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

namespace TextTool
{
    public partial class Form1 : Form
    {
        private Thread workThread = null; //工作线程
        private List <string> fileList = new List <string>();
        public Form1()
        {
            InitializeComponent();
        }

        // 导入 可多选
        private void button1_Click(object sender, EventArgs e)
        {
            Stream mystream;
            OpenFileDialog openfiledialog1 = new OpenFileDialog();
            openfiledialog1.Multiselect = true;//允许同时选择多个文件
            openfiledialog1.Filter = "txt files(*.txt)|*.txt|All files(*.*)|*.*" ;
            openfiledialog1.FilterIndex = 1;
            openfiledialog1.RestoreDirectory = true;
            if (openfiledialog1.ShowDialog() == DialogResult.OK)
            {
                if ((mystream = openfiledialog1.OpenFile()) != null)
                {
                    fileList.Clear();
                    for (int fi = 0; fi < openfiledialog1.FileNames.Length; fi++)
                    {
                        fileList.Add(openfiledialog1.FileNames[fi]);
                    }
                    mystream.Close();
                }

                listView1.Items.Clear();
                foreach (var filePath in fileList)
                {
                    ListViewItem lvi = new ListViewItem(filePath);
                    listView1.Items.Add(lvi);
                }
            }
        }

        // 合并处理并输出
        private void button2_Click(object sender, EventArgs e)
        {
            if (fileList.Count == 0)
            {
                MessageBox.Show("请先添加要处理的文件" );
                return;
            }

            SaveFileDialog savefiledialog1 = new SaveFileDialog();
            savefiledialog1.Filter = "txt files(*.txt)|*.txt" ;
            if (savefiledialog1.ShowDialog() == DialogResult.OK)
            {
                button2.Text = "处理中..." ;
                button2.Enabled = false;

                workThread = new Thread (WorkThread);
                workThread.Start(savefiledialog1.FileName);
            }
        }

        private int MyCompareString(string x, string y)
        {
            int pos1 = x.IndexOf("(" );
            int pos2 = x.IndexOf(")" );
            int cnt1 = Convert .ToInt32(x.Substring(pos1 + 1, pos2-pos1 -1));

            pos1 = y.IndexOf( "(");
            pos2 = y.IndexOf( ")");
            int cnt2 = Convert .ToInt32(y.Substring(pos1 + 1, pos2 - pos1 - 1));
 
            if (cnt1 > cnt2)
            {
                return -1;
            }
            else if (cnt1< cnt2)
            {
                return 1;
            }
            else
            {
                return 0;
            }
        }

        public void WorkThread(object savePath)
        {
            List<string > allLines = new List< string>();
            foreach (var filePath in fileList)
            {
                allLines.AddRange( File.ReadAllLines(filePath));
            }

            int totalCount = 0;
            List<string > outputList = new List< string>();
            foreach (var line in (from t in allLines where t.Trim() != "" select t).Distinct())
            {
                int count = (from c in allLines where c == line select c).Count();
                totalCount += count;
                outputList.Add( $"{line} ( {count})");
            }
            outputList.Sort(MyCompareString);
            outputList.Add( $"总数：{totalCount}" );
            File.WriteAllLines(savePath.ToString(), outputList);

            Invoke( new MethodInvoker (delegate ()
            {
                button2.Text = "合并处理输出" ;
                button2.Enabled = true;
                MessageBox.Show("处理完成！" );
            }));
        }

        private void Form1_FormClosing(object sender, FormClosingEventArgs e)
        {
            if (workThread != null && workThread.IsAlive)
            {
                workThread.Abort();
            }
        }
    }
}

如果是WPF程序，则在线程中像下面这样操作控件

this.Dispatcher.Invoke(new Action( delegate
{
    lblState.Content = "正在检测域名" + fullUrl;
}));