Estou desenvolvendo um aplicativo localizável. No meu arquivo de recurso "local", tenho o idioma usado por padrão (inglês) e, se possível, carrego a preferência e a cultura do usuário e carrego as strings traduzidas no idioma.

Então, o que eu fiz:

private static CultureInfo _culture = CultureInfo.CurrentUICulture;
private static ResourceManager _manager;

private static void ToNeutralCulture()
{
    while (!_culture.IsNeutralCulture)
    {
        _culture = _culture.Parent;
    }
}

private static void LoadCulture()
{
    ResourceManager manager = Properties.Resources.ResourceManager;

    try
    {
        ToNeutralCulture();

        string assembly = Assembly.GetCallingAssembly().GetName().CodeBase;
        string assemblyDir = Path.GetDirectoryName(assembly);
        string assemblyName = Path.GetFileNameWithoutExtension(assembly);
        string resourceFileName = string.Format(CultureInfo.InvariantCulture,
            @"{0}\{1}_{2}.dll",
            assemblyDir,
            assemblyName,
            _culture.Name.ToUpper());

        FileInfo resourceFile = new FileInfo(resourceFileName);
        if (resourceFile.Exists)
        {
            Assembly resourceAssembly = Assembly.LoadFrom(resourceFile.FullName);
            string[] manifests = resourceAssembly.GetManifestResourceNames();

            if (manifests.Length == 1)
            {
                manager = new ResourceManager(manifests[0], resourceAssembly);
            }

            using (ResourceReader reader = new ResourceReader(resourceAssembly.GetManifestResourceStream(manifests[0])))
            {
                IDictionaryEnumerator dict = reader.GetEnumerator();
                while (dict.MoveNext())
                {
                    string key = dict.Key as string;
                    object val = dict.Value;

                    //string mVal = manager.GetString(key);
                }
            }
        }
    }
    catch (Exception ex)
    {
        Trace.WriteLine(ex.Message);

        Trace.WriteLine(string.Format(CultureInfo.InvariantCulture,
            "Fail to loading culture {0}", 
            (_culture == null) ? "--" : _culture.EnglishName));
    }

    _manager = manager;
}

O assembly está carregado corretamente e o enumerador me exibirá todos os recursos presentes no arquivo de recursos, bem, funciona bem, exceto:

string mVal = manager.GetString(key);

Quando tirei o comentário desta linha, tenho umSystem.Resources.MissingManifestResourceException, Alguém pode me dizer o porquê?

Obrigado !

[EDITAR]

Projeto "MyApp"

namespace MyApp
{
    Assembly resourceAssembly = Assembly.LoadFrom(resourceFileName);
    string[] manifests = resourceAssembly.GetManifestResourceNames();

    if (manifests.Length == 1)
    {
        manager = new ResourceManager(manifests[0], resourceAssembly);
    }

    // Throws the exception
    manager.GetString("PleaseCallIT", null);

    // Works
    using (ResourceReader reader = new ResourceReader(resourceAssembly.GetManifestResourceStream(manifests[0])))
    {
        IDictionaryEnumerator dict = reader.GetEnumerator();
        while (dict.MoveNext())
        {
            string key = dict.key as string; // PleaseCallIT
            object val = dict.value; // Please call IT.
        }
    }
}

Projeto "MyApp_FR" (arquivo gerado automaticamente pelo Resources.Designer.cs)

namespace MyApp.Properties {
    // ...
    internal static string PleaseCallIT {
        get {
            return ResourceManager.GetString("PleaseCallIT", resourceCulture);
        }
    }
}

Eu não entendo ...