checked.cs

// why masking is a good habit
// the following fails are runtime
static byte[] ToByteArray(int i)
{
    checked
    {
        var b = new byte[4];
        b[0] = (byte)(i); 
        b[1] = (byte)((i >> 8));
        b[2] = (byte)((i >> 16));
        b[3] = (byte)((i >> 24));
        return b;
    }
}

gistfile1.cs

// assumes little endian
static int ToInt32(byte[] b) // must be 4 bytes
{
    return b[0] | (b[1] << 8) | (b[2] << 16) | (b[3] << 24); // could be + too
}

static byte[] ToByteArray(int i)
{
    var b = new byte[4];
    b[0] = (byte)(i & 0xFF); // must mask higher bits
    b[1] = (byte)((i >> 8) & 0xFF);
    b[2] = (byte)((i >> 16) & 0xFF);
    b[3] = (byte)((i >> 24) & 0xFF);
    return b;
}

unsafe.cs

// assumes little endian
static unsafe int ToInt32(byte[] b) // must be 4 bytes
{
    fixed (byte* bp = b)
    {
        return *((int*)bp);
    }
}

static unsafe byte[] ToByteArray(int i)
{
    var b = new byte[4];
    fixed (byte* bp = b)
    {
        *((int*)bp) = i;
    }
    return b;
}