以下是使用AES 256位加密和解密的Java和.NET兼容的代码示例：

Java示例：

import javax.crypto.Cipher;
import javax.crypto.spec.IvParameterSpec;
import javax.crypto.spec.SecretKeySpec;
import java.nio.charset.StandardCharsets;
import java.util.Base64;

public class AESUtils {

    private static final String AES_ALGORITHM = "AES/CBC/PKCS5Padding";
    private static final String AES_KEY = "0123456789abcdef0123456789abcdef";
    private static final String AES_IV = "0123456789abcdef";

    public static String encrypt(String plaintext) throws Exception {
        SecretKeySpec secretKey = new SecretKeySpec(AES_KEY.getBytes(StandardCharsets.UTF_8), "AES");
        IvParameterSpec ivParameterSpec = new IvParameterSpec(AES_IV.getBytes(StandardCharsets.UTF_8));
        Cipher cipher = Cipher.getInstance(AES_ALGORITHM);
        cipher.init(Cipher.ENCRYPT_MODE, secretKey, ivParameterSpec);
        byte[] encryptedBytes = cipher.doFinal(plaintext.getBytes(StandardCharsets.UTF_8));
        return Base64.getEncoder().encodeToString(encryptedBytes);
    }

    public static String decrypt(String encryptedText) throws Exception {
        SecretKeySpec secretKey = new SecretKeySpec(AES_KEY.getBytes(StandardCharsets.UTF_8), "AES");
        IvParameterSpec ivParameterSpec = new IvParameterSpec(AES_IV.getBytes(StandardCharsets.UTF_8));
        Cipher cipher = Cipher.getInstance(AES_ALGORITHM);
        cipher.init(Cipher.DECRYPT_MODE, secretKey, ivParameterSpec);
        byte[] decryptedBytes = cipher.doFinal(Base64.getDecoder().decode(encryptedText));
        return new String(decryptedBytes, StandardCharsets.UTF_8);
    }

    public static void main(String[] args) throws Exception {
        String plaintext = "Hello World!";
        String encryptedText = encrypt(plaintext);
        System.out.println("Encrypted Text: " + encryptedText);
        String decryptedText = decrypt(encryptedText);
        System.out.println("Decrypted Text: " + decryptedText);
    }
}

.NET示例：

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

public class AESUtils
{
    private static readonly string AES_ALGORITHM = "AES/CBC/PKCS5Padding";
    private static readonly string AES_KEY = "0123456789abcdef0123456789abcdef";
    private static readonly string AES_IV = "0123456789abcdef";

    public static string Encrypt(string plaintext)
    {
        byte[] keyBytes = Encoding.UTF8.GetBytes(AES_KEY);
        byte[] ivBytes = Encoding.UTF8.GetBytes(AES_IV);
        byte[] plaintextBytes = Encoding.UTF8.GetBytes(plaintext);

        using (Aes aes = Aes.Create())
        {
            aes.Mode = CipherMode.CBC;
            aes.Padding = PaddingMode.PKCS7;
            aes.KeySize = 256;
            aes.Key = keyBytes;
            aes.IV = ivBytes;

            using (ICryptoTransform encryptor = aes.CreateEncryptor())
            {
                byte[] encryptedBytes = encryptor.TransformFinalBlock(plaintextBytes, 0, plaintextBytes.Length);
                return Convert.ToBase64String(encryptedBytes);
            }
        }
    }

    public static string Decrypt(string encryptedText)
    {
        byte[] keyBytes = Encoding.UTF8.GetBytes(AES_KEY);
        byte[] ivBytes = Encoding.UTF8.GetBytes(AES_IV);
        byte[] encryptedBytes = Convert.FromBase64String(encryptedText);

        using (Aes aes = Aes.Create())
        {
            aes.Mode = CipherMode.CBC;
            aes.Padding = PaddingMode.PKCS7;
            aes.KeySize = 256;
            aes.Key = keyBytes;
            aes.IV = ivBytes;

            using (ICryptoTransform decryptor = aes.CreateDecryptor())
            {
                byte[] decryptedBytes = decryptor.TransformFinalBlock(encryptedBytes, 0, encryptedBytes.Length);
                return Encoding.UTF8.GetString(decryptedBytes);
            }
        }
    }

    public static void Main(string[] args)
    {
        string plaintext = "Hello World!";
        string encryptedText = Encrypt(plaintext);
        Console.WriteLine("Encrypted Text: " + encryptedText);
        string decryptedText = Decrypt(encryptedText);
        Console.WriteLine("Decrypted Text: " + decryptedText);
    }
}

请确保在使用代码示例时，将AES_KEY和AES_IV替换为您自己的密钥和初始向量。