://C:/path/to/my/test/file as the request parameter. They might look something like this:

    public void SomeRandomTest()
    {
        string testFileLocalPath = @"C:\path\to\my\test\file";
        // Code to create an XML file with expected data goes here ...

        UriBuilder ub = new UriBuilder();
        ub.Scheme = "file";
        ub.Host = "";
        ub.Path = testFileLocalPath;
        string request = ub.ToString();

        var target = new SomethingThatReadsXml();
        var details = target.ParseUserDetails(request, "dummy", "whocares");

        // Compare returned user details to expected values here ...
    }