how to build a double quoted string

For this you need to seach in google with keyword
"escape sequence for double quotes"
I can explain you with example you can make changes as per your need

Consider i need string as => My friend told "being late is better than never"
so now to add this string to variable

string str =  "My friend told" +"\"being late is better than never\"";

StringBuffer strBuf = new StringBuffer();

strBuf.Append(str);

Where \ is used to escape the next character to it.

Hope this helps you
Cheers