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public IActionResult Index()
    {

        ViewBag.Test = "Hello world!!!";
        string htmlContent = "<html><head><title>Test page</title><body>@ViewBag.Test</body></html>";

        return GetViewFromString(htmlContent);
    }

services.SetupOptions<RazorViewEngineOptions>(options =>
{
    var expander = new LanguageViewLocationExpander(
        context => context.HttpContext.Request.Query["language"]);
    options.ViewLocationExpanders.Insert(0, expander);
});