第一个Winform 程序 （附一个需求实现，望大家帮忙）

我现在的 选择 .pdf 可以筛选出 .pdf的文件，但是选择 .txt之后只会筛选出 .txt文件，如何实现出  选择2个 都可以实现的代码

代码如下：

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.IO;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form    //partial可以使一个程序的代码分别处在不同的文件中
    {
        string currentDirectory;//用于存路径
        string currentextens;//用于存 选择扩展名
        
        
        public Form1()
        {
            InitializeComponent();
        }

        private void label1_Click(object sender, EventArgs e)
        {

        }

        private void button1_Click(object sender, EventArgs e)
        {
            if (textBox1.Text != string.Empty && !Directory.Exists(textBox1.Text))//判断输入为非空 并且 不存在的时弹出信息
            {
                MessageBox.Show("Invalid Directory","Error",MessageBoxButtons.OK,MessageBoxIcon.Error);
            }

            else if (textBox1.Text == string.Empty)
            {
                currentDirectory = textBox1.Text;   
                MessageBox.Show("Your Enter is Empty","Error");
            }

            else if (Directory.Exists(textBox1.Text))
            {
               
                textBox2.Clear();//清空
                richTextBox1.Clear();//清空
                richTextBox2.Clear();//清空
                richTextBox3.Clear();//清空
                richTextBox4.Clear();//清空
                currentDirectory = textBox1.Text;
                //textBox2.Text = textBox1.Text;//无扩展性
                textBox2.AppendText("The Path :"+currentDirectory);
                SeachDirectory(currentDirectory);
            }
        }

    
        /*方法*/
        private void SeachDirectory(string path)
        {
            textBox3.AppendText("恭喜你:目录存在!");
            string[] files = Directory.GetFiles(path);
            string[] directorys = Directory.GetDirectories(path);

            
            foreach (var i in files)
            {
                richTextBox1.AppendText(i + "\n");
            }

          
            foreach (var i in directorys)
            {
                richTextBox2.AppendText(i + "\n");
            }
            var exten_name =
                from file in files
                where Path.GetExtension(file) == currentextens
                select file;
           
            foreach (var i in exten_name)
            {

                richTextBox4.AppendText(Path.GetFileName(i) + "\n");
            }

            var exten_name2=
                (from file in files
                select Path.GetExtension(file)).Distinct();
     
            foreach(var i in exten_name2)
            {
                richTextBox3.AppendText(i+"\n");
            }
        }

        private void textBox1_TextChanged(object sender, EventArgs e) { /*即使是空也不可删*/}
        private void textBox2_TextChanged(object sender, EventArgs e) {/*即使是空也不可删*/}
        private void richTextBox1_TextChanged(object sender, EventArgs e) {/*即使是空也不可删*/}

        private void textBox3_TextChanged(object sender, EventArgs e)
        {

        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }

        private void checkBox1_CheckedChanged(object sender, EventArgs e)
        {
            currentextens = ".pdf";
        }

        private void checkBox2_CheckedChanged(object sender, EventArgs e)
        {
            currentextens = ".txt";
        }

        private void checkBox4_CheckedChanged(object sender, EventArgs e)
        {
            currentextens = ".rar";
        }

        private void checkBox3_CheckedChanged(object sender, EventArgs e)
        {
            currentextens = ".exe";
        }

        private void checkBox5_CheckedChanged(object sender, EventArgs e)
        {
            currentextens = ".jpg";
        }
    }
}