What Would Be the Fastest Way to Concatenate Three Files in C#

What would be the fastest way to concatenate three files in C#?

void CopyStream(Stream destination, Stream source) {
   int count;
   byte[] buffer = new byte[BUFFER_SIZE];
   while( (count = source.Read(buffer, 0, buffer.Length)) > 0)
       destination.Write(buffer, 0, count);
}

CopyStream(outputFileStream, fileStream1);
CopyStream(outputFileStream, fileStream2);
CopyStream(outputFileStream, fileStream3);

Efficient way to combine multiple text files

Do it in chunks:

const int chunkSize = 2 * 1024; // 2KB
var inputFiles = new[] { "file1.dat", "file2.dat", "file3.dat" };
using (var output = File.Create("output.dat"))
{
    foreach (var file in inputFiles)
    {
        using (var input = File.OpenRead(file))
        {
            var buffer = new byte[chunkSize];
            int bytesRead;
            while ((bytesRead = input.Read(buffer, 0, buffer.Length)) > 0)
            {
                output.Write(buffer, 0, bytesRead);
            }
        }
    }
}

Combine multiple files into single file

General answer

Why not just use the Stream.CopyTo(Stream destination) method?

private static void CombineMultipleFilesIntoSingleFile(string inputDirectoryPath, string inputFileNamePattern, string outputFilePath)
{
    string[] inputFilePaths = Directory.GetFiles(inputDirectoryPath, inputFileNamePattern);
    Console.WriteLine("Number of files: {0}.", inputFilePaths.Length);
    using (var outputStream = File.Create(outputFilePath))
    {
        foreach (var inputFilePath in inputFilePaths)
        {
            using (var inputStream = File.OpenRead(inputFilePath))
            {
                // Buffer size can be passed as the second argument.
                inputStream.CopyTo(outputStream);
            }
            Console.WriteLine("The file {0} has been processed.", inputFilePath);
        }
    }
}

Buffer size adjustment

Please, note that the mentioned method is overloaded.

There are two method overloads:

1. CopyTo(Stream destination).
2. CopyTo(Stream destination, int bufferSize).

The second method overload provides the buffer size adjustment through the bufferSize parameter.

How to merge efficiently gigantic files with C#

So "merging" is really just writing the files one after the other? That's pretty straightforward - just open one output stream, and then repeatedly open an input stream, copy the data, close. For example:

static void ConcatenateFiles(string outputFile, params string[] inputFiles)
{
    using (Stream output = File.OpenWrite(outputFile))
    {
        foreach (string inputFile in inputFiles)
        {
            using (Stream input = File.OpenRead(inputFile))
            {
                input.CopyTo(output);
            }
        }
    }
}

That's using the Stream.CopyTo method which is new in .NET 4. If you're not using .NET 4, another helper method would come in handy:

private static void CopyStream(Stream input, Stream output)
{
    byte[] buffer = new byte[8192];
    int bytesRead;
    while ((bytesRead = input.Read(buffer, 0, buffer.Length)) > 0)
    {
        output.Write(buffer, 0, bytesRead);
    }
}

There's nothing that I'm aware of that is more efficient than this... but importantly, this won't take up much memory on your system at all. It's not like it's repeatedly reading the whole file into memory then writing it all out again.

EDIT: As pointed out in the comments, there are ways you can fiddle with file options to potentially make it slightly more efficient in terms of what the file system does with the data. But fundamentally you're going to be reading the data and writing it, a buffer at a time, either way.

What is the fastest method to merge a number of files into a file in c#?

Your code looks fine but ElementAt is a code smell. Convert that to an array and use [i] instead. If you have 10K elements I'm positive you're wasting a lot of time.

What is the fastest way to combine two xml files into one

The easiest way to do this is using LINQ to XML. You can use either Union or Concat depending on your needs.

var xml1 = XDocument.Load("file1.xml");
var xml2 = XDocument.Load("file2.xml");

//Combine and remove duplicates
var combinedUnique = xml1.Descendants("AllNodes")
                          .Union(xml2.Descendants("AllNodes"));

//Combine and keep duplicates
var combinedWithDups = xml1.Descendants("AllNodes")
                           .Concat(xml2.Descendants("AllNodes"));

What is the most efficient way in C# to merge more than 2 xml files with the same schema together?

I'm going to go out on a limb here and assume that your xml looks something like:

<records>
  <record>
    <dataPoint1/>
    <dataPoint2/>
  </record>
</records>

If that's the case, I would open a file stream and write the <records> part, then sequentially open each XML file and write all lines (except the first and last) to disk. That way you don't have huge strings in memory and it should all be very, very quick to code and run.

public void ConsolidateFiles(List<String> files, string outputFile)
{
  var output = new StreamWriter(File.Open(outputFile, FileMode.Create));
  output.WriteLine("<records>");
  foreach (var file in files)
  {
    var input = new StreamReader(File.Open(file, FileMode.Open));
    string line;
    while (!input.EndOfStream)
    {
      line = input.ReadLine();
      if (!line.Contains("<records>") &&
          !line.Contains("</records>"))
      {
        output.Write(line);
      }
    }
  }
  output.WriteLine("</records>");
}

What's the fastest way of appending text from one file to another with huge files

If maintaining order is not important, and if the potential characters are limited (eg A-Z), a possibility would be to say, "OK, let's start with the As".

So you start with each file, and go through line by line until you find a line starting with 'A'. If you find one, add it to a new file and a HashSet. Each time you find a new line starting with 'A', check if it is in the HashSet, and if not add it to both the new file and the HashSet. Once you've processed all files, dispose the HashSet and skip to the next letter (B).

You're going to iterate through the files 26 times this way.

Of course you can optimise it even further. Check how much memory is available and divide the possible characters by ranges, so for example with the first iteration your HashSet might contain anything starting with A-D.

What is the best way to merge large files?

Suppose you have a condition which must be true (i.e. a predicate) for each line in one file that you want to append to another file.

You can efficiently process that as follows:

var filteredLines = 
    File.ReadLines("MySourceFileName")
    .Where(line => line.Contains("Target")); // Put your own condition here.

File.AppendAllLines("MyDestinationFileName", filteredLines);

This approach scales to multiple files and avoids loading the entire file into memory.

If instead of appending all the lines to a file, you wanted to replace the contents, you'd do:

File.WriteAllLines("MyDestinationFileName", filteredLines);

instead of

File.AppendAllLines("MyDestinationFileName", filteredLines);

Also note that there are overloads of these methods that allow you to specify the encoding, if you are not using UTF8.

Finally, don't be thrown by the inconsistent method naming.File.ReadLines() does not read all lines into memory, but File.ReadAllLines() does. However, File.WriteAllLines() does NOT buffer all lines into memory, or expect them to all be buffered in memory; it uses IEnumerable<string> for the input.

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