New Post: Problem on storing ulong

Trying to implement the followwing method

   [Cudafy]
    public static ulong ByteArrayToUlong(int n, byte[] array)
    {
        int i;
        ulong k = 0;
        for (i = 0; i < n; k += array[i++])
            k <<= 8;
        return k;
    }

I realised that there is a problem on storing ulong numbers. It keeps only the 32 bits e.g. 0x32877FB0A17C40 it will convert it to 0xffffffffB0A17C40.
Does anyone else face something like it ??