以下是一个使用C#解密AES Rijndael的示例代码：

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

public class AESDecryptor
{
    public static string DecryptString(string cipherText, string key, string iv)
    {
        byte[] encryptedBytes = Convert.FromBase64String(cipherText);
        byte[] keyBytes = Encoding.UTF8.GetBytes(key);
        byte[] ivBytes = Encoding.UTF8.GetBytes(iv);

        string plaintext = null;

        using (Aes aes = Aes.Create())
        {
            aes.Key = keyBytes;
            aes.IV = ivBytes;

            ICryptoTransform decryptor = aes.CreateDecryptor(aes.Key, aes.IV);
            using (var ms = new System.IO.MemoryStream(encryptedBytes))
            {
                using (var cs = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
                {
                    using (var sr = new System.IO.StreamReader(cs))
                    {
                        plaintext = sr.ReadToEnd();
                    }
                }
            }
        }

        return plaintext;
    }
}

使用示例：

string encryptedText = "AesEncryptedText";
string key = "YourEncryptionKey";
string iv = "YourIV";

string decryptedText = AESDecryptor.DecryptString(encryptedText, key, iv);

Console.WriteLine(decryptedText);

请注意，您需要将 "AesEncryptedText" 替换为要解密的实际密文，将 "YourEncryptionKey" 替换为您的实际密钥，将 "YourIV" 替换为您的实际初始化向量。