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Adding GetInt64/SetInt64 to ASP.NET Core Session

I need to add GetInt64 and SetInt64 instructions to the ISession interface in ASP.NET Core so we're able to store some long values.
The existing code for GetInt32 and SetInt32 is available on Github in SessionExtensions.cs.
I am trying to understand the pattern that is in use:
public static void SetInt32(this ISession session, string key, int value)
{
var bytes = new byte[]
{
(byte)(value >> 24),
(byte)(0xFF & (value >> 16)),
(byte)(0xFF & (value >> 8)),
(byte)(0xFF & value)
};
session.Set(key, bytes);
}
public static int? GetInt32(this ISession session, string key)
{
var data = session.Get(key);
if (data == null || data.Length < 4)
{
return null;
}
return data[0] << 24 | data[1] << 16 | data[2] << 8 | data[3];
}
I had expected to see BitConverter.GetBytes, but for whatever reason the set is doing a bunch of right shifts against each octet, and the read does left shifts against each octet. I'm guessing that this relates to keeping the endianness neutral as the BitConverter methods return different values depending on the CPU architecture in use.
Is there an obvious reason the code is written like this?
Would the following be a correct implementation for SetInt64/GetInt64?
public static void SetInt64(this ISession session, string key, long value)
{
var bytes = new byte[]
{
(byte)(value >> 56),
(byte)(0xFF & (value >> 48)),
(byte)(0xFF & (value >> 40)),
(byte)(0xFF & (value >> 32)),
(byte)(0xFF & (value >> 24)),
(byte)(0xFF & (value >> 16)),
(byte)(0xFF & (value >> 8)),
(byte)(0xFF & value)
};
session.Set(key, bytes);
}
public static long? GetInt64(this ISession session, string key)
{
var data = session.Get(key);
if (data == null || data.Length < 8)
{
return null;
}
return data[0] << 56 | data[1] << 48 | data[2] << 40 | data[3] << 32 | data[4] << 24 | data[5] << 16 | data[6] << 8 | data[7];
}

You are almost right. The only problem is that in GetInt64 you need to cast byte to long before shifting it.
public static void SetInt64(this ISession session, string key, long value)
{
var bytes = new byte[]
{
(byte)(value >> 56),
(byte)(0xFF & (value >> 48)),
(byte)(0xFF & (value >> 40)),
(byte)(0xFF & (value >> 32)),
(byte)(0xFF & (value >> 24)),
(byte)(0xFF & (value >> 16)),
(byte)(0xFF & (value >> 8)),
(byte)(0xFF & value)
};
session.Set(key, bytes);
}
public static long? GetInt64(this ISession session, string key)
{
var data = session.Get(key);
if (data == null || data.Length < 8)
{
return null;
}
return (long)data[0] << 56 | (long)data[1] << 48 | (long)data[2] << 40 | (long)data[3] << 32 | (long)data[4] << 24 | (long)data[5] << 16 | (long)data[6] << 8 | (long)data[7];
}
Eventually this code and BitConverter has the same result. BitConverter is using Unsafe to do this operation, maybe that's the reason it's not being used.
https://source.dot.net/#System.Private.CoreLib/BitConverter.cs,107
Why the shifting?
Let's say you want to store 472 (a 3 digit number) in one digit place holders. This is what you do.
Take the first digit from right and put it in first place holder.
Shift the number one digit to the right and repeat the first step.
So it goes
472 ---> 2
047 ---> 7
004 ---> 4
This is exactly what's happening here except place holder is a byte which holds 8 bits and that is why the shifting is happening 8 bits at a time.

Get integer from last 12 bits of 2 bytes in C#

I suspect this is an easy one.
I need to get a number from the first 4 bits and another number from the last 12 bits
of 2 bytes.
So here is what I have but it doesn't seem to be right:
byte[] data = new byte[2];
//assume byte array contains data
var _4bit = data[0] >> 4;
var _12bit = data[0] >> 8 | data[1] & 0xff;

data[0]>>8 is 0. Remember that your data is defined as byte[] so it has 8bits per single item, so you are effectively cutting ALL bits off the data[0].
You want rather to take the lowest 4 bits from that byte by bitwise AND (00001111 = 0F) and then shift it leftwards as needed.
So try this:
var _4bit = data[0] >> 4;
var _12bit = ((data[0] & 0x0F) << 8) | (data[1] & 0xff);
It's also worth noting that the last & 0xFF is not needed, as the data[1] is already a byte.
On bits, step by step:
byte[2] data = { aaaabbbb, cccccccc }
var _4bit = data[0] >> 4;
= aaaabbbb >> 4
= 0000aaaa
var _12bit = ( (data[0] & 0x0F) << 8) | ( data[1] & 0xff);
= ((aaaabbbb & 0x0F) << 8) | (cccccccc & 0xff);
= ( 0000bbbb << 8) | ( cccccccc );
= ( 0000bbbb000000000 ) | ( cccccccc );
= 0000bbbbcccccccc;
BTW. also, note that results of & and | operators are typed as int, so 32bits, I've omitted the zeroes for clarity and written it as 8bit only to make it brief!

Bitshift over Int32.max fails

Ok, so I have two methods
public static long ReadLong(this byte[] data)
{
if (data.Length < 8) throw new ArgumentOutOfRangeException("Not enough data");
long length = data[0] | data[1] << 8 | data[2] << 16 | data[3] << 24
| data[4] << 32 | data[5] << 40 | data[6] << 48 | data[7] << 56;
return length;
}
public static void WriteLong(this byte[] data, long i)
{
if (data.Length < 8) throw new ArgumentOutOfRangeException("Not enough data");
data[0] = (byte)((i >> (8*0)) & 0xFF);
data[1] = (byte)((i >> (8*1)) & 0xFF);
data[2] = (byte)((i >> (8*2)) & 0xFF);
data[3] = (byte)((i >> (8*3)) & 0xFF);
data[4] = (byte)((i >> (8*4)) & 0xFF);
data[5] = (byte)((i >> (8*5)) & 0xFF);
data[6] = (byte)((i >> (8*6)) & 0xFF);
data[7] = (byte)((i >> (8*7)) & 0xFF);
}
So WriteLong works correctly(Verified against BitConverter.GetBytes()). The problem is ReadLong. I have a fairly good understanding of this stuff, but I'm guessing what's happening is the or operations are happening as 32 bit ints so at Int32.MaxValue it rolls over. I'm not sure how to avoid that. My first instinct was to make an int from the lower half and an int from the upper half and combine them, but I'm not quite knowledgeable to know even where to start with that, so this is what I tried....
public static long ReadLong(byte[] data)
{
if (data.Length < 8) throw new ArgumentOutOfRangeException("Not enough data");
long l1 = data[0] | data[1] << 8 | data[2] << 16 | data[3] << 24;
long l2 = data[4] | data[5] << 8 | data[6] << 16 | data[7] << 24;
return l1 | l2 << 32;
}
This didn't work though, at least not for larger numbers, it seems to work for everything below zero.
Here's how I run it
void Main()
{
var larr = new long[5]{
long.MinValue,
0,
long.MaxValue,
1,
-2000000000
};
foreach(var l in larr)
{
var arr = new byte[8];
WriteLong(ref arr,l);
Console.WriteLine(ByteString(arr));
var end = ReadLong(arr);
var end2 = BitConverter.ToInt64(arr,0);
Console.WriteLine(l + " == " + end + " == " + end2);
}
}
and here's what I get(using the modified ReadLong method)
0:0:0:0:0:0:0:128
-9223372036854775808 == -9223372036854775808 == -9223372036854775808
0:0:0:0:0:0:0:0
0 == 0 == 0
255:255:255:255:255:255:255:127
9223372036854775807 == -1 == 9223372036854775807
1:0:0:0:0:0:0:0
1 == 1 == 1
0:108:202:136:255:255:255:255
-2000000000 == -2000000000 == -2000000000

The problem is not the or, it is the bitshift. This has to be done as longs. Currently, the data[i] are implicitely converted to int. Just change that to long and that's it. I.e.
public static long ReadLong(byte[] data)
{
if (data.Length < 8) throw new ArgumentOutOfRangeException("Not enough data");
long length = (long)data[0] | (long)data[1] << 8 | (long)data[2] << 16 | (long)data[3] << 24
| (long)data[4] << 32 | (long)data[5] << 40 | (long)data[6] << 48 | (long)data[7] << 56;
return length;
}

You are doing int arithmetic and then assigning to long, try:
long length = data[0] | data[1] << 8L | data[2] << 16L | data[3] << 24L
| data[4] << 32L | data[5] << 40L | data[6] << 48L | data[7] << 56L;
This should define your constants as longs forcing it to use long arithmetic.
EDIT: Turns out this may not work according to comments below as while bitshift takes many operators on the left it only takes int on the right. Georg's should be the accepted answer.

Convert 3 Hex (byte) to Decimal

How do I take the hex 0A 25 10 A2 and get the end result of 851.00625? This must be multiplied by 0.000005. I have tried the following code without success:
byte oct6 = 0x0A;
byte oct7 = 0x25;
byte oct8 = 0x10;
byte oct9 = 0xA2;
decimal BaseFrequency = Convert.ToDecimal((oct9 | (oct8 << 8) | (oct7 << 16) | (oct6 << 32))) * 0.000005M;
I am not getting 851.00625 as the BaseFrequency.

oct6 is being shifted 8 bits too far (32 instead of 24)
decimal BaseFrequency = Convert.ToDecimal((oct9 | (oct8 << 8) | (oct7 << 16) | (oct6 << 24))) * 0.000005M;

How can I combine 4 bytes into a 32 bit unsigned integer?

I'm trying to convert 4 bytes into a 32 bit unsigned integer.
I thought maybe something like:
UInt32 combined = (UInt32)((map[i] << 32) | (map[i+1] << 24) | (map[i+2] << 16) | (map[i+3] << 8));
But this doesn't seem to be working. What am I missing?

Your shifts are all off by 8. Shift by 24, 16, 8, and 0.

Use the BitConverter class.
Specifically, this overload.

BitConverter.ToInt32()
You can always do something like this:
public static unsafe int ToInt32(byte[] value, int startIndex)
{
fixed (byte* numRef = &(value[startIndex]))
{
if ((startIndex % 4) == 0)
{
return *(((int*)numRef));
}
if (IsLittleEndian)
{
return (((numRef[0] | (numRef[1] << 8)) | (numRef[2] << 0x10)) | (numRef[3] << 0x18));
}
return ((((numRef[0] << 0x18) | (numRef[1] << 0x10)) | (numRef[2] << 8)) | numRef[3]);
}
}
But this would be reinventing the wheel, as this is actually how BitConverter.ToInt32() is implemented.