How to unit test methods that construct a string?

Question

I have a method which constructs a string from List . I want to test the method with a unit test. What's a good way to test that method?

I need to use the database to get matchList. I don't know how to use a mock object to do that.

public class File_IO
{
    private string _rootDirectory;
    private string _taskName;

    private File_IO()
    {

    }

    public File_IO(string rootDirectory, string taskName)
    {
        _rootDirectory = rootDirectory;
        _taskName = taskName;

    }

    public string RootDir
    {
        get { return _rootDirectory; }
    }

    public string TaskName
    {
        get { return _taskName; }
    }

    public string constructString()
    {
        string result = string.Empty;
        string pipe = "|";
        StringBuilder stringBuilder = new StringBuilder();
        bool isFirst = true;

        IList matchList = new List<string> { "*.Ext","*.ext" };

        // Iterate over the results and construct the string.
        foreach (var item in matchList)
        {
            if (!isFirst)
            {
                stringBuilder.Append(pipe);
            }

            isFirst = false;
            stringBuilder.Append(item);
        }

        result = stringBuilder.ToString();
        return result;
    }

}

Answer 1

Nunit is a pretty popular framework to do unit testing in C#. http://nunit.org/

Use nuget to install it and create your test cases. It'll look more or less like this:

[Test]
public static void TestFile_IO()
{
    var expectedString = "what ever is supposed to come out of the method";
    var result = new File_IO.constructString();
    Assert.AreEqual(expectedString, result)
}

Answer 2

I'm not sure if I understand question correctly or not... May be you need something like this:

interface IRepository
{
    List<string> GetList();    
}

class Repository
{
    List<string> GetList() { return /*Query database here*/ ;}
}

public class File_IO
{
    private IRepository _repository;

    private File_IO(IRepository repository)
    {
         _repository = repository;
    }

    public string constructString()
    {
         IList matchList = _repository.GetList();
         .........................
         return result;
    }
}

In this case you will be able to test this using one of mock frameworks (Moq, NSubstitute). Example below use NSubstitute.

public void TestMethod()
{
    var repositoryMock = Substitute.For<IRepository>();
    repositoryMock.GetList().Returns(new List<string> {"str1", "str2"});
    var f_io = new File_IO(repositoryMock);

    var result = f_io.constructString();

    Assert.AreEqual("Expected string", result);
}