RazorViewToStringRenderer.cs

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Mvc.Abstractions;
using Microsoft.AspNetCore.Mvc.ModelBinding;
using Microsoft.AspNetCore.Mvc.Razor;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.ViewEngines;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Routing;
using System;
using System.IO;
using System.Linq;
using System.Threading.Tasks;

/*
 *  Source: https://github.com/aspnet/Entropy/blob/dev/samples/Mvc.RenderViewToString/RazorViewToStringRenderer.cs
 */

namespace RazorHtmlEmail.lib
{

    public class RazorViewToStringRenderer : IRazorViewToStringRenderer
    {
        private IRazorViewEngine _viewEngine;
        private ITempDataProvider _tempDataProvider;
        private readonly IHttpContextAccessor _httpContextAccessor;

        public RazorViewToStringRenderer(
            IRazorViewEngine viewEngine,
            ITempDataProvider tempDataProvider,
            IHttpContextAccessor httpContextAccessor)
        {
            _viewEngine = viewEngine;
            _tempDataProvider = tempDataProvider;
            _httpContextAccessor = httpContextAccessor;
        }

        public async Task<string> RenderViewToStringAsync<TModel>(string viewName, TModel model)
        {
            var actionContext = GetActionContext();
            var view = FindView(actionContext, viewName);

            using (var output = new StringWriter())
            {
                var viewContext = new ViewContext(
                    actionContext,
                    view,
                    new ViewDataDictionary<TModel>(
                        metadataProvider: new EmptyModelMetadataProvider(),
                        modelState: new ModelStateDictionary())
                    {
                        Model = model
                    },
                    new TempDataDictionary(
                        _httpContextAccessor.HttpContext,
                        _tempDataProvider),
                    output,
                    new HtmlHelperOptions());
                viewContext.RouteData = _httpContextAccessor.HttpContext.GetRouteData();

                await view.RenderAsync(viewContext);

                return output.ToString();
            }
        }

        private IView FindView(ActionContext actionContext, string viewName)
        {
            ViewEngineResult getViewResult = _viewEngine.GetView(executingFilePath: null, viewPath: viewName, isMainPage: true);
            if (getViewResult.Success)
            {
                return getViewResult.View;
            }

            var findViewResult = _viewEngine.FindView(actionContext, viewName, isMainPage: true);
            if (findViewResult.Success)
            {
                return findViewResult.View;
            }

            var searchedLocations = getViewResult.SearchedLocations.Concat(findViewResult.SearchedLocations);
            var errorMessage = string.Join(
                Environment.NewLine,
                new[] { $"Unable to find view '{viewName}'. The following locations were searched:" }.Concat(searchedLocations)); ;

            throw new InvalidOperationException(errorMessage);
        }

        private ActionContext GetActionContext()
        {
            return new ActionContext(_httpContextAccessor.HttpContext, new RouteData(), new ActionDescriptor());
        }

    }

    public interface IRazorViewToStringRenderer
    {
        Task<string> RenderViewToStringAsync<TModel>(string viewName, TModel model);
    }

}