ViewRenderService.cs

using System;
using System.IO;
using System.Threading.Tasks;

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.ViewFeatures;
using Microsoft.AspNetCore.Routing;

public interface IViewRenderService
{
    Task<string> RenderToStringAsync(string viewName, object model);
}

public class ViewRenderService : IViewRenderService
{
    private readonly IRazorViewEngine razorViewEngine;
    private readonly ITempDataProvider tempDataProvider;
    private readonly IServiceProvider serviceProvider;

    public ViewRenderService(
        IRazorViewEngine razorViewEngine,
        ITempDataProvider tempDataProvider,
        IServiceProvider serviceProvider)
    {
        this.razorViewEngine = razorViewEngine;
        this.tempDataProvider = tempDataProvider;
        this.serviceProvider = serviceProvider;
    }

    public async Task<string> RenderToStringAsync(string viewName, object model)
    {
        var httpContext = new DefaultHttpContext { RequestServices = this.serviceProvider };
        var actionContext = new ActionContext(httpContext, new RouteData(), new ActionDescriptor());

        using (var sw = new StringWriter())
        {
            var viewResult = this.razorViewEngine.GetView(null, 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, this.tempDataProvider),
                sw,
                new HtmlHelperOptions());

            await viewResult.View.RenderAsync(viewContext);
            return sw.ToString();
        }
    }
}