How to get user input from a textarea using C#

var mystr = document.getElementById(id).value;

you can put that in a javascript function then gets called on onlcick (like a submit button) and or on the textarea as OnTextChanged.

You have to provide the textarea's Id or use the Html helper method to get it. Here is a working example:

using System.IO;
using System.Security;
using System.Text.RegularExpressions;
using System.Web.Mvc;
using System.Windows.Forms;
using System.Xml.Linq;
using System;

@using System.IO;
using Microsoft.AspNetCore.Mvc;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using System;
public class HomeController : Controller
{

    IConfiguration Configuration { get; }

    public HomeController(IConfiguration configuration) => Configuration = configuration;

    public IActionResult Index() => View();

    // Here you can specify the path of the html file and a text for it in this case will be used to grab the value of an input text area named 'Text'. But it works as well if you want to grab an input from a button and save it as another one in a new html file or somewhere else if you want to do something like that
    [HttpPost]
    public IActionResult Index(string path, string text)
    {
        try
        {
            var files = Directory.GetFiles(path);
        }
        catch (DirectoryNotFoundException)
        {
            return View("Error");
        }
        catch (UnauthorizedAccessException)
        {
            return View("Error");
        }
        catch (ArgumentNullException)
        {
            return View("Error");
        }
        catch (ArgumentException)
        {
            return View("Error");
        }
        catch (PathTooLongException)
        {
            return View("Error");
        }
        catch (NotSupportedException)
        {
            return View("Error");
        }
        catch (SecurityException)
        {
            return View("Error");
        }

        // Here you can get the value of your textarea and save it for further use
        var input = Request.Form["Text"]; // This is an example

        foreach (var file in files)
        { // Here you can loop through all the files, edit them or delete them
            if (file.Contains(".cshtml"))
            { // You can just look for a specific file extension here if you want or loop through all the files in your directory, edit or delete any file
                var htmlDocument = new HtmlDocument();
                htmlDocument.LoadHtml(File.ReadAllText(file));
                var formElement = htmlDocument.GetElementbyId("example-form").OuterHtml;
                formElement = Regex.Replace(formElement,
                $"<input name=\"Text\" type=\"text\" value=\"\s*?.*?\" />",
                $"<input name=\"Text\" type=\"text\" value=\"{input}\" />");

                var streamWriter = new StreamWriter(file); // Replace the content of the file with a string created on the previous line
                streamWriter.WriteLineAsync(formElement);
            }
        }
        return View("Index"); // Return your view to be called after saving or editing a file or not depending on what you are trying to do.
    }

}