C＃wpf如何将变量Excel.Worksheet从Button_Click传递给另一个Button_Click

Question

我正在编写用于对excel中的特定列进行排序的C＃WPF简易程序，我在将变量excel.worksheet从Button_Click传递到另一个Button_Click时遇到了问题。 代码如下：

using Microsoft.Win32;
using System.IO;
using System;
using System.Windows;
using Excel = Microsoft.Office.Interop.Excel;

namespace Sorting
{
    public partial class MainWindow : Window
    {

        int column, CounterRow;
        string filepath;
        Excel.Worksheet x = new Excel.Worksheet();

        public MainWindow()
        {
            InitializeComponent();
        }

        private void Choose_Files_Button_Click(object sender, RoutedEventArgs e)
        {
            OpenFileDialog ofd = new OpenFileDialog();
            ofd.Multiselect = false;
            ofd.Filter = "All files (*.*)|*.*";
            ofd.InitialDirectory = Environment.GetFolderPath(Environment.SpecialFolder.Desktop);
            if (ofd.ShowDialog() == true)
            {
                ListBox_Files.Items.Clear();
                filepath = ofd.FileName;
                ListBox_Files.Items.Add(Path.GetFileName(filepath));

            }
            if (ListBox_Files.Items.Count != 0) ListBox_Files.IsEnabled = Otworz.IsEnabled = true;
        }

        private void Button_Click_0(object sender, RoutedEventArgs e)
        {
            Excel.Application excel = new Excel.Application();
            excel.Visible = true;
            excel.DisplayAlerts = false;

            Excel.Workbook sheet = excel.Workbooks.Open(filepath);
            Excel.Worksheet x = excel.ActiveSheet as Excel.Worksheet;

            MessageBox.Show("select column");

            Sort(x,1);

            MessageBox.Show("end");

            excel.Quit();
        }

        void Button_Click(object sender, RoutedEventArgs e)
        {
            Excel.Application excel = new Excel.Application();
            excel.Visible = true;
            excel.DisplayAlerts = false;

            Excel.Workbook sheet = excel.Workbooks.Open(filepath);
            Excel.Worksheet x = excel.ActiveSheet as Excel.Worksheet;

        }

        private void Button_Click_1(object sender, RoutedEventArgs e)
        {
            Sort(x, 1);
        }

        private void Button_Click_2(object sender, RoutedEventArgs e)
        {
           Sort(x, 2);
        }

        private void Sort(Excel.Worksheet excel, int info)
        {
            Excel.Range rng = excel.Application.ActiveCell;
            column = rng.Column;            

            Excel.Range userRange = excel.UsedRange;
            CounterRow = userRange.Rows.Count;

            Excel.Range range = excel.Range[excel.Cells[7, 1], excel.Cells[CounterRow - 1, 8]];

            if (info == 1) range.Sort(range.Columns[column], Excel.XlSortOrder.xlAscending);
            else if (info == 2) range.Sort(range.Columns[column], Excel.XlSortOrder.xlDescending);
        }
    }
}

任何人都可以解释一下，为什么Click_Button_0正常工作，当我通过Click_Button打开文件然后我将通过Click_Button_1排序列并单击Button_2不起作用？ Visual Studio说：＆＃34; System.InvalidCastException＆＃34;