I am porting an old .Net 4.8 app to .Net core and the old app has custom code which extends the @Html.EditorFor to wrap the within a and appends a after the tag (basically to add a “%” sign after the input field). I have added the older code below.

        public static MvcHtmlString Addon(this MvcHtmlString htmlText, string text, string classes)
        {
            var wrapper = new TagBuilder("div");
            wrapper.AddCssClass("input-group");
            wrapper.AddCssClass(classes);
            wrapper.InnerHtml = htmlText.ToHtmlString();
            var span = new TagBuilder("span");
            span.AddCssClass("input-group-addon");
            span.InnerHtml = text;
            wrapper.InnerHtml += span.ToString();

            return new MvcHtmlString(wrapper.ToString());
        }

The above code is used in conjunction with the @Html.EditorFor as below.

@Html.EditorFor(model => model.PercentageOfControlledSubstances, new { htmlAttributes = new { @class = "form-control short-textfield percent" } }).**Addon**("%", "med-textfield")

When compiling I get the error an error stating -> “Reference to type ‘HtmlString’ claims it is defined in ‘System.Web’, but it could not be found”.

Below is a screenshot of what the final rendering should look like.

tag followed by

I know this is a conversion from 4.8 to .Net Core and I have been searching how to convert my code but cannot find documentation on how to do this in .Net Core. After a lot of searching it looks like .Net Core uses either IHtmlContent or HtmlHelper(s) for this type coding but I am not sure – since I can find no documentation.