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Why StringBuilder Size is greater than string(~250MB).

Please read the question. I want to know the reason of size constraint in the string, but not in stringbuilder. I have fixed the problem of reading file.

Yes, I know there are operation, we can perform on string builder like append, replace, remove, etc. But what is the use of it when we can't get ToString() from it and we can't write it directly in the file. We had to get ToString() to actually use it, but because its size is out of string range it throws exception.

So in particular is there any use of string builder having size greated than string as i read a file of around 1 gb into string builder but cant get it into string. I read all the pros and cons of StringBuilder over String but I cant anything explaning this

Update: I want to load XMLDocument from file if reading in chunk then data cannot be loaded because root level node needs its closing tag which will be in other chunk block

Update: I know it is not a correct aproach now i am different process but still i want to know the reason of size constraing in string but not in stringbuilder

Update: I have Fixed my proble and want to know the reason why there is no memory constraint on stringbuilder.

XYZ
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    In first place why are you even loading 1GB file into memory? – Sriram Sakthivel May 05 '15 at 07:41
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    Read [this](http://stackoverflow.com/questions/6464628/processing-large-text-file-in-c-sharp) SO thread. It says about processing large text files. StringBuilder does not seem to be good solution in this scenario. – sszarek May 05 '15 at 07:42
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    You can generate smaller strings using `ToString(int,int)`. Why do you want to put so much text in memory though? What are you trying to do? There *are* better alternatives than doing everything inside a StringBuilder, eg using memory mapped files, parsing, using text generators etc – Panagiotis Kanavos May 05 '15 at 07:42
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    It's just not the right tool for the job, and/or not the right approach the the problem. – weston May 05 '15 at 07:42
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    This sounds like a case of the [XY Problem](http://meta.stackexchange.com/questions/66377/what-is-the-xy-problem). You have a problem with X and assume Y is the solution, so you ask about Y instead of X when you run into trouble. What is the actual problem you are trying to solve? – Panagiotis Kanavos May 05 '15 at 07:45
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    Could you explain what you want to do with the data you're reading? Maybe you could process it in chunks, so you don't have to have the whole GB in memory at the same time. – Stefan May 05 '15 at 07:45
  • What version of .net? As `StringBuilder` implementations differ greatly. – weston May 05 '15 at 07:54
  • I am using 4.5. Please see the updates in Question – XYZ May 05 '15 at 08:00
  • "it throws exception." What exception? – weston May 05 '15 at 08:02
  • @weston OutOfMemoryException – XYZ May 05 '15 at 08:04
  • @PanagiotisKanavos i want to know the reason for this. why there is no size constraint. I have fixed my problem. – XYZ May 05 '15 at 08:07
  • Remember that XML files are often UTF8, while `StringBuilder` is UTF16, so it often "doubles" the amount of memory used. – xanatos May 05 '15 at 08:10
  • @AmanSeth your real problem is how to handle large XML files, not StringBuilder - there's nothing wrong with its behavior. You *can* process large files using XmlReader to process one element at a time. XmlReader can read from a Stream so you can control how the data is read, how much is buffered etc – Panagiotis Kanavos May 05 '15 at 08:20

3 Answers3

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Why StringBuilder Size is greater than string(~250MB).

The reason depends on the version of .net.

There are two implementations Eric Lippert mentions here: https://stackoverflow.com/a/6524401/360211

Internally a string builder maintains a char[]. When you append it may have to resize this array. In order to stop it needing to be resized every time you append it resizes to a larger size to anticipate future appends (it actually doubles in size). So the StringBuilder often ends up larger than it's content, as much as double the size.

A newer implementation maintains a linked list of char[]. If you do many small appends, the overhead of the linked list may account for the extra 250MB.

In normal use, an extra 100% size on a string temporarily doesn't make one bit of difference given the performance benefits, but when you are dealing with a GB, it becomes significant and that is not its intended usage.

Why you get OutOfMemoryException

The linked list implementation can fit more in memory than a string because it does not need one continuous block of 1GB. When you ToString it would force it to try to find another GB, which is also continuous and that is the problem.

Why is there no constraint preventing this?

Well there is. The constraint is if there is not enough memory to create a string during ToString, throw an OutOfMemoryException.

You may want this to happen during Append operations, but that would be impossible to determine. StringBuilder could look at the free memory, but that might change before you call ToString. So the author of StringBuilder could have set an arbitrary limit, but that can't suit all systems equally, as some will have more memory than others.

You also might want to do operations that reduce the size of the StringBuilder before calling ToString, or not call ToString at all! So just because StringBuilder is too large to ToString at any point is not a reason to throw an exception.

Community
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weston
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  • Shouldn't be there any constraint if it is not intended to use like this – XYZ May 05 '15 at 08:03
  • Well where would the designers draw the line? And how would they enforce it at runtime. You could have situation where it works one day, then the next day the code has to deal with input that is just one byte larger than the limit and then it throws exception, that would be very bad. – weston May 05 '15 at 08:05
  • What designer you r talking about. thanks this is a very good explanation that string requires a single block of memory. But Still, this is not the answer to the question. when we know that we can't get that big data then this in string so why made the string that big – XYZ May 05 '15 at 08:12
  • The designer of the .net framework who wrote StringBuilder code. – weston May 05 '15 at 08:12
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    "this is not the answer to the question" well you've been changing your question! Anyway I have added to answer. – weston May 05 '15 at 08:18
  • Thanks Weston. This helps. I might be not have expressed the question in correct manner. – XYZ May 05 '15 at 08:22
  • Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/76967/discussion-between-aman-seth-and-weston). – XYZ May 05 '15 at 08:31
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You can use StringBuilder.ToString(int, int) to get smaller-sized chunks of your huge content out of of the StringBuilder.

In addition, you might want to consider whether you are really using the right tool for the job. StringBuilder's purpose is to build and modify strings, not to load huge files to memory.

Heinzi
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  • I have to read a xml file that big and if i split that data than it will not be possible to load that into XMLDocument – XYZ May 05 '15 at 07:57
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    @AmanSeth You should use XmlReader instead of XMLDocument. XmlReader uses the SAX mode of processings, reading elements and attributes as they appear in its input stream. It doesn't load the entire file in memory - in fact it can easily work with network streams or any kind of stream – Panagiotis Kanavos May 05 '15 at 08:26
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You can try the following to handle large XML files. CodeProject

abhinav pandey
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  • I dont want a solution to read xml file. I have made a new code that reads the huge file very efficiently and will share that in few time. Here i want to know the reason why there is no memory constraint on stringbuilder. – XYZ May 05 '15 at 08:16