a question about String

[smartnose]

I defined a string: dim str as string and I assigned it a long string with more than 300 bytes: str="kfa;sdlkjfakdjflakjsdfkd... when the program runs, I found that str contains only 253 bytes and no more, as if the string is truncated. And I try to use str=str + "asldkk" to increase the length of this string graduately, but it still cannot be longer than 253 bytes. But MSDN tells me that strings can be much longer than that. So how could this happen? Thank you.