arrays - Converting byte[] to string[] in c# -

i have byte array of length 56 , converted string using function:

str = bitconverter.tostring(bytes).replace("-", ""); 

now, need copy first twenty characters of "str" string "keydata" or string[] , when use

array.copy(str, 0, keydata, 0, 20);  

i error stating

parameter needs string[] , not string

how past this?

string anotherstring = str.substring(0, 20); 

or

array arr = str.substring(0, 20).toarray();