在C#中，可以使用System.Security.Cryptography命名空間下的類來實現字符串的加密和解密。以下是一個使用AES加密和解密的示例：

首先，確保已經安裝了System.Security.Cryptography.Aes NuGet包。如果沒有安裝，可以通過以下命令安裝：

dotnet add package System.Security.Cryptography.Aes

然后，可以使用以下代碼實現加密和解密：

using System;
using System.Text;
using System.Security.Cryptography;
using System.IO;

class AesEncryptionExample
{
    static void Main()
    {
        string key = "your-secret-key"; // 密鑰應該是16、24或32個字符長度的字符串
        string plaintext = "Hello, World!"; // 要加密的字符串

        string encrypted = Encrypt(plaintext, key);
        Console.WriteLine("Encrypted text: " + encrypted);

        string decrypted = Decrypt(encrypted, key);
        Console.WriteLine("Decrypted text: " + decrypted);
    }

    static string Encrypt(string plaintext, string key)
    {
        byte[] iv = new byte[16]; // 初始化向量（IV）長度為16字節（AES-128）
        byte[] array;

        using (Aes aes = Aes.Create())
        {
            aes.Key = Encoding.UTF8.GetBytes(key);
            aes.IV = iv;

            ICryptoTransform encryptor = aes.CreateEncryptor(aes.Key, aes.IV);

            using (MemoryStream memoryStream = new MemoryStream())
            {
                using (CryptoStream cryptoStream = new CryptoStream(memoryStream, encryptor, CryptoStreamMode.Write))
                {
                    using (StreamWriter streamWriter = new StreamWriter(cryptoStream))
                    {
                        streamWriter.Write(plaintext);
                    }
                    array = memoryStream.ToArray();
                }
            }
        }

        return Convert.ToBase64String(array);
    }

    static string Decrypt(string ciphertext, string key)
    {
        byte[] iv = new byte[16]; // 初始化向量（IV）長度為16字節（AES-128）
        byte[] array;

        using (Aes aes = Aes.Create())
        {
            aes.Key = Encoding.UTF8.GetBytes(key);
            aes.IV = iv;

            ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);

            using (MemoryStream memoryStream = new MemoryStream(Convert.FromBase64String(ciphertext)))
            {
                using (CryptoStream cryptoStream = new CryptoStream(memoryStream, decryptor, CryptoStreamMode.Read))
                {
                    using (StreamReader streamReader = new StreamReader(cryptoStream))
                    {
                        array = Encoding.UTF8.GetBytes(streamReader.ReadToEnd());
                    }
                }
            }
        }

        return Encoding.UTF8.GetString(array);
    }
}

請注意，這個示例使用了AES-128加密算法。你可以根據需要選擇其他加密算法，例如AES-192或AES-256。但是，密鑰和IV的長度將相應地變化。例如，對于AES-256，密鑰應該是32個字符長度的字符串，IV應該是16字節（AES-128）或24字節（AES-192或AES-256）長度的字符串。