string and array are immutable?

Question

Hello everyone I read from some documents that string/array are immutable means when we change the content of a string (e.g. append text) or change the content of an array (e.g. append new element) then a new instance be created and the old instance will be discarded? thanks in advance George

Ryan Alford · Answer

in your original question you are half right. yes strings are immutable. you never actually modify the string variable after it's initial instantiation. The part that you are wrong is the the old instances will be discarded. The old instances will stay in memory. example: string value = string.Empty; value += hey; value += there; value += how; value += are; value += you; value += doing; in memory you have now created 7 seperate variables. these instances will stay in memory. because of this you are recommended to use the StringBuilder object when appending to a string.

Niradhip Chakraborty · Answer

String str1 = “xyz”; String str2 = “xyz”; Now compiler will check in String Pool where there is same characters are available or not. If two Strings are match the compiler will refer str1 address to str2. Now the str1 and str2 referring the same characters in same memory location. This is a good idea for increasing memory efficiency. When we change the str1 characters the changes will be reflected to str2. Because both str1 and str2 variables are referring the same memory location. For avoiding this we are keeping String as immutable. However we can use StringBuffer if you want to do modifications in the string.

Rasik Bihari Tiwari · Answer

The concept of Pool format which @Niradip is referring to is conceptually known as string interning in C#. You can refer to Eric's blog here for more details to understand string interning. https://blogs.msdn.microsoft.com/ericlippert/2009/09/28/string-interning-and-string-empty/

George George · Answer

Thanks Niradhip In your case then the content of the string pointed by str1 changes I think the underlying operation is there is a new instance created to reflect the new string content and str1 will be binded to the new string and str2 still binded to the old string correct? regards George

George George · Answer

Thanks Niradhip! 1. What means pool format? 2. I think for array we can not change its content (e.g. when change some specific char inside a string a new instance will be created) correct? regards George

Niradhip Chakraborty · Answer

String in a class it is used to holding the array of characters.String is immutable. Means we can not change the value of the string. Strings are handling in Pool format. For example: String str1 = “xyz”; This string(str1) will be stored into memory in particular address. Check Out the following articles: 1. http://www.braintique.com/csharp/chapters/03ch01.shtml 2.http://blogs.msdn.com/ericlippert/archive/2007/11/13/immutability-in-c-part-one-kinds-of-immutability.aspx

George George · Answer

Thanks Ryan question answered. regards George