What is the correct way of coping one text box to another using += ...

Question

Hi,

Bit of an odd one I am reading back some data an loading it to a rich text box
using the method:

for(int i = 0; i < Max; i++)
        rtbData_Back += stringParts[i] +','+'\n' ; 


I have noticed some of the data missing when I change it to: 
for(int i = 0; i < Max; i++)
        rtbData_Back = rtbData_Back + stringParts[i]  +','+'\n';
 
I have not seen error since.   I thought += was inherited from C++/Java,
so that can't be the problem, can it?

Glenn