CODEWITHSUNDEEP
c#text-filesio
jessegavin
jessegavin

Reputation: 75690

How to efficiently write a large text file in C#?

I am creating a method in C# which generates a text file for a Google Product Feed. The feed will contain upwards of 30,000 records and the text file currently weighs in at ~7Mb.

Here's the code I am currently using (some lines removed for brevity's sake).

public static void GenerateTextFile(string filePath) {

  var sb = new StringBuilder(1000);
  sb.Append("availability").Append("\t");
  sb.Append("condition").Append("\t");
  sb.Append("description").Append("\t");
  // repetitive code hidden for brevity ...
  sb.Append(Environment.NewLine);

  var items = inventoryRepo.GetItemsForSale();

  foreach (var p in items) {
    sb.Append("in stock").Append("\t");
    sb.Append("used").Append("\t");
    sb.Append(p.Description).Append("\t");
    // repetitive code hidden for brevity ...
    sb.AppendLine();
  }

  using (StreamWriter outfile = new StreamWriter(filePath)) {
      result.Append("Writing text file to disk.").AppendLine();
      outfile.Write(sb.ToString());
  }
}

I am wondering if StringBuilder is the right tool for the job. Would there be performance gains if I used a TextWriter instead?

I don't know a ton about IO performance so any help or general improvements would be appreciated. Thanks.

Upvotes: 51

Views: 54526

Answers (4)

mxdog
mxdog

Reputation: 51

This might be old but I had a file to write with about 17 million lines so I ended up batching the writes every 10k lines similar to these lines

for (i6 = 1; i6 <= ball; i6++) 
{ //this is middle of 6 deep nest ..
  counter++;
  // modus to get a value at every so often 10k lines
  divtrue = counter % 10000; // remainder operator % for 10k
  //  build the string of fields with \n at the end 
  lineout = lineout + whatever 
  // the magic 10k block here
  if (divtrue.Equals(0))  
  {
     using (StreamWriter outFile = new StreamWriter(@filepath, true))
     { 
         //  write the 10k lines with .write NOT writeline..
         outFile.Write(lineout); 
     } 
     // reset the string so we dont do silly like memory overflow
     lineout = ""; 
  }
}

In my case it was MUCH faster then one line at a time.

Upvotes: 5

LBushkin
LBushkin

Reputation: 131806

File I/O operations are generally well optimized in modern operating systems. You shouldn't try to assemble the entire string for the file in memory ... just write it out piece by piece. The FileStream will take care of buffering and other performance considerations.

You can make this change easily by moving:

using (StreamWriter outfile = new StreamWriter(filePath)) {

to the top of the function, and getting rid of the StringBuilder writing directly to the file instead.

There are several reasons why you should avoid building up large strings in memory:

  1. It can actually perform worse, because the StringBuilder has to increase its capacity as you write to it, resulting in reallocation and copying of memory.
  2. It may require more memory than you can physically allocate - which may result in the use of virtual memory (the swap file) which is much slower than RAM.
  3. For truly large files (> 2Gb) you will run out of address space (on 32-bit platforms) and will fail to ever complete.
  4. To write the StringBuilder contents to a file you have to use ToString() which effectively doubles the memory consumption of the process since both copies must be in memory for a period of time. This operation may also fail if your address space is sufficiently fragmented, such that a single contiguous block of memory cannot be allocated.

Upvotes: 82

Alex Humphrey
Alex Humphrey

Reputation: 6219

Write one string at a time using StreamWriter.Write rather than caching everything in a StringBuilder.

Upvotes: 13

Jon Skeet
Jon Skeet

Reputation: 1503779

Just move the using statement so it encompasses the whole of your code, and write directly to the file. I see no point in keeping it all in memory first.

Upvotes: 27

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