在C#中实现SM4加密可以通过使用第三方库来实现。一个常用的库是Bouncy Castle，可以通过NuGet安装。

首先，安装Bouncy Castle库：

Install-Package BouncyCastle

然后，使用以下代码示例实现SM4加密：

using System;using System.IO;using System.Text;using System.Security.Cryptography;using Org.BouncyCastle.Crypto;using Org.BouncyCastle.Crypto.Parameters;using Org.BouncyCastle.Crypto.Engines;public class SM4Encryption{public static string Encrypt(string plaintext, string key){byte[] keyBytes = Encoding.UTF8.GetBytes(key);byte[] plaintextBytes = Encoding.UTF8.GetBytes(plaintext);SM4Engine sm4 = new SM4Engine();KeyParameter keyParam = new KeyParameter(keyBytes);sm4.Init(true, keyParam);byte[] ciphertextBytes = new byte[plaintextBytes.Length];for (int i = 0; i < plaintextBytes.Length; i++){sm4.ProcessBlock(plaintextBytes, i, ciphertextBytes, i);}return Convert.ToBase64String(ciphertextBytes);}public static string Decrypt(string ciphertext, string key){byte[] keyBytes = Encoding.UTF8.GetBytes(key);byte[] ciphertextBytes = Convert.FromBase64String(ciphertext);SM4Engine sm4 = new SM4Engine();KeyParameter keyParam = new KeyParameter(keyBytes);sm4.Init(false, keyParam);byte[] plaintextBytes = new byte[ciphertextBytes.Length];for (int i = 0; i < ciphertextBytes.Length; i++){sm4.ProcessBlock(ciphertextBytes, i, plaintextBytes, i);}return Encoding.UTF8.GetString(plaintextBytes);}}

使用示例：

string plaintext = "Hello, SM4!";string key = "0123456789abcdeffedcba9876543210";string ciphertext = SM4Encryption.Encrypt(plaintext, key);Console.WriteLine("Ciphertext: " + ciphertext);string decryptedText = SM4Encryption.Decrypt(ciphertext, key);Console.WriteLine("Decrypted Text: " + decryptedText);

请注意，以上代码示例仅供参考，并不保证在所有情况下都能正确工作。在实际应用中，建议根据具体需求进行适当调整和优化。