Create a Razor View at runtime from string content

I managed to go down the rabbit hole of a DynamicContentFileProvider with a degree of success so thought I would post it here for any interested parties. There are still some holes in it but it is working correctly as a proof of concept.

First I created a DynamicContentFileProvider

public class DynamicContentFileProvider : IFileProvider
{
    private readonly Regex PathSelector = new Regex(@"(dynamic)", RegexOptions.IgnoreCase);
    public IDirectoryContents GetDirectoryContents(string subpath)
    {
        return new NotFoundDirectoryContents();
    }

    public IFileInfo GetFileInfo(string subpath)
    {
        if (PathSelector.IsMatch(subpath.ToLower() ?? string.Empty))
        {
            var result = new DynamicContentFileInfo(subpath, "<p>My silly page - @Current.Channel.Id</p>");
            return result.Exists ? result as IFileInfo : new NotFoundFileInfo(subpath);
        }
        return new NotFoundFileInfo(subpath);
    }

    public IChangeToken Watch(string filter)
    {
        if (PathSelector.IsMatch(filter.ToLower() ?? string.Empty))
            return new DynamicContentChangeToken(true);
        return new DynamicContentChangeToken(false);
    }
}

This class implements IFileProvider. It has a Regex to match a specific path pattern. I will use this as a convention when generating "dynamic" type views. The important parts are the GetFileInfo call which creates and returns a DynamicContentFileInfo object. This object takes in a path (this is a psuedo path used to allow the view to be cached after compilation ) and it takes in the actual razor template as a string.( it's just hard-coded for now till I work out how to inject that part dynamically or fetch from a database, not sure )

Then I created a DynamicContentFileInfo

public class DynamicContentFileInfo : IFileInfo
{
    public string Name { get; set; }

    public string? PhysicalPath => null;

    public byte[] ViewContent { get; set; }
    public bool Exists { get; set; }

    public bool IsDirectory => false;

    public DateTimeOffset LastModified { get; set; }

    public DynamicContentFileInfo(string name, string viewContent)
    {
        Name = name;
        ViewContent = Encoding.UTF8.GetBytes(viewContent);
        LastModified = DateTimeOffset.Now;
        Exists = !string.IsNullOrEmpty(viewContent);
    }
    public long Length
    {
        get
        {
            using (var stream = new MemoryStream(this.ViewContent))
            {
                return stream.Length;
            }
        }
    }

    public Stream CreateReadStream()
    {
        return new MemoryStream(ViewContent);
    }
}

This class simply wraps the given razor template and masquerades as a file by wrapping it in a MemoryStream

Then I created a DynamicContentChangeToken

public class DynamicContentChangeToken : IChangeToken
{
    public bool ActiveChangeCallbacks => false;

    public bool HasChanged { get; set; }

    public IDisposable RegisterChangeCallback(Action<object?> callback, object? state) => EmptyDisposable.Instance;

    public DynamicContentChangeToken(bool hasChanged)
    {
        HasChanged = hasChanged;
    }

    internal class EmptyDisposable : IDisposable
    {
        public static EmptyDisposable Instance { get; } = new EmptyDisposable();
        private EmptyDisposable() { }
        public void Dispose() { }
    }
}

This class takes in a bool flag to force the content to be marked as "changed" or not. This is returned in the IChangeToken Watch(string filter) method call in the DynamicContentFileProvider class. I again use a regex to match on the path and then either force the change or not. This allows unmatched views to remain in the cache whilst forcing recompilation of the dynamic stuff ( I will smarten this up later )

I had to register my file providers in the startup process

.AddRazorRuntimeCompilation(opts =>
            {
                opts.FileProviders.Clear();
                opts.FileProviders.Add(new CompositeFileProvider(new ViewFileProvider(configuration), new DynamicContentFileProvider()));
            });

Then finally, in order to render my dynamic content I need to use a path convention of dynamic when rendering dynamic content and it all works out

return View($"/dynamic/{UniqueIdOfContentFromDb}");

What's missing?

1.Well I still need to work out how to get the content from the Controller/ViewComponent/Page into the `DynamicContentFileProvider'. I can use the key in the path and load from the database but that would mean a redundant call, so still pondering this 2.I need a cool way to invalidate the view cache when a user edits the template in the database. I figure I can use the ModifiedDate in the database and compare, but again, that's another database call on every render, so who knows.

For now it's working in principle, so happy days!