C# how do I force text to display in a text box during loop?

The problem is that your long-running method is blocking the UI thread. I would suggest that the solution is to use the Task-based Asynchronous Pattern.You need to run your long-running method on its own thread and get it to report its progress to the UI thread in the form of a string. You can then stick the returned string into a TextBlock. Here's a demo console project to show you the basic idea.

using System;
using System.Threading;
using System.Threading.Tasks;

namespace DemoAsync
{
    // To get async Main to run you may need to change the language version
    //In Solution Explorer/Properties/Build/Advanced 
    //Select 'C# latest supported minor version (latest)' in the Language Version list
    class Program
    {
        private static async Task Main()
        {
            var progress = new Progress<string>();
            //Define the handler. It will be run on the UI  thread
            //In this example the handler is writing the text to the console
            //But you can put it in a TextBox
            progress.ProgressChanged +=
                 ((sender, text) => Console.WriteLine(text));
            await Task.Run(() => MyLongRunningMethod(progress));
            Console.WriteLine("Hit 'return' to finish");
            Console.ReadLine();

        }

        private static void MyLongRunningMethod(IProgress<string> progress)
        {
            int max = 10;
            //simulation of a long-running method
            //It reports progress every second
            for (int i = 1; i <= max; i++)
            {
                Thread.Sleep(TimeSpan.FromSeconds(1));

                progress.Report(i.ToString());
            }


        }
    }
}