IsDevelopment()
.I'm using ASP.NET Core 3.1
I figured I'd try the new Razor Pages as they're advertised as super easy.
@page
@using MyProject.Pages.Pdf
@model IndexModel
<h2>Test</h2>
<p>
@Model.Message
</p>
namespace MyProject.Pages.Pdf
{
public class IndexModel : PageModel
{
private readonly MyDbContext _context;
public IndexModel(MyDbContext context)
{
_context = context;
}
public string Message { get; private set; } = "PageModel in C#";
public async Task<IActionResult> OnGetAsync()
{
var count = await _context.Foos.CountAsync();
Message += $" Server time is { DateTime.Now } and the Foo count is { count }";
return Page();
}
}
}
This works in the browser - yay!
I found Render a Razor Page to string which appears to do what I want.
But this is where the trouble start:(
First off, I find it very odd that when you find the page via _razorViewEngine.FindPage
it doesn't know how to populate the ViewContext
or the Model
. I'd think the job of IndexModel
was to populate these. I was hoping it was possible to ask ASP.NET for the IndexModel
Page and that would be that.
Anyway... the next problem. In order to render the Page I have to manually create a ViewContext
and I have to supply it with a Model
. But the Page is the Model, and since it's a Page it isn't a simple ViewModel. It rely on DI and it expects OnGetAsync()
to be executed in order to populate the Model. It's pretty much a catch-22.
I also tried fetching the View instead of the Page via _razorViewEngine.FindView
but that also requires a Model, so we're back to catch-22.
Another issue. The purpose of the debug/tweaking page was to easily see what was generated. But if I have to create a Model
outside IndexModel
then it's no longer representative of what is actually being generated in a service somewhere.
All this have me wondering if I'm even on the right path. Or am I missing something?
Please refer to the following steps to render A Partial View To A String:
Add an interface to the Services folder named IRazorPartialToStringRenderer.cs.
public interface IRazorPartialToStringRenderer { Task<string> RenderPartialToStringAsync<TModel>(string partialName, TModel model); }
Add a C# class file to the Services folder named RazorPartialToStringRenderer.cs with the following code:
using System; using System.IO; using System.Linq; 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.ViewEngines; using Microsoft.AspNetCore.Mvc.ViewFeatures; using Microsoft.AspNetCore.Routing; namespace RazorPageSample.Services { public class RazorPartialToStringRenderer: IRazorPartialToStringRenderer { private IRazorViewEngine _viewEngine; private ITempDataProvider _tempDataProvider; private IServiceProvider _serviceProvider; public RazorPartialToStringRenderer( IRazorViewEngine viewEngine, ITempDataProvider tempDataProvider, IServiceProvider serviceProvider) { _viewEngine = viewEngine; _tempDataProvider = tempDataProvider; _serviceProvider = serviceProvider; } public async Task<string> RenderPartialToStringAsync<TModel>(string partialName, TModel model) { var actionContext = GetActionContext(); var partial = FindView(actionContext, partialName); using (var output = new StringWriter()) { var viewContext = new ViewContext( actionContext, partial, new ViewDataDictionary<TModel>( metadataProvider: new EmptyModelMetadataProvider(), modelState: new ModelStateDictionary()) { Model = model }, new TempDataDictionary( actionContext.HttpContext, _tempDataProvider), output, new HtmlHelperOptions() ); await partial.RenderAsync(viewContext); return output.ToString(); } } private IView FindView(ActionContext actionContext, string partialName) { var getPartialResult = _viewEngine.GetView(null, partialName, false); if (getPartialResult.Success) { return getPartialResult.View; } var findPartialResult = _viewEngine.FindView(actionContext, partialName, false); if (findPartialResult.Success) { return findPartialResult.View; } var searchedLocations = getPartialResult.SearchedLocations.Concat(findPartialResult.SearchedLocations); var errorMessage = string.Join( Environment.NewLine, new[] { $"Unable to find partial '{partialName}'. The following locations were searched:" }.Concat(searchedLocations)); ; throw new InvalidOperationException(errorMessage); } private ActionContext GetActionContext() { var httpContext = new DefaultHttpContext { RequestServices = _serviceProvider }; return new ActionContext(httpContext, new RouteData(), new ActionDescriptor()); } } }
Register the services in the ConfigureServices
method in the Startup
class:
public void ConfigureServices(IServiceCollection services) { services.AddRazorPages(); services.AddTransient<IRazorPartialToStringRenderer, RazorPartialToStringRenderer>(); }
Using the RenderPartialToStringAsync() method to render Razor Page as HTML string:
public class ContactModel: PageModel { private readonly IRazorPartialToStringRenderer _renderer; public ContactModel(IRazorPartialToStringRenderer renderer) { _renderer = renderer; } public void OnGet() { } [BindProperty] public ContactForm ContactForm { get; set; } [TempData] public string PostResult { get; set; } public async Task<IActionResult> OnPostAsync() { var body = await _renderer.RenderPartialToStringAsync("_ContactEmailPartial", ContactForm); //transfer model to the partial view, and then render the Partial view to string. PostResult = "Check your specified pickup directory"; return RedirectToPage(); } } public class ContactForm { public string Email { get; set; } public string Message { get; set; } public string Name { get; set; } public string Subject { get; set; } public Priority Priority { get; set; } } public enum Priority { Low, Medium, High }
The debug screenshot as below:
More detail steps, please check this blog Rendering A Partial View To A String .
I managed to crack it. I was on the wrong path after all... the solution is to use a ViewComponent
. But it's still funky!
Thanks to
namespace MyProject.ViewComponents
{
public class MyViewComponent : ViewComponent
{
private readonly MyDbContext _context;
public MyViewComponent(MyDbContext context)
{
_context = context;
}
public async Task<IViewComponentResult> InvokeAsync()
{
var count = await _context.Foos.CountAsync();
var message = $"Server time is { DateTime.Now } and the Foo count is { count }";
return View<string>(message);
}
}
}
and the view is placed in Pages/Shared/Components/My/Default.cshtml
@model string
<h2>Test</h2>
<p>
@Model
</p>
using System;
using System.IO;
using System.Text.Encodings.Web;
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.Rendering;
using Microsoft.AspNetCore.Mvc.ViewEngines;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Routing;
public class RenderViewComponentService
{
private readonly IServiceProvider _serviceProvider;
private readonly ITempDataProvider _tempDataProvider;
private readonly IViewComponentHelper _viewComponentHelper;
public RenderViewComponentService(
IServiceProvider serviceProvider,
ITempDataProvider tempDataProvider,
IViewComponentHelper viewComponentHelper
)
{
_serviceProvider = serviceProvider;
_tempDataProvider = tempDataProvider;
_viewComponentHelper = viewComponentHelper;
}
public async Task<string> RenderViewComponentToStringAsync<TViewComponent>(object args)
where TViewComponent : ViewComponent
{
var viewContext = GetFakeViewContext();
(_viewComponentHelper as IViewContextAware).Contextualize(viewContext);
var htmlContent = await _viewComponentHelper.InvokeAsync<TViewComponent>(args);
using var stringWriter = new StringWriter();
htmlContent.WriteTo(stringWriter, HtmlEncoder.Default);
var html = stringWriter.ToString();
return html;
}
private ViewContext GetFakeViewContext(ActionContext actionContext = null, TextWriter writer = null)
{
actionContext ??= GetFakeActionContext();
var viewData = new ViewDataDictionary(new EmptyModelMetadataProvider(), new ModelStateDictionary());
var tempData = new TempDataDictionary(actionContext.HttpContext, _tempDataProvider);
var viewContext = new ViewContext(
actionContext,
NullView.Instance,
viewData,
tempData,
writer ?? TextWriter.Null,
new HtmlHelperOptions());
return viewContext;
}
private ActionContext GetFakeActionContext()
{
var httpContext = new DefaultHttpContext
{
RequestServices = _serviceProvider,
};
var routeData = new RouteData();
var actionDescriptor = new ActionDescriptor();
return new ActionContext(httpContext, routeData, actionDescriptor);
}
private class NullView : IView
{
public static readonly NullView Instance = new NullView();
public string Path => string.Empty;
public Task RenderAsync(ViewContext context)
{
if (context == null) { throw new ArgumentNullException(nameof(context)); }
return Task.CompletedTask;
}
}
}
Note there is no code behind file
@page
@using MyProject.ViewComponents
@await Component.InvokeAsync(typeof(MyViewComponent))
@page "{id}"
@using MyProject.ViewComponents
@await Component.InvokeAsync(typeof(MyViewComponent), RouteData.Values["id"])
[HttpGet]
public async Task<IActionResult> Get()
{
var html = await _renderViewComponentService
.RenderViewComponentToStringAsync<MyViewComponent>();
// do something with the html
return Ok(new { html });
}
[HttpGet("{id}")]
public async Task<IActionResult> Get([FromRoute] int id)
{
var html = await _renderViewComponentService
.RenderViewComponentToStringAsync<MyViewComponent>(id);
// do something with the html
return Ok(new { html });
}
It's very unfortunate that the injected IViewComponentHelper
doesn't work out of the box.
So we have do this very unintuitive thing to make it work.
(_viewComponentHelper as IViewContextAware).Contextualize(viewContext);
which causes a cascade of odd things like the fake ActionContext
and ViewContext
which require a TextWriter
but it isn't used for ANYTHING! In fact the hole ViewContext
isn't used at all. It just needs to exist:(
Also the NullView
... for some reason Microsoft.AspNetCore.Mvc.ViewFeatures.NullView
is Internal
so we basically have to copy/paste it into our own code.
Perhaps it'll be improved in the future.
Anyway: IMO this is simpler then using IRazorViewEngine
which turns up in pretty much every web search:)
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