C# AES Encryption/Decryption or byte[] array with a custom Key/IV
ctor
Neither key.Length nor iv.Length can be smaller than 0 you only need to check if it is equal to 0. But for me as a user of that class it would look strange to receive an ArgumentNullException if I pass e.g a key with Length == 0. I would expect an ArgumentOutOfRangeException.
Omitting braces althougth they might be optional can lead to hidden and therfor hard to find bugs.
Encrypt()

The default Mode of RijndaelManaged is already CipherMode.CBC so there is no need to set it again.

By returning out of the most inner using you can remove byte[] encrypted.

By stacking the using's you save some levels of indentation.

You can use the parameterless CreateEncryptor() method because you already set the Key and IV.

As the method is public you should validate its parameter.


Decrypt()

The default Mode of RijndaelManaged is already CipherMode.CBC so there is no need to set it again.

By returning out of the most inner using you can remove byte[] decrypted.

By stacking the using's you save some levels of indentation.

You can use the parameterless CreateDecryptor() method because you already set the Key and IV.

You shouldn't mix styles. In the using's you sometimes use the concrete type and sometimes you use var.


General
Naming things in a good way is hard. Mostly it is sufficient and just more readable to just use a classname in camelCase casing. If you stumble in a few months over the variable rijAlg you will need to ask yourself what it stands for. By naming it rijndaelManaged you will see at first glance what it is about.
Another example of &quot;bad&quot;  naming is in Encrypt() you name the MemoryStream msEncrypt but in Decrypt() you name it ms the same is true for csEncrypt vs cryptoStream.
Implementing the mentioned points leads to
public class AesGenericEncryptionService
{
    private readonly byte[] m_Key;
    private readonly byte[] m_IV;

    public AesGenericEncryptionService(byte[] key, byte[] iv)
    {
        if (key == null)
        {
            throw new ArgumentNullException(nameof(key));
        }
        else if (key.Length == 0)
        {
            throw new ArgumentOutOfRangeException(nameof(key));
        }

        if (iv == null || iv.Length &lt;= 0)
        {
            throw new ArgumentNullException(nameof(iv));
        }
        else if (iv.Length == 0)
        {
            throw new ArgumentOutOfRangeException(nameof(iv));
        }

        m_Key = key;
        m_IV = iv;
    }

    public byte[] Encrypt(byte[] data)
    {
        if (data == null)
        {
            throw new ArgumentNullException(nameof(data));
        }

        using (var rijndaelManaged = new RijndaelManaged())
        {
            rijndaelManaged.KeySize = m_Key.Length * 8;
            rijndaelManaged.Key = m_Key;
            rijndaelManaged.BlockSize = m_IV.Length * 8;
            rijndaelManaged.IV = m_IV;

            using (var encryptor = rijndaelManaged.CreateEncryptor())
            using (var ms = new MemoryStream())
            using (var cryptoStream = new CryptoStream(ms, encryptor, CryptoStreamMode.Write))
            {
                cryptoStream.Write(data, 0, data.Length);
                cryptoStream.FlushFinalBlock();

                return ms.ToArray();
            }
        }
    }
    public byte[] Decrypt(byte[] cipher)
    {
        // Check arguments.
        if (cipher == null)
        {
            throw new ArgumentNullException(nameof(cipher));
        }
        else if (cipher.Length == 0)
        {
            throw new ArgumentOutOfRangeException(nameof(cipher));
        }

        using (var rijndaelManaged = new RijndaelManaged())
        {
            rijndaelManaged.KeySize = m_Key.Length * 8;
            rijndaelManaged.Key = m_Key;
            rijndaelManaged.BlockSize = m_IV.Length * 8;
            rijndaelManaged.IV = m_IV;

            using (var decryptor = rijndaelManaged.CreateDecryptor())
            using (var ms = new MemoryStream(cipher))
            using (var cryptoStream = new CryptoStream(ms, decryptor, CryptoStreamMode.Read))
            {

                var dycrypted = new byte[cipher.Length];
                var bytesRead = cryptoStream.Read(dycrypted, 0, cipher.Length);

                return dycrypted.Take(bytesRead).ToArray();
            }
        }
    }
}