C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace TestCopyFolder
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
    }
}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace TestCopyFolder
{
    public partial class ucCopyDir : UserControl
    {
        public ucCopyDir()
        {
            InitializeComponent();
        }

        private void txtBoxCpSrc_DragEnter(object sender, DragEventArgs e)
        {
            // Drag and drop effect in windows
            if (e.Data.GetDataPresent(DataFormats.FileDrop, false) == true)
            {
                e.Effect = DragDropEffects.All;
            }
        }

        // Source Drag and Drop section
        private void txtBoxCpSrc_DragDrop(object sender, DragEventArgs e)
        {
            // This just allows you to drop a file into the source textbox and display it's path.
            string[] files1 = (string[])e.Data.GetData(DataFormats.FileDrop,false);
            foreach (string file1 in files1)
                txtBoxCpSrc.Text = file1;
        }

        private void btnCpSrcBrowse_Click(object sender, EventArgs e)
        {
            FolderBrowserDialog fbd = new FolderBrowserDialog();
            if (fbd.ShowDialog() == DialogResult.OK)
            {
                txtBoxCpSrc.Text = (fbd.SelectedPath);
            }
        }
    }
}

public string RemoveSpecialCharacters(string str)
       {
           StringBuilder sb = new StringBuilder();
           for(int i=0; i<str.Length;i++)
           {
               if((str[i] >= '0' && str[i] <='9')
                   ||( str[i] >='A' &&  str[i]<='z'|| (str[i]=='-' || str[i] == '+' || str[i] == '*' || str[i] == '/' || str[i] == '%'|| str[i] ==')' || str[i] == '(')))
               {
                   sb.Append(str[i]);
               }
           }
           return sb.ToString();
       }
       private void button8_Click(object sender, EventArgs e)
       {
           string refine_string = RemoveSpecialCharacters(textBox1.Text);
           string[] split_var = refine_string.Split('+', '-', '/', '%', '^', '*', '=',')','(');
           double value;
           string str = "";
           for (int i = 0; i < split_var.Length; i++)
           {
               if (double.TryParse(split_var[i], out value))
               {
                   split_var[i] = " ";
               }
           }
           split_var = split_var.Where(s => s != " ").ToArray();
           string[] variables = split_var.Distinct().ToArray();

           for (int j = 0; j < variables.Length; j++)
           {

               str = str + variables[j] + '=' + ";";

           }
           textBox2.Text = str.TrimEnd(';');

       }