c#asp.net-mvc asp.net-core razor html-helper

ASP .NET Core - Extending @Html.EditorFor to append <span>

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 <input> within a and appends a <span> after the <input> 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 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 .NET 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) but I am unsure - since I can find no documentation.

Solution

For .NET Core, you need to migrate from MvcHtmlString which only supports .NET Framework MVC to IHtmlContent.

The implementation is almost similar, just need to migrate following:

No	.NET Framework MVC	.NET Core MVC
1	`HtmlMvcString`	`IHtmlContent`
2	Append HTML with `<Tag Builder instance>.InnerHtml += <HTML string>`	Apend HTML with `<Tag Builder instance>.InnerHtml.AppendHtml(<HTML string>)`
3	Retrieve parsed HTML from `Html.Editor()` with `htmlText`	Retrieve parsed HTML from `Html.Editor()` by using `StringWriter`
4	Return value with `MvcHtmlString` instance	Return value with `HtmlString` instance

using Microsoft.AspNetCore.Html;
using Microsoft.AspNetCore.Mvc.Rendering;
using System.IO;
using System.Text.Encodings.Web;

public static class HtmlHelperEditorExtensions
{
    public static IHtmlContent Addon(this IHtmlContent htmlContent, string text, string classes)
    {
        var wrapper = new TagBuilder("div");
        wrapper.AddCssClass("input-group");
        wrapper.AddCssClass(classes);

        using (var writer = new StringWriter())
        {
            htmlContent.WriteTo(writer, HtmlEncoder.Default);
            wrapper.InnerHtml.AppendHtml(writer.ToString());
        }

        var span = new TagBuilder("span");
        span.AddCssClass("input-group-addon");
        span.InnerHtml.AppendHtml(text);

        wrapper.InnerHtml.AppendHtml(span);

        using (var writer = new StringWriter())
        {
            wrapper.WriteTo(writer, HtmlEncoder.Default);
            return new HtmlString(writer.ToString());
        }
    }
}