IHtmlHelper<Model> does not contain a definition for <Foo>

Question

I am migrating an application from ASP.NET 4.8 to ASP.NET Core 6.

I have the following in a ButtonExtensions class:

public static IHtmlContent HelpButton(this HtmlHelper htmlHelper, Enums.ButtonSize buttonSize = Enums.ButtonSize.Normal)
{
    var button = new TagBuilder("button");

    button.MergeAttribute("type", "button");
    button.MergeAttribute("aria-label", "Help Button");
    button.MergeAttribute("tabindex", "100");

    button.AddCssClass("btn");
    button.AddCssClass("btn-outline-info");
    button.AddCssClass("btn-light");
    button.AddCssClass("help-button");

    if (buttonSize == Enums.ButtonSize.Small)
        button.AddCssClass("btn-sm");
    else if (buttonSize == Enums.ButtonSize.Large)
        button.AddCssClass("btn-lg");

    return BuildButton("Help", BuildInternalIconTag("fa-question-circle"), button);
}

I try to use it in an Index.cshtml file as so:

@Html.HelpButton()

I have the following in a FormLabelExtensions class:

public static IHtmlContent FormLabelFor<TModel, TValue>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TValue>> expression, object htmlAttributes = null)
{
    return FormLabelFor(htmlHelper, expression, null, htmlAttributes);
}

private static IHtmlContent FormLabelFor<TModel, TValue>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TValue>> expression, string labelText, object htmlAttributes)
{
    var attributes = HtmlHelper.AnonymousObjectToHtmlAttributes(htmlAttributes);

    if (attributes.ContainsKey("class"))
        attributes["class"] = $"{attributes["class"]} col-form-label";
    else
        attributes.Add("class", "col-form-label");

    var modelExpressionProvider = htmlHelper.ViewContext?.HttpContext?.RequestServices?.GetService<ModelExpressionProvider>()
        ?? new ModelExpressionProvider(htmlHelper.MetadataProvider);

    var modelExpression = modelExpressionProvider.CreateModelExpression(htmlHelper.ViewData, expression);

    if (!modelExpression.Metadata.IsRequired)
        return htmlHelper.LabelFor(expression, labelText, attributes);

    if (attributes.ContainsKey("class"))
        attributes["class"] = $"{attributes["class"]} required";
    else
        attributes.Add("class", "required");

    return htmlHelper.LabelFor(expression, labelText, attributes);
}

I try to use it in an Index.cshtml file as so:

@Html.FormLabelFor(m => model)

Both give me the same error.

Answer 1

Looks like you just need to change HtmlHelper<TModel> to use the interface version of that IHtmlHelper<TModel> as this is the proper way in .NET Core.

For example:

public static IHtmlContent HelpButton(this IHtmlHelper htmlHelper, Enums.ButtonSize buttonSize = Enums.ButtonSize.Normal)
{
    ...
}