How to unit test methods that construct the string?

I have method which construct the string from List. I want to test the method with Unit Test Method. What's the good idea to test that method? I need to use data base to get matchList. I don't know how to use mock object to do that, please help.

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;
    }

}