.NET 4.x Bitmap Недопустимый параметр (String filename) - C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
 
using System.Threading.Tasks;
using System.Threading;
using System.IO;
 
namespace DataParallelismWithForEach
{
    public partial class MainForm : Form
    {
        public MainForm()
        {
            InitializeComponent();
        }
      
        private void ProcessFiles()
        {
            string[] files = Directory.GetFiles(@"G:\source", "*.jpg", SearchOption.AllDirectories);
            string newDir = @"G:\ModifiPictures";
            Directory.CreateDirectory(newDir);
 
            foreach (string currentFile in files)
            {
                string filename = Path.GetFileName(currentFile);
 
                try
                {
                    Bitmap bitmap = new Bitmap((String)filename);
                }
                catch (Exception ex)
                {
                    textBox1.Text += ex.Message+" message ";
                    textBox1.Text += ex.StackTrace+" stack trace ";
                    textBox1.Text += ex.Data+" data ";
                }
                //using (Bitmap bitmap = new Bitmap(currentFile))
                //{
                //    bitmap.RotateFlip(RotateFlipType.Rotate180FlipNone);
                //    bitmap.Save(Path.Combine(newDir, filename));
                //    this.Text = string.Format(" Processing {0} on thread {1} ", filename, Thread.CurrentThread.ManagedThreadId);
                //}
            }
            this.Text = " All done ";
        }
 
        private void button1_Click(object sender, EventArgs e)
        {
            ProcessFiles();
        }
    }
}

 Bitmap bitmap = new Bitmap((String)filename);