Example

//namespace for Input Output file stream
using System.IO;
//name space for Cryptography
using System.Security.Cryptography;
private void btnSelFile_Click(object sender, EventArgs e)
        {
            try
            {
               //opening file.
                openFileDialog1.Title = "Open File";
                if (openFileDialog1.ShowDialog() == DialogResult.OK)
                    txtFileName.Text = openFileDialog1.FileName;
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
           //Encrypting 
        private void btnEncrypt_Click(object sender, EventArgs e)
        {
            try
            {
                saveKeyDialog2.Title = "Save Key";
                saveFileDialog1.Title = "Save Encrypted file";
               // after the user chose where he wants the key file saved
                if (saveKeyDialog2.ShowDialog() == DialogResult.OK)
                {
                // after the user chose where he wants the encrypted file saved
                    if (saveFileDialog1.ShowDialog() == DialogResult.OK)
                    {
                      FileStream fs = File.Create(saveFileDialog1.FileName);
                      // the cryptographic service provider  we're going to use
                     TripleDESCryptoServiceProvider tc = new TripleDESCryptoServiceProvider();
                      // this object links data streams to cryptographic values
                    CryptoStream cs = new CryptoStream(fs, tc.CreateEncryptor(), CryptoStreamMode.Write);
                     // This stream reader will read the file to encrypt
                        StreamReader sr = new StreamReader(txtFileName.Text);
                      // this stream writer will write the new file
                        StreamWriter sw = new StreamWriter(cs);
                        string curline = sr.ReadLine();
                        while (curline != null)
                        {
                        // Write to the encryption stream
                            sw.Write(curline);
                            curline = sr.ReadLine();
                        }
                        sr.Close();
                        sw.Flush();
                        sw.Close();
                        //creating key file
                        FileStream fsKey = File.Create(saveKeyDialog2.FileName);
                        BinaryWriter bw = new BinaryWriter(fsKey);
                        bw.Write(tc.Key);
                        bw.Write(tc.IV);
                        bw.Flush();
                        bw.Close();
                        MessageBox.Show("File Encrypted Successfully");
                        txtFileName.Text = "";
                    }
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
 
         //Decrypting 
        private void btnDecrypt_Click(object sender, EventArgs e)
        {
            try
            {
                openFileDialog1.Title = "Open Key File";
                saveFileDialog1.Title = "Save Decrypted File";
                if (openFileDialog1.ShowDialog() == DialogResult.OK)
                {
                    if (saveFileDialog1.ShowDialog() == DialogResult.OK)
                    {
                        // The encrypted file
                        FileStream fsFile = File.OpenRead(txtFileName.Text);
                     // The key
                        FileStream fsKey = File.OpenRead(openFileDialog1.FileName);
                       // The decrypted file
                        FileStream fsSave = File.Create(saveFileDialog1.FileName);
                        //Prepare the encryption algorithm and read the key from the key file
                        TripleDESCryptoServiceProvider csAlgo = new TripleDESCryptoServiceProvider();
                        BinaryReader br = new BinaryReader(fsKey);
                        csAlgo.Key = br.ReadBytes(24);
                        csAlgo.IV = br.ReadBytes(8);
                      // The cryptographic stream takes in the encrypted file
                        CryptoStream cs = new CryptoStream(fsFile, csAlgo.CreateDecryptor(), CryptoStreamMode.Read);
                         // Write the new unecrypted file
                        StreamReader srCleanStream = new StreamReader(cs);
                        StreamWriter swCleanStream = new StreamWriter(fsSave);
                        swCleanStream.Write(srCleanStream.ReadToEnd());
                        swCleanStream.Close();
                        fsSave.Close();
                        srCleanStream.Close();
                        MessageBox.Show("File Decrypted Successfully");
                    }
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }            
        }