#if DEBUG for exception handling

Question

I have an open method ChangeLanguage, which is in the class library, which should be used by other people who do not know what the source code is, but know what it can do.

public string ChangeLanguage(string language) 
{
    #if DEBUG
        // Check if the new language is not null or an empty string.
        language.ThrowArgumentNullExceptionIfNullOrEmpty("language", GetString("0x000000069"));
    #else
        if (string.IsNullOrEmpty(language))
            language = _fallbackLanguage;
    #endif 
}

For me it was obvious to check whether the languagemethod was actually passed, and if not, issue an error to the developer. This is a very small performance loss, I think it’s better not to throw an exception, but just use a spare language when the language has not been provided.

I am wondering if this is a good approach to design, and if I continue to use it elsewhere in the library, like here:

    _appResDicSource = Path.Combine("\\" + _projectName + ";component", _languagesDirectoryName, _fileBaseName + "_" + language + ".xaml");
    _clsLibResDicSource = "\\Noru.Base;component\\Languages\\Language_" + language + ".xaml";

    ResourceDictionary applicationResourceDictionary;
    ResourceDictionary classLibraryResourceDictionary;

    try { applicationResourceDictionary = new ResourceDictionary { Source = new Uri(_appResDicSource, UriKind.RelativeOrAbsolute) }; }
    catch
    {
#if DEBUG
        throw new IOException(string.Format(GetString("1x00000006A"), _appResDicSource));
#else
        return ChangeLanguage(_fallbackLanguage);
#endif
    }
    try { classLibraryResourceDictionary = new ResourceDictionary { Source = new Uri(_clsLibResDicSource, UriKind.RelativeOrAbsolute) }; }
    catch
    {
#if DEBUG
        throw new IOException(string.Format(GetString("1x00000006B"), _clsLibResDicSource));
#else
        return ChangeLanguage(_fallbackLanguage);
#endif
    }

Answer 1

, Debug.Fail ( , DEBUG )

public string ChangeLanguage(string language) 
{
    if (string.IsNullOrEmpty(language))
    {
        Debug.Fail("language is NOK");
        language = _fallbackLanguage;
    }
}

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Answer 2

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