CODEWITHSUNDEEP
c#serial-portrangechecksummodulo
Mathias Van den Bossche
Mathias Van den Bossche

Reputation: 13

Problems with checksum modulo 256 C#

I am a student industrial engineering in the field of electrotechnics. I am not used to work with c#. I am trying to make a checksum tha consists of 2 bytes. The checksum is computed with the following algorithm: 

A = 0xFF 
B = 0 

For I FROM 0 TO number_of_bytes -1 DO 
   A := (A + DATA[I]) mod 0x100; 
   B := (B + A) mod 0x100; 
END 

checksum[0] := A 
checksum[1] := B

A and B are byte values and the addition is made modulo 256.

When i run the program it breaks at A += byteToCalculate[i]; This warning occures:

"An exception of type 'System.IndexOutOfRangeException' occurred in communicatie.exe but was not handled in user code."

Can someone help me with this problem?

    public void ChecksumBerekenen(object sender, EventArgs e)
    {

        byte[] byteToCalculate = new byte[] { 0x00, 0x02, 0x02, 0x00, 0x58, 0x04, 0x00, 0x00, 0x05, 0x00, bytewaarde_2[3], bytewaarde_2[2], bytewaarde_2[1], bytewaarde_2[0] };

        int A = 0xff;
        int B = 0;

        for (int i = 0; i <= byteToCalculate.Length ; i++)
        {
            A += byteToCalculate[i];
            A %= 0x100;

            B += A ;
            B %= 0x100;
        }
        VerstuurdeData.AppendText(A.ToString("x") + " " + B.ToString("x"));
    }

Upvotes: 1

Views: 990

Answers (2)

Dmitrii Bychenko
Dmitrii Bychenko

Reputation: 186813

In order to avoid such pesky errors (<= instead of required <) use foreach loop:

  foreach (var item in byteToCalculate) {
    A += item;
    A %= 0x100;

    B += A;
    B %= 0x100;
  }

and you'll have a more readable code as a bonus.

Upvotes: 2

Marco Fatica
Marco Fatica

Reputation: 937

The error is here: for (int i = 0; i <= byteToCalculate.Length ; i++)

You want this for (int i = 0; i < byteToCalculate.Length ; i++)

Array indices are 0-based so you want to iterate to 1 less than the length.

Upvotes: 0

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