Partial file encryption with AES

Question

Task is to encrypt part of the file using aes encryption algorithm. I did

public static byte[] AES_(int len, byte[] passwordBytes, byte[] saltBytes, FileStream fSin)
{

    byte[] encryptedBytes = null;
    byte[] encryptedBytesAndIV = null;

    using (System.IO.MemoryStream ms = new System.IO.MemoryStream())
    {
        using (AesCryptoServiceProvider AES = new AesCryptoServiceProvider())
        {
            AES.KeySize = 256;
            AES.BlockSize = 128;

            var key = new Rfc2898DeriveBytes(passwordBytes, saltBytes, 100);
            AES.Key = key.GetBytes(AES.KeySize / 8);
            AES.IV = generateIV();

            AES.Mode = CipherMode.CBC;

            using (var cs = new CryptoStream(ms, AES.CreateEncryptor(), CryptoStreamMode.Write))
            {
                byte[] buffer = new byte[1048576];
                int read,read2=0;


                while (read2 < len)
                {
                    read = fSin.Read(buffer, 0, buffer.Length);
                    cs.Write(buffer, 0, read);
                    read2 += read;
                }
                cs.Close();


            }
            encryptedBytes = ms.ToArray();
            encryptedBytesAndIV = new byte[encryptedBytes.Length + AES.IV.Length];
            AES.IV.CopyTo(encryptedBytesAndIV, 0);
            encryptedBytes.CopyTo(encryptedBytesAndIV, IV_LENGTH);
        }
    }

    return encryptedBytesAndIV;
}
private static byte[] generateIV()
{
    using (RNGCryptoServiceProvider rng = new RNGCryptoServiceProvider())
    {
        byte[] nonce = new byte[IV_LENGTH];
        rng.GetBytes(nonce);
        return nonce;
    }
}

call function

  FileStream fsIn = new FileStream(path, FileMode.Open);
len = Convert.ToInt32(fsIn.Length) / 3;
 var resultb = AES_(len, passwordBytes, saltBytes, fsIn);


    fsIn.Seek(0, SeekOrigin.Begin);
    fsIn.Write(resultb, 0, resultb.Length);

    fsIn.Seek(fsIn.Length, SeekOrigin.Begin);
    fsIn.Write(saltBytes, 0, saltBytes.Length);
using (var writer = new BinaryWriter(fsIn))
{
    writer.Seek(Convert.ToInt32(fsIn.Length), SeekOrigin.Begin);
    writer.Write(resultb.Length);

}

The problem is that when a large file consumes a lot of memory and the computer freezes.Is it possible to alter this code so that the memory consumption of the PC was the minimum ?

As I understand it, that is the memory write bytes to the MemoryStream, but how different ? I need to overwrite the bytes in the same file if the same record byte immediately into a new file, it will have to write all the remaining bytes, and it is a waste of time and also memory