IRazorViewEngine.FindView with GetView can't find view

Though I was building things from scratch in Core 3.1 and not upgrading from an earlier version, I ran into the same issue. I got things working by the doing the following:

I created an implementation of IWebHostEnvironment (I called mine DummyWebHostEnvironment.cs). I left all but one of the interface's properties with the default implementation; for that one property, I used the name of the project containing the views. (I just hardcoded it into the sample below for brevity; there are obviously slicker ways to obtain it.)

 public class DummyWebHostEnvironment : IWebHostEnvironment
    {
        public IFileProvider WebRootFileProvider { get => throw new System.NotImplementedException(); set => throw new System.NotImplementedException(); }
        public string WebRootPath { get => throw new System.NotImplementedException(); set => throw new System.NotImplementedException(); }
        public string ApplicationName { get => "TheProjectContainingMyViews.RazorClassLibrary"; set => throw new System.NotImplementedException(); }
        public IFileProvider ContentRootFileProvider { get => throw new System.NotImplementedException(); set => throw new System.NotImplementedException(); }
        public string ContentRootPath { get => throw new System.NotImplementedException(); set => throw new System.NotImplementedException(); }
        public string EnvironmentName { get => throw new System.NotImplementedException(); set => throw new System.NotImplementedException(); }
}

Note: As is evident from the above code, the project containing the Views is a RazorClassLibrary. (I was using this and this as guidesfor getting the RazorViewEngine to work in a console application.)

One I had the implementation above, I added it to my services collection along with some other goodies:

private static RazorViewToStringRenderer GetRenderer()
        {
            var services = new ServiceCollection();
            var applicationEnvironment = PlatformServices.Default.Application;
            services.AddSingleton(applicationEnvironment);

            var appDirectory = Directory.GetCurrentDirectory();
    
            var environment = new DummyWebHostEnvironment();
            services.AddSingleton<IWebHostEnvironment>(environment);

            services.AddSingleton<ObjectPoolProvider, DefaultObjectPoolProvider>();

            var diagnosticSource = new DiagnosticListener("Microsoft.AspNetCore");
            services.AddSingleton<DiagnosticSource>(diagnosticSource);
            services.AddSingleton<DiagnosticListener>(diagnosticSource);

            services.AddLogging();
            services.AddMvc();
            services.AddSingleton<RazorViewToStringRenderer>();
            var provider = services.BuildServiceProvider();
            return provider.GetRequiredService<RazorViewToStringRenderer>();
        }

Note: See the first of the links above for the code for RazorViewToStringRenderer. Here's the interface:

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

Then, in Program.cs, I can just do something like this:

static async Task Main(string[] args)
        {
            var dto = BuildDto();
            var renderer = GetRenderer();

            var renderedString = await renderer.RenderViewToStringAsync("Views/Path/To/Some.cshtml", dto);
// ...

        }

I had the same issue. Although I am writing in .NET Core 5 already. I am assuming you are writing based on this or similar solution: https://scottsauber.com/2018/07/07/walkthrough-creating-an-html-email-template-with-razor-and-razor-class-libraries-and-rendering-it-from-a-net-standard-class-library/

You have

var getViewResult = _viewEngine.GetView(executingFilePath: null, viewPath: viewName, isMainPage: true);

where executingFilePath is null.

Add executingFilePath so it leads to the view location on disk.

In my solution I have:

var getViewResult = _viewEngine.GetView(executingFilePath: executingFilePath, viewPath: viewName, isMainPage: true);

where executingFilePath is passed to RenderViewToStringAsync as additional parameter:

public class MessageBodyBuilderService : IMessageBodyBuilderService
{
    private readonly IRazorViewToStringRenderer _razorViewToStringRenderer;
    private readonly IWebHostEnvironment _hostingEnv;
    private readonly string _pathToEmailTemplates = $"/Views/EmailTemplates/";

    public MessageBodyBuilderService(
        IWebHostEnvironment hostingEnv, 
        IRazorViewToStringRenderer razorViewToStringRenderer)
    {
        _hostingEnv = hostingEnv;
        _razorViewToStringRenderer = razorViewToStringRenderer;
    }

    public async Task<BodyBuilder> BuildMessage<T>(string templateName, T modelForReplacement, bool isHtml = true)
    {
        string viewName = $"{_pathToEmailTemplates}{templateName}.cshtml";
        string body = await _razorViewToStringRenderer.RenderViewToStringAsync(viewName, modelForReplacement, _hostingEnv.ContentRootPath);

        var builder = new BodyBuilder()
        {
            HtmlBody = body
        };

        return builder;
    }
}

where _hostingEnv.ContentRootPath comes from the ContentRootPath I declared on Startup:

AppDomain.CurrentDomain.SetData("ContentRootPath", webHostEnvironment.ContentRootPath);

and then you can pass executingFilePath (in your RazorViewToStringRenderer's RenderViewToStringAsync method) to FindView as additional parameter:

var view = FindView(executingFilePath, actionContext, viewName);

I hope it helps.