"；无法对空引用“”执行运行时绑定；读取/显示Excel

我准备了一个简单的代码，用于将excel文件读取到c#Wpf表单。

但我得到了错误：

{"Cannot perform runtime binding on a null reference"}.

下面是代码。。。。

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;
using Excel = Microsoft.Office.Interop.Excel;
namespace WindowsFormsApplication9
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }
        private void button1_Click(object sender, EventArgs e)
        {
            Excel.Application xlApp = new Excel.Application();
            Excel.Workbook xlWorkbook = xlApp.Workbooks.Open("Book1.xlsx");
            Excel._Worksheet xlWorksheet = xlWorkbook.Sheets[1];
            Excel.Range xlRange = xlWorksheet.UsedRange;
            int rowCount = xlRange.Rows.Count;
            int colCount = xlRange.Columns.Count;
            for (int i = 1; i <= rowCount; i++)
            {
                for (int j = 1; j<= colCount; j++)
                {
                    MessageBox.Show(xlRange.Cells[i, j].Value2.ToString());
                }
            }
        }
    }
}