Validate HTML using Fluent Assertions

In this case, it makes sense to test the validity of the returned value as part of the automated tests. As I’m a big fan of Fluent Assertions and use them for all my assertions, I created a simple extension to validate the HTML using HtmlAgilityPack.

Assertion Class

public class HtmlStringAssertions : ReferenceTypeAssertions<HtmlString, HtmlStringAssertions> {

    public HtmlStringAssertions(HtmlString instance) {
        Subject = instance;
    }

    protected override string Identifier => "HtmlString";

    public AndConstraint<HtmlStringAssertions> BeValidHtml(string because = "", params object[] becauseArgs) {
        Execute.Assertion
            .BecauseOf(because, becauseArgs)
            .Given(() => {
                var htmlDocument = new HtmlDocument();
                htmlDocument.LoadHtml(Subject.ToString());

                return htmlDocument;
            })
            .ForCondition(htmlDocument => !htmlDocument.ParseErrors.Any())
            .FailWith("Expected {context:HtmlString} to contain valid HTML{reason}.");

        return new AndConstraint<HtmlStringAssertions>(this);
    }

}

Extension Method

public static class HtmlStringExtensions  {

    public static HtmlStringAssertions Should(this HtmlString instance) {
        return new HtmlStringAssertions(instance);
    }

}

Usage

// ...
var htmlString = myHelper.CreateSomeMarkup(param1, param2);

htmlString.Should().BeValidHtml();