A sample code that demonstrates a typical AES Encryption/Decryption with C#

SampleAESEncryptDecrypt.cs

static class SampleAESEncryptDecrypt
{
	static void Main()
	{
		string original = "The data to encrypt.";

		using (AesCryptoServiceProvider aes = new AesCryptoServiceProvider())
		{
			// Setting a key size disposes the previously-set key. 
			// Setting a key size is redundant if a key going to be set after this statement. 
			// According to https://en.wikipedia.org/wiki/Advanced_Encryption_Standard, Supported key sizes are 128, 192 and 256
			aes.KeySize = 256;

			// aes.BlockSize = 128; // According to https://en.wikipedia.org/wiki/Advanced_Encryption_Standard: Block size for AES is always 128

			byte[] key = aes.Key;

			byte[] encrypted = EncryptStringToBytes(original, key);

			string decrypted = DecryptStringFromBytes(encrypted, key);

			Console.WriteLine(decrypted);
		}
	}

	static byte[] EncryptStringToBytes(string str, byte[] keys)
	{
		byte[] encrypted;
		using (AesCryptoServiceProvider aes = new AesCryptoServiceProvider())
		{
			aes.Key = keys;

			aes.GenerateIV(); // The get method of the 'IV' property of the 'SymmetricAlgorithm' automatically generates an IV if it is has not been generate before. 

			aes.Mode = CipherMode.CBC;
			aes.Padding = PaddingMode.PKCS7;
			
			using (MemoryStream msEncrypt = new MemoryStream())
			{
				msEncrypt.Write(aes.IV, 0, aes.IV.Length);
				ICryptoTransform encoder = aes.CreateEncryptor();
				using (CryptoStream csEncrypt = new CryptoStream(msEncrypt, encoder, CryptoStreamMode.Write))
				using (StreamWriter swEncrypt = new StreamWriter(csEncrypt))
				{
					swEncrypt.Write(str);
				}
				encrypted = msEncrypt.ToArray();
			}
		}

		return encrypted;
	}

	static string DecryptStringFromBytes(byte[] cipherText, byte[] key)
	{
		string decrypted;
		using (AesCryptoServiceProvider aes = new AesCryptoServiceProvider())
		{
			// Setting a key size disposes the previously-set key. 
			// Setting a key size will generate a new key. 
			// Setting a key size is redundant if a key going to be set after this statement. 
			// aes.KeySize = 256; 

			aes.Key = key;
			aes.Mode = CipherMode.CBC;
			aes.Padding = PaddingMode.PKCS7;

			using (MemoryStream msDecryptor = new MemoryStream(cipherText))
			{
				byte[] readIV = new byte[16];
				msDecryptor.Read(readIV, 0, 16);
				aes.IV = readIV;
				ICryptoTransform decoder = aes.CreateDecryptor();
				using (CryptoStream csDecryptor = new CryptoStream(msDecryptor, decoder, CryptoStreamMode.Read))
				using (StreamReader srReader = new StreamReader(csDecryptor))
				{
					decrypted = srReader.ReadToEnd();
				}
			}
		}
		return decrypted;
	}
}

This does not work in Asp.Net core: System.Security.Cryptography.CryptographicException: 'Padding is invalid and cannot be removed.'

Change to key size 128