public async Task ProcessAsync_ItemAlreadyHasTitleThrowsInvalidOperationException() { // Arrange var panelContext = new PanelContext(); panelContext.TrySetTitle(content: new HtmlString("Existing title")); var context = new TagHelperContext( tagName: "govuk-panel-title", allAttributes: new TagHelperAttributeList(), items: new Dictionary <object, object>() { { typeof(PanelContext), panelContext } }, uniqueId: "test"); var output = new TagHelperOutput( "govuk-panel-title", attributes: new TagHelperAttributeList(), getChildContentAsync: (useCachedResult, encoder) => { var tagHelperContent = new DefaultTagHelperContent(); tagHelperContent.SetHtmlContent("Title"); return(Task.FromResult <TagHelperContent>(tagHelperContent)); }); var tagHelper = new PanelTitleTagHelper(); // Act & Assert var ex = await Assert.ThrowsAsync <InvalidOperationException>(() => tagHelper.ProcessAsync(context, output)); Assert.Equal("Cannot render <govuk-panel-title> here.", ex.Message); }