Render View to String caching issue

Question

For my project I currently implemented a ViewRender for my asp.net core application. It generates Views without a controller to html, this works fine using the following code:

public class ViewRenderService : IViewRenderService
{
    private readonly IRazorViewEngine _razorViewEngine;
    private readonly ITempDataProvider _tempDataProvider;
    private readonly IServiceProvider _serviceProvider;

    public ViewRenderService(IRazorViewEngine razorViewEngine,
        ITempDataProvider tempDataProvider,
        IServiceProvider serviceProvider)
    {
        _razorViewEngine = razorViewEngine;
        _tempDataProvider = tempDataProvider;
        _serviceProvider = serviceProvider;
    }

    public async Task<string> RenderToStringAsync(string viewName, object model)
    {
        var httpContext = new DefaultHttpContext { RequestServices = _serviceProvider };

        var actionContext = new ActionContext(httpContext, new RouteData(), new ActionDescriptor());
        string viewgerendered = "";
        try
        {
            using (var sw = new StringWriter())
            {
                var viewResult = _razorViewEngine.GetView(viewName, viewName, false);

                if (viewResult.View == null)
                {
                    throw new ArgumentNullException($"{viewName} does not match any available view");
                }

                var viewDictionary = new ViewDataDictionary(new EmptyModelMetadataProvider(), new ModelStateDictionary())
                {
                    Model = model
                };        

                var viewContext = new ViewContext(
                    actionContext,
                    viewResult.View,
                    viewDictionary,
                    new TempDataDictionary(actionContext.HttpContext, _tempDataProvider),
                    sw,
                    new HtmlHelperOptions()
                );

                await viewResult.View.RenderAsync(viewContext);

                viewgerendered = sw.ToString();
                return viewgerendered;
            }
        }
        catch (Exception e)
        {
            object temp = e.Message + " - " + e.StackTrace;
            return temp.ToString();
        }
    }

    public Task RenderToStringAsync(string v)
    {
        throw new NotImplementedException();
    }
}

Source: https://ppolyzos.com/2016/09/09/asp-net-core-render-view-to-string/

Changes which are made to views which use this renderer are not updated without restarting the application itself. Diving further into it, the views are cached. A comment within the source mentions using the _razorViewEngine.GetView method should get rid of my caching issue. However this doesn't work.

What I got, trying to figure out a way to register a new ViewRender, with a slight modification of the ViewRenderService.

//Seems not to be available on asp.net core 2.0...
services.AddMvc().Configure<MvcViewOptions>(options =>
            {
                options.ViewEngines.Clear();
                options.ViewEngines.Add(typeof(CustomViewEngine));
            });

And to overload the RazorViewEngine to expose the ViewLookupCache, where supposedly the view cache is located.

  public class CustomViewEngine : RazorViewEngine
    {
        public CustomViewEngine(
            IRazorPageFactoryProvider pageFactory, 
            IRazorPageActivator pageActivator,
            HtmlEncoder htmlEncoder, 
            IOptions<RazorViewEngineOptions> optionsAccessor, 
            Microsoft.AspNetCore.Razor.Language.RazorProject razorProject, 
            ILoggerFactory loggerFactory, 
            System.Diagnostics.DiagnosticSource diagnosticSource) : 
            base(pageFactory, pageActivator, htmlEncoder, optionsAccessor,razorProject,loggerFactory, diagnosticSource){ }

        public void RemoveCachedView(string view)
        { 
            this.ViewLookupCache.Remove(view);
        }
    }

There's not a lot to find on how caching is done within asp.net core 2.0 for views and clearing a particular view / set of. Basically I want to find a way how I can flush an entire selection of cached views as a command, for performance reasons.

Edit 13-04-2018

As suggested by K Finley, I tried emptying the ViewLookupCache as suggested. The code in short;

In my startup.cs ConfigureServices (not entirely sure if this is how a custom viewengine is registered).

    services.AddSingleton<IRazorViewEngine, CustomViewEngine>();
    services.AddSingleton<IViewRenderService, ViewRenderService>();

The custom view engine:

public class CustomViewEngine : RazorViewEngine
{
    public CustomViewEngine(
        IRazorPageFactoryProvider pageFactory,
        IRazorPageActivator pageActivator,
        HtmlEncoder htmlEncoder,
        IOptions<RazorViewEngineOptions> optionsAccessor,
        Microsoft.AspNetCore.Razor.Language.RazorProject razorProject,
        ILoggerFactory loggerFactory,
        System.Diagnostics.DiagnosticSource diagnosticSource) :
        base(pageFactory, pageActivator, htmlEncoder, optionsAccessor, razorProject, loggerFactory, diagnosticSource)
    { }


    public void RemoveViewFromCache(string viewName, string controller, bool isLayout, bool isPartial = false, string pageName = null, string areaName = null)
    {   
        var key = new ViewLocationCacheKey(viewName, controller, areaName, pageName, !isLayout | !isPartial, isLayout ? null : new Dictionary<string, string>(StringComparer.Ordinal));        
        base.ViewLookupCache.Remove(key);
    }

    public void RemoveViewFromCache(string viewName, bool isLayout)
    {
        //Code uses this one
        var key = new ViewLocationCacheKey(viewName, isLayout);  
        base.ViewLookupCache.Remove(key);    
    }
}

And modified the original ViewRenderService...

    public class ViewRenderService : IViewRenderService
    {
        private CustomViewEngine _razorViewEngine;
        private ITempDataProvider _tempDataProvider;
        private IServiceProvider _serviceProvider;
        private IHostingEnvironment _hostingEnvironment;


        public ViewRenderService(IRazorViewEngine razorViewEngine,
            ITempDataProvider tempDataProvider,
            IServiceProvider serviceProvider, 
            IHostingEnvironment hostingEnvironment)
        {
            _razorViewEngine = (CustomViewEngine)razorViewEngine;
...

try
            {
                using (var sw = new StringWriter())
                {
                    _razorViewEngine.RemoveViewFromCache(viewName, false);
                    var viewResult = _razorViewEngine.GetView(viewName, viewName, false);

These modifications do delete the ViewLookupCache using the second method. However it still doesn't properly update my views. I do have to note the views don't have their own controller.

Answer 1

You need to enable file watcher:
Add the environment variable DOTNET_USE_POLLING_FILE_WATCHER=true or ENV DOTNET_USE_POLLING_FILE_WATCHER=true in the Dockerfile.