Converting an array of int to an array of bytes

your right, but the values in the array span byte boundries i.e. the fist value is contained in the first 9 bits. So it eisier to take in multiple bytes and shift left and right to get the value. Thanks again.

Convert.ToInt32(byteValue)  is equivalent to casting the byte value to an int.

So, if you have a byte with a value of 32, it just 'widens' it to a 4 byte Int32 containing the same value.  

You can use:

int i  = BitConverter.ToInt32(ba, 2);

which takes the 4 bytes of the byte array starting at index 2 (zero-based) and converts them to an int.

Sorry, I forgot about that one :)

        int[] ia  = new int[ba.Length / 4];
        for(int i = 0;  i < ia.Length;  i++)
        {
           int j = 4 * i;
           ia[i] = ((int)ba[j+3] << 24) + ((int)ba[j+2] << 16) + ((int)ba[j+1] << 8) + ((int)ba[j]);
        }

In C#, I think most people would just do something like this:

        int[] ia = new int[someSize];   
        // code to initialize ia
        byte[] ba = new byte[4 * someSize];

        for(int i = 0; i < ia.Length ; i++)
        {
            int j = 4 * i;
            ba[j] = (byte)(ia[i] & 0xFF);
            ba[j+1] = (byte)(ia[i] >> 8 & 0xFF);
            ba[j+2] = (byte)(ia[i] >>16 & 0xFF);
            ba[j+3] = (byte)(ia[i] >> 24 & 0xFF);
       }

You can also use the BitConverter.GetBytes(someInt) method to get the byte representation of a single integer though using this class isn't as fast as bit arithmetic. 

Although it's possible to simulate unions, you can't overlap reference types within them which, of course, is what arrays are in .NET. You might be able to do something using pointers in 'unsafe' code but I doubt whether it would be much quicker than the above.