Creating a Random File in C#

Question

I am creating a file of a specified size - I don't care what data is in it, although random would be nice. Currently I am doing this:

        var sizeInMB = 3; // Up to many Gb         using (FileStream stream = new FileStream(fileName, FileMode.Create))         {             using (BinaryWriter writer = new BinaryWriter(stream))             {                 while (writer.BaseStream.Length <= sizeInMB * 1000000)                 {                     writer.Write("a"); //This could be random. Also, larger strings improve performance obviously                 }                 writer.Close();             }         } 

This isn't efficient or even the right way to go about it. Any higher performance solutions?

Thanks for all the answers.

Edit

Ran some tests on the following methods for a 2Gb File (time in ms):

Method 1: Jon Skeet

byte[] data = new byte[sizeInMb * 1024 * 1024]; Random rng = new Random(); rng.NextBytes(data); File.WriteAllBytes(fileName, data); 

N/A - Out of Memory Exception for 2Gb File

Method 2: Jon Skeet

byte[] data = new byte[8192]; Random rng = new Random(); using (FileStream stream = File.OpenWrite(fileName)) {     for (int i = 0; i < sizeInMB * 128; i++)     {          rng.NextBytes(data);          stream.Write(data, 0, data.Length);     } } 

@1K - 45,868, 23,283, 23,346

@128K - 24,877, 20,585, 20,716

@8Kb - 30,426, 22,936, 22,936

Method 3 - Hans Passant (Super Fast but data isn't random)

using (var fs = new FileStream(fileName, FileMode.Create, FileAccess.Write, FileShare.None)) {     fs.SetLength(sizeInMB * 1024 * 1024); } 

257, 287, 3, 3, 2, 3 etc.

Answer 1

Well, a very simple solution:

byte[] data = new byte[sizeInMb * 1024 * 1024]; Random rng = new Random(); rng.NextBytes(data); File.WriteAllBytes(fileName, data); 

A slightly more memory efficient version :)

// Note: block size must be a factor of 1MB to avoid rounding errors :) const int blockSize = 1024 * 8; const int blocksPerMb = (1024 * 1024) / blockSize; byte[] data = new byte[blockSize]; Random rng = new Random(); using (FileStream stream = File.OpenWrite(fileName)) {     // There      for (int i = 0; i < sizeInMb * blocksPerMb; i++)     {         rng.NextBytes(data);         stream.Write(data, 0, data.Length);     } } 

However, if you do this several times in very quick succession creating a new instance of Random each time, you may get duplicate data. See my article on randomness for more information - you could avoid this using System.Security.Cryptography.RandomNumberGenerator... or by reusing the same instance of Random multiple times - with the caveat that it's not thread-safe.