在C#中实现加密功能，您可以使用.NET框架提供的System.Security.Cryptography命名空间下的多种加密算法类。这里我将给出一个使用AES（Advanced Encryption Standard）对称加密算法的简单示例：

using System;
using System.IO;
using System.Security.Cryptography;
using System.Text;public class AesEncryptionExample
{public static void Main(){// 加密部分string plainText = "这是一段需要加密的文本";byte[] keyBytes = Encoding.UTF8.GetBytes("这是一个16字节的密钥"); // AES密钥必须为16/24/32个字节byte[] ivBytes = new byte[16]; // 初始化向量，AES要求16个字节// 使用随机数生成器初始化IVRandomNumberGenerator.Create().GetBytes(ivBytes);using (AesManaged aes = new AesManaged()){aes.Key = keyBytes;aes.IV = ivBytes;ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);using (MemoryStream ms = new MemoryStream()){using (CryptoStream cs = new CryptoStream(ms, encryptor, CryptoStreamMode.Write)){using (StreamWriter sw = new StreamWriter(cs)){sw.Write(plainText);}byte[] cipherTextBytes = ms.ToArray();Console.WriteLine("加密后的数据：");Console.WriteLine(Convert.ToBase64String(cipherTextBytes));}}}// 解密部分byte[] encryptedData = Convert.FromBase64String("这里是加密后Base64编码的数据");using (AesManaged aesDecryptor = new AesManaged()){aesDecryptor.Key = keyBytes;aesDecryptor.IV = ivBytes;ICryptoTransform decryptor = aesDecryptor.CreateDecryptor();using (MemoryStream msDecrypt = new MemoryStream(encryptedData)){using (CryptoStream csDecrypt = new CryptoStream(msDecrypt, decryptor, CryptoStreamMode.Read)){using (StreamReader srDecrypt = new StreamReader(csDecrypt)){string decryptedText = srDecrypt.ReadToEnd();Console.WriteLine("\n解密后的数据：");Console.WriteLine(decryptedText);}}}}}
}

这个例子展示了如何使用AES加密和解密字符串。注意在实际应用中，密钥应通过安全的方式存储和传递，并且可能需要使用密码派生函数（如Rfc2898DeriveBytes）从用户输入的密码生成密钥和初始化向量。此外，在传输或存储密文时，通常会一起保存IV，以便于解密。