A template service based on the ASP.NET Core razor engine

ITemplateService.cs

/// <summary>
/// Renders email content based on razor templates
/// </summary>
public interface ITemplateService
{
    /// <summary>
    /// Renders a template given the provided view model
    /// </summary>
    /// <typeparam name="TViewModel"></typeparam>
    /// <param name="filename">Filename of the template to render</param>
    /// <param name="viewModel">View model to use for rendering the template</param>
    /// <returns>Returns the rendered template content</returns>
    Task<string> RenderTemplateAsync<TViewModel>(string filename, TViewModel viewModel);
}

TemplateService.cs

public class TemplateService: ITemplateService
{
    private IRazorViewEngine _viewEngine;
    private readonly IServiceProvider _serviceProvider;
    private readonly ITempDataProvider _tempDataProvider;

    public TemplateService(IRazorViewEngine viewEngine, IServiceProvider serviceProvider, ITempDataProvider tempDataProvider)
    {
        _viewEngine = viewEngine;
        _serviceProvider = serviceProvider;
        _tempDataProvider = tempDataProvider;
    }

    public async Task<string> RenderTemplateAsync<TViewModel>(string filename, TViewModel viewModel)
    {
        var httpContext = new DefaultHttpContext
        {
            RequestServices = _serviceProvider
        };

        var actionContext = new ActionContext(httpContext, new RouteData(), new ActionDescriptor());

        using (var outputWriter = new StringWriter())
        {
            var viewResult = _viewEngine.FindView(actionContext, filename, false);
            var viewDictionary = new ViewDataDictionary<TViewModel>(new EmptyModelMetadataProvider(), new ModelStateDictionary())
            {
                Model = viewModel
            };
            
            var tempDataDictionary = new TempDataDictionary(httpContext, _tempDataProvider);

            if (!viewResult.Success)
            {
                throw new TemplateServiceException($"Failed to render template {filename} because it was not found.");
            }

            try
            {
                var viewContext = new ViewContext(actionContext, viewResult.View, viewDictionary,
                    tempDataDictionary, outputWriter, new HtmlHelperOptions());

                await viewResult.View.RenderAsync(viewContext);
            }
            catch (Exception ex)
            {
                throw new TemplateServiceException("Failed to render template due to a razor engine failure", ex);
            }

            return outputWriter.ToString();
        }
    }
}

TemplateServiceException.cs

/// <summary>
/// Gets thrown when the template service was unable to render the template
/// </summary>
public class TemplateServiceException: Exception
{
    /// <summary>
    /// Initializes a new instance of <see cref="TemplateServiceException"/>
    /// </summary>
    /// <param name="message"></param>
    public TemplateServiceException(string message) : base(message)
    {
    }

    /// <summary>
    /// Initializes a new instance of <see cref="TemplateServiceException"/>
    /// </summary>
    /// <param name="message"></param>
    /// <param name="innerException"></param>
    public TemplateServiceException(string message, Exception innerException) : base(message, innerException)
    {
    }
}

For anyone that comes across this and is wondering why their models are null in the view, causing exceptions, just add this line in, after line 26 in TemplateService.cs:

viewDictionary.Model = viewModel;

Thanks. It's working.

This works great if I run it with dotnet run or inside of visual studio. But if I publish the app I get an error that the "Template" was not found. Using .net core 2.0 the views will be pre-compiled. Do I need to copy the files separately?

[Edit]
Okay, so it seems that it will not work with PreCompiled views. Had to add the following to my csproj

<MvcRazorCompileOnPublish>false</MvcRazorCompileOnPublish>

Nice code, worked perfectly. I just changed a little to use a list of viewmodels. Thanks

In rendering the view I use Viewbag and javascript in the view. Is that something that can be used as well and how do I specify those?
any thoughts? ideas?