HtmlSanitizer + ASP.NET Core 2 with DI

Question

When I use HtmlSanitizer without DI it works well.

HtmlSanitizer without DI:

But when I want to get HtmlSanitizer using DI.

1. I added to Startup.cs file:

   services.AddSingleton<IHtmlSanitizer, HtmlSanitizer>();
   

2. I used constructor of repository to get instance of IHtmlSanitizer but in the injected HtmlSanitizer instance, AllowedTags, and AllowAttributes are empty.

HtmlSanitizer with DI:

How can I get HtmlSanitizer with filled properties using DI?

Answer 1

The .Net framework dependency injection is trying to inject the optional constructor parameters

    public HtmlSanitizer(IEnumerable<string> allowedTags = null, IEnumerable<string> allowedSchemes = null,
        IEnumerable<string> allowedAttributes = null, IEnumerable<string> uriAttributes = null, IEnumerable<string> allowedCssProperties = null, IEnumerable<string> allowedCssClasses = null)
    {
        AllowedTags = new HashSet<string>(allowedTags ?? DefaultAllowedTags, StringComparer.OrdinalIgnoreCase);
        AllowedSchemes = new HashSet<string>(allowedSchemes ?? DefaultAllowedSchemes, StringComparer.OrdinalIgnoreCase);
        AllowedAttributes = new HashSet<string>(allowedAttributes ?? DefaultAllowedAttributes, StringComparer.OrdinalIgnoreCase);
        UriAttributes = new HashSet<string>(uriAttributes ?? DefaultUriAttributes, StringComparer.OrdinalIgnoreCase);
        AllowedCssProperties = new HashSet<string>(allowedCssProperties ?? DefaultAllowedCssProperties, StringComparer.OrdinalIgnoreCase);
        AllowedAtRules = new HashSet<CssRuleType>(DefaultAllowedAtRules);
        AllowedCssClasses = allowedCssClasses != null ? new HashSet<string>(allowedCssClasses) : null;
    }

Source

which results in empty collections being used by the DI container to initialize the target HtmlSanitizer class.

In this case, use the factory delegate when registering the class and call the constructor (just as was done when not using DI)

services.AddSingleton<IHtmlSanitizer>(_ => new HtmlSanitizer());

Or simply create the instance and register it with the DI container

IHtmlSanitizer sanitizer = new HtmlSanitizer();
services.AddSingleton<IHtmlSanitizer>(sanitizer);