How to pass substring of byte array using index in C#?

Question

I want to transmit one APDU and I get back the response. I want to check the last two bytes by an API which will log the comparison.

byte[] response = Transmit(apdu);

//response here comes will be 0x00 0x01 0x02 0x03 0x04 
//response.Length will be 5


byte[] expectedResponse = { 0x03, 0x04 };

int index = (response.Length)-2;

Log.CheckLastTwoBytes(response[index],expectedResponse);

//The declaration of CheckLastTwoBytes is 
//public static void CheckLastTwoBytes(byte[] Response, byte[] ExpResp)

This is an error of invalid arguments. How can I pass the last 2 bytes to APIs?

Actually I wanted to know, if it is possible to pass substring using index without using temporary array. (like how we can do in C) — SHRI
– SHRI, Commented Nov 29, 2012 at 3:31
ArraySegment is a way to reference a slice of an array without making a copy of it — gordy
– gordy, Commented Nov 29, 2012 at 4:18

Tilak · Accepted Answer · 2012-11-29 03:18:39Z

3

Use Array.Copy

byte[] newArray = new byte[2];
Array.Copy(response, response.Length-2, newArray, 2);
Log.CheckLastTwoBytes(newArray,expectedResponse);

answered Nov 29, 2012 at 3:18

Tilak

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2 Comments

YoryeNathan Over a year ago

Or... you could just have two assignments? It's just two bytes. No need for actually Array.Copy...

Tilak Over a year ago

yeah, Array.Copy can be replaced with 2 assignments.

gordy · Accepted Answer · 2012-11-29 04:24:26Z

1

new ArraySegment<byte>(response, response.Length - 2, 2).Array

EDIT: nevermind this, apparently .Array just returns the original entire array and not the slice. You would have to modify your other method to accept ArraySegment instead of byte[]

edited Nov 29, 2012 at 4:24

answered Nov 29, 2012 at 3:26

gordy

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Comments

Edmund Schweppe · Accepted Answer · 2012-11-29 03:23:41Z

1

Since the type of response[index] is byte (not byte[]), it's not surprising that you'd get that error.

If Log.CheckLastTwoBytes really does check just the last two bytes of its Response parameter, then you should just pass response:

   Log.CheckLastTwoBytes(response, expectedResponse)

answered Nov 29, 2012 at 3:23

Edmund Schweppe

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Comments

YoryeNathan · Accepted Answer · 2012-11-29 03:52:42Z

1

You can't have a subarray just like that, no...

First solution, obvious one:

var tmp = new byte[] { response[response.Length - 2],
                       response[response.Length - 1] };

Log.CheckLastTwoBytes(tmp, expectedResponse);

Or, you could do this:

response[0] = response[response.Length - 2];
response[1] = response[response.Length - 1];

Log.CheckLastTwoBytes(response, expectedResponse);

It might be that this function doesn't check for exact lengths, etc, so you could just put the last two bytes as the first two, if you don't care about destroying the data.

edited Nov 29, 2012 at 3:52

answered Nov 29, 2012 at 3:44

YoryeNathan

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RAS · Accepted Answer · 2012-11-29 03:22:29Z

0

Or, alternatively, you can use linq:

byte[] lastTwoBytes = response.Skip(response.Length-2).Take(2).ToArray();

answered Nov 29, 2012 at 3:22

RAS

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