Cannot concat string in C#

Question

I have a code to read data block inside MIFARE card.

The method rfidM1.ReadDataFromCardM1 will read a block and return value in string.

string memQuery = string.Empty;
int i = 0, j = 0;
sector = 4;
block = 4;

for (i = 0; i < block; i++)
{
    for (j = 0; j < sector; j++)
    {                  
        memQuery += rfidM1.ReadDataFromCardM1(Convert.ToByte(j), Convert.ToByte(i), _Key1) + ",";
    }
}

My intention is concating memQuery with comma. Example output here:

,0,,,,,True,,C0-12320,0,,,,,,

I concat memQuery with various ways, for example, using +=, StringBuilder or ArrayList but they didn't work because it always has an output like this when I put it in MessageBox.

,0

It looks like string after that 0 cannot concat with other string after it. Why?

Try and use a List<string> (which contains real strings) instead of an ArrayList (which contains just objects) — Hans Keﬆing
– Hans Keﬆing, Commented May 4, 2018 at 8:44
I'm getting confused with that memQuery.Add. Is that a collection? Try refactoring your code. What are the variables and values you want to concat to that string? K ? You're near it smells weird near that .ToString().Replace() . Try creating different methods for each purpose so it will be easier to debug the problem. — Gonzo345
– Gonzo345, Commented May 4, 2018 at 8:46
OT the usual way to declare an index variable of a for statement is inside it: for (int i=0; .... And you could also declare it as byte if that is what you really need — Hans Keﬆing
– Hans Keﬆing, Commented May 4, 2018 at 8:50
@HansKesting I tried it with List<string> and got the same result. — Veerakran Sereerungruangkul
– Veerakran Sereerungruangkul, Commented May 4, 2018 at 8:53
if memQuery[k] returns null, you will get a NullReferenceException when trying to execute any method on it, such as GetType() or ToString(). Could that ReadDataFromCardM1 return nulls or control characters? — Hans Keﬆing
– Hans Keﬆing, Commented May 4, 2018 at 9:00

bommelding · Accepted Answer · 2018-05-04 09:05:09Z

1

My intention is concating memQuery with comma

Wel, first get rid of the ArrayList and replace it with var memQuery = new List<string>();.

Inside your for-loops, decide what to with null or empty results. Add a null or skip the Add or ...

And then when the memQuery is filled correctly, you can do

 string result = string.Join(",", memQuery);

string.Join() can handle nulls in the input sequence.

edited May 4, 2018 at 9:05

answered May 4, 2018 at 8:59

bommelding

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2 Comments

Veerakran Sereerungruangkul Over a year ago

Thanks for the answer. It turned out those return string have control characters inside them. I removed them by following this thread stackoverflow.com/a/15259355/4535654 and it works now.

bommelding Over a year ago

Then maybe they were not strings at all... It all depends on what you want to show.

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