bigLarry bigLarry - 3 months ago 11
C# Question

Translating bitwise operations from Visual Basic to C#

Having mixed results when trying to translate a function using bitwise operations from Visual Basic to C#.

I'm sure I'm missing something really (visually) basic.

Here's the original VB code:

Dim CRCresult As UInt16 = 0
Dim CRC_16_POLY As UInt16 = &HA001

Private Sub CRC16(newByte As Byte)
CRCresult = CRCresult Xor newByte
For i As Integer = 0 To 7
If (CRCresult And &H1) Then
CRCresult = (CRCresult >> 1) Xor CRC_16_POLY
Else
CRCresult = (CRCresult >> 1)
End If
Next
End Sub


When I feed it the first 8 terms of this array:

{&H47, &H41, &H50, &H53, &H41, &HC, &H0, &H1, &H0, &H0, &H0, &HFF}


CRCresult appears as follows:

12864, 49395, 31104, 40248, 9949, 23782, 35549, 39307


Below is my translation to C#:

private UInt16 CRCresult = 0;
private UInt16 CRC_16_POLY = 0xA001;

private void CRC16(byte newByte)
{
CRCresult = (UInt16)(CRCresult ^ newByte);
for (int i = 0; i <= 7; i++) {
if((CRCresult & 0x1) != 0)
{
CRCresult = (UInt16)((CRCresult >> 1) ^ CRC_16_POLY);
} else
{
CRCresult = (UInt16)(CRCresult >> 1);
}
}
}


When I feed it the first 8 terms of this array:

{ 0x47, 0x41, 0x50, 0x53, 0x41, 0xC, 0x0, 0x1, 0x0, 0x0, 0x0, 0xFF }


CRCresult appears as follows:

12864, 49395, 31104, 40248, 57948, 15586, 18876, 29065


In summary

newBytes CRCresult
VB | C# | VB | C#
------------- ---------------
&H47 | 0x47 | 12864 | 12864
&H41 | 0x41 | 49395 | 49395
&H50 | 0x50 | 31104 | 31104
&H53 | 0x53 | 40248 | 40248
&H41 | 0x41 | 9949 | 57948
&HC | 0xC | 23782 | 15586
&H0 | 0x0 | 35549 | 18876
&H1 | 0x1 | 39307 | 29065
&H0 | 0x0 |
&H0 | 0x0 |
&H0 | 0x0 |
&HFF | 0xFF |


What am I missing in my translation that's causing the differences in the results?

Answer

both implementations are correct and produce same results

i found a sequence with a different value at pos 5 (index 4) &H4f instead &H41

thisgenerate your (VB) results:

{&H47, &H41, &H50, &H53, &H4f, &HC, &H0, &H1, &H0, &H0, &H0, &HFF}

and this for (C#) results:

{&H47, &H41, &H50, &H53, &H41, &HC, &H0, &H1, &H0, &H0, &H0, &HFF}

maybe a small typing error in your test data? :-)

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