C# how to decrypt an encrypted excel file before displaying it to a datagridview?

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;
using System.Data.OleDb;
using System.Security.Cryptography;

namespace EnDecrytion_tryout
{
publicpartialclassReadExcel : Form
{
	privatestaticstring key = "asdfghjklzxcasdfghjklzxcasdfghjk";
	privatestaticstring IV = "qwertyuiopasdfgh";
	public ReadExcel()
        {
            InitializeComponent();
            browse.Click += newEventHandler(browse_Click);
            btnloadexcel.Click += newEventHandler(btnloadexcel_Click);
        } 

	void btnloadexcel_Click(object sender, EventArgs e)
        {

		string pathcon = @"Provider=Microsoft.ACE.OLEDB.12.0;Data Source =" + 			
		label1.Text + @";Extended Properties =""Excel 8.0;IMEX=1 				
		HDR=Yes;ImportMixedTypes=Text;TypeGuessRows=0""";
		OleDbConnection conn = newOleDbConnection(pathcon);

		var adapter = newOleDbDataAdapter("SELECT CATEGORY, 				
		ITEM_NAME, SUBNAME_or_BRAND, ITEM_SIZE, ITEM_QUANTITY_DESCRIPTION, 				ITEM_UNIT, QUANTITY, REFERENCE, TRANSACTION, TRANSACTION_DATE" +")"+ "FROM 			[SHEET1$]", conn);
		var ds = newDataSet();

            	adapter.Fill(ds);

		DataTable QueryTable = ds.Tables[0];

            dataGridView1.DataSource = QueryTable;
        }

	void browse_Click(object sender, EventArgs e)
        {
	if (openFileDialog1.ShowDialog() == System.Windows.Forms.DialogResult.OK)
            {
                label1.Text = openFileDialog1.FileName;
            }
        }
	publicstaticstring decrypt(string adap)
	{
		byte[] encryptedbytes = Convert.FromBase64String(adap);
		AesCryptoServiceProvider aes = newAesCryptoServiceProvider();
            	aes.BlockSize = 128;
           	 aes.KeySize = 256;
            	aes.Key = System.Text.ASCIIEncoding.ASCII.GetBytes(key);
            	aes.IV = System.Text.ASCIIEncoding.ASCII.GetBytes(IV);
            	aes.Padding = PaddingMode.PKCS7;
            	aes.Mode = CipherMode.CBC;
		ICryptoTransform crypto = aes.CreateDecryptor(aes.Key, aes.IV);
		byte[] secret = crypto.TransformFinalBlock(encryptedbytes, 0, 				
		encryptedbytes.Length);
        	crypto.Dispose();
		return System.Text.ASCIIEncoding.ASCII.GetString(secret);
	}
    }
}

- this is my code for connection and decryption but i don't know where i will implement the decryption code to decrypt the excel file before displaying it to datagridview.

(by the way i also used AES for encrypting every cell entry in the excel table)

-- I appreciate some help. thanks