[c#] How to read a text file in project's root directory?

You can have it embedded (build action set to Resource) as well, this is how to retrieve it from there:

private static UnmanagedMemoryStream GetResourceStream(string resName)
{
    var assembly = Assembly.GetExecutingAssembly();
    var strResources = assembly.GetName().Name + ".g.resources";
    var rStream = assembly.GetManifestResourceStream(strResources);
    var resourceReader = new ResourceReader(rStream);
    var items = resourceReader.OfType<DictionaryEntry>();
    var stream = items.First(x => (x.Key as string) == resName.ToLower()).Value;
    return (UnmanagedMemoryStream)stream;
}

private void Button1_Click(object sender, RoutedEventArgs e)
{
    string resName = "Test.txt";
    var file = GetResourceStream(resName);
    using (var reader = new StreamReader(file))
    {
        var line = reader.ReadLine();
        MessageBox.Show(line);
    }
}

(Some code taken from this answer by Charles)