Asp.Net MVC Core built-in tag helpers won't process attributes provided from custom taghelpers - asp.net-mvc

Question

Out of boredom of writing the same boilerplate forms, I thought I'd rather write a tag helper that would produce attributes that can be processed from stock tag helpers. But, even though I managed to get my tag helper before work before form tag helpers, form tag helpers won't process the ones that I produce.
Here is the cshtml:
#model City
#{
Layout = "_Layout";
ViewData["Title"] = "Create City";
}
<form method="post" asp-action="Create">
#foreach (string propName in BaseModel.GetProperties<City>()) {
<formgroup for="#propName" />
}
<div class="form-group">
<label asp-for="Name">Name:</label>
<input class="form-control" asp-for="Name" />
</div>
<div class="form-group">
<label asp-for="Country">Country:</label>
<input class="form-control" asp-for="Country" />
</div>
<div class="form-group">
<label asp-for="Population">Population:</label>
<input class="form-control" asp-for="Population" />
</div>
<button type="submit" class="btn btn-primary">Create</button>
<a class="btn btn-secondary" asp-controller="Home" asp-action="Index">Cancel</a>
</form>
Here is the output:
<form method="post" action="/City/Create">
<div class="form-group"><label asp-for="Name"></label><input asp-for="Name" class="form-control"></div>
<div class="form-group"><label asp-for="Country"></label><input asp-for="Country" class="form-control"></div>
<div class="form-group"><label asp-for="Population"></label><input asp-for="Population" class="form-control"></div>
<div class="form-group">
<label for="Name">Name:</label>
<input class="form-control" type="text" id="Name" name="Name" value="">
</div>
<div class="form-group">
<label for="Country">Country:</label>
<input class="form-control" type="text" id="Country" name="Country" value="">
</div>
<div class="form-group">
<label for="Population">Population:</label>
<input class="form-control" type="number" data-val="true" data-val-required="The Population field is required." id="Population" name="Population" value="">
</div>
<button type="submit" class="btn btn-primary">Create</button>
<a class="btn btn-secondary" href="/">Cancel</a>
<input name="__RequestVerificationToken" type="hidden" value="CfDJ8M_6usK6CRRNkwluTiTW8uaAAfhMcU9tAxyCT7z55zQKmpUwpi_lfSDIN4FrlMo9cE3Ka9zgX4WdpXHUdlBFVGsLIw7h_cR3FjJb6Vjqnjm8mQmtKTey_9l188p9E2sKgiksO_OB6K9-F1D7SP2lX0g"></form>
Here is my tag helper:
using Microsoft.AspNetCore.Html;
using Microsoft.AspNetCore.Mvc.Rendering;
using Microsoft.AspNetCore.Mvc.ViewFeatures;
using Microsoft.AspNetCore.Razor.TagHelpers;
using System;
using System.Collections.Generic;
using System.Reflection;
using System.Threading.Tasks;
namespace ViewComponents.Infrastructure.TagHelpers {
[HtmlTargetElement("formgroup", Attributes = "for", TagStructure = TagStructure.NormalOrSelfClosing)]
public class FormGroupTagHelper : TagHelper {
/// <inheritdoc />
public override int Order => -2000;
[HtmlAttributeNotBound]
[ViewContext]
public ViewContext ViewContext { get; set; }
protected IHtmlGenerator Generator { get; }
public string For { get; set; }
public override async Task ProcessAsync(TagHelperContext context, TagHelperOutput output) {
if (context == null) {
throw new ArgumentNullException(nameof(context));
}
if (output == null) {
throw new ArgumentNullException(nameof(output));
}
// So that other tag helpers are processed after me...
var childContent = await output.GetChildContentAsync();
// Starting up...
// Replace the tag name, include the form-group bootstrap class.
output.TagName = "div";
output.TagMode = TagMode.StartTagAndEndTag;
output.Attributes.Add("class", "form-group");
PropertyInfo propInfo = ViewContext.ViewData.ModelMetadata.ModelType.GetTypeInfo().GetDeclaredProperty(For);
output.Content.AppendHtml(GenerateLabel(new Dictionary<string, string> { ["asp-for"] = propInfo.Name }));
output.Content.AppendHtml(GenerateInput(new Dictionary<string, string> { ["asp-for"] = propInfo.Name, ["class"] = "form-control" }));
}
public static IHtmlContent GenerateLabel(IReadOnlyDictionary<string, string> attrDict) {
TagBuilder tBuilder = new TagBuilder("label");
foreach (var kvp in attrDict)
tBuilder.Attributes.Add(kvp);
return tBuilder;
}
public static IHtmlContent GenerateInput(IReadOnlyDictionary<string, string> attrDict) {
TagBuilder tBuilder = new TagBuilder("input");
foreach (var kvp in attrDict)
tBuilder.Attributes.Add(kvp);
return tBuilder;
}
}
}
Any help will be appreciated.
Have a nice one!

Answer 1

Tag Helpers aren't processed in a specific order, so you cannot count on one being able to do something based on the output of the other. Each tag helper needs to be designed to work independently of any other, so if you need to render HTML, you'll need to handle that yourself in your tag helper. However, since you're trying to have one tag generate any kind of form field, that's going to be a heavy lift. This is basically beyond the scope of what a tag helper is intended for.