Reading large text files with streams in C#

问题:

I\'ve got the lovely task of working out how to handle large files being loaded into our application\'s script editor (it\'s like VBA for our internal product for quick macros). Most files are about 300-400 KB which is fine loading. But when they go beyond 100 MB the process has a hard time (as you\'d expect).

What happens is that the file is read and shoved into a RichTextBox which is then navigated - don\'t worry too much about this part.

The developer who wrote the initial code is simply using a StreamReader and doing

[Reader].ReadToEnd()

which could take quite a while to complete.

My task is to break this bit of code up, read it in chunks into a buffer and show a progressbar with an option to cancel it.

Some assumptions:

• Most files will be 30-40 MB
• The contents of the file is text (not binary), some are Unix format, some are DOS.
• Once the contents is retrieved we work out what terminator is used.
• No-one\'s concerned once it\'s loaded the time it takes to render in the richtextbox. It\'s just the initial load of the text.

Now for the questions:

• Can I simply use StreamReader, then check the Length property (so ProgressMax) and issue a Read for a set buffer size and iterate through in a while loop WHILST inside a background worker, so it doesn\'t block the main UI thread? Then return the stringbuilder to the main thread once it\'s completed.
• The contents will be going to a StringBuilder. can I initialise the StringBuilder with the size of the stream if the length is available?

Are these (in your professional opinions) good ideas? I\'ve had a few issues in the past with reading content from Streams, because it will always miss the last few bytes or something, but I\'ll ask another question if this is the case.

回答1:

You can improve read speed by using a BufferedStream, like this:

using (FileStream fs = File.Open(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (BufferedStream bs = new BufferedStream(fs))
using (StreamReader sr = new StreamReader(bs))
{
    string line;
    while ((line = sr.ReadLine()) != null)
    {

    }
}

March 2013 UPDATE

I recently wrote code for reading and processing (searching for text in) 1 GB-ish text files (much larger than the files involved here) and achieved a significant performance gain by using a producer/consumer pattern. The producer task read in lines of text using the BufferedStream and handed them off to a separate consumer task that did the searching.

I used this as an opportunity to learn TPL Dataflow, which is very well suited for quickly coding this pattern.

Why BufferedStream is faster

A buffer is a block of bytes in memory used to cache data, thereby reducing the number of calls to the operating system. Buffers improve read and write performance. A buffer can be used for either reading or writing, but never both simultaneously. The Read and Write methods of BufferedStream automatically maintain the buffer.

December 2014 UPDATE: Your Mileage May Vary

Based on the comments, FileStream should be using a BufferedStream internally. At the time this answer was first provided, I measured a significant performance boost by adding a BufferedStream. At the time I was targeting .NET 3.x on a 32-bit platform. Today, targeting .NET 4.5 on a 64-bit platform, I do not see any improvement.

Related

I came across a case where streaming a large, generated CSV file to the Response stream from an ASP.Net MVC action was very slow. Adding a BufferedStream improved performance by 100x in this instance. For more see Unbuffered Output Very Slow

回答2:

You say you have been asked to show a progress bar while a large file is loading. Is that because the users genuinely want to see the exact % of file loading, or just because they want visual feedback that something is happening?

If the latter is true, then the solution becomes much simpler. Just do reader.ReadToEnd() on a background thread, and display a marquee-type progress bar instead of a proper one.

I raise this point because in my experience this is often the case. When you are writing a data processing program, then users will definitely be interested in a % complete figure, but for simple-but-slow UI updates, they are more likely to just want to know that the computer hasn\'t crashed. :-)

回答3:

If you read the performance and benchmark stats on this website, you\'ll see that the fastest way to read (because reading, writing, and processing are all different) a text file is the following snippet of code:

using (StreamReader sr = File.OpenText(fileName))
{
    string s = String.Empty;
    while ((s = sr.ReadLine()) != null)
    {
        //do your stuff here
    }
}

All up about 9 different methods were bench marked, but that one seem to come out ahead the majority of the time, even out performing the buffered reader as other readers have mentioned.

回答4:

For binary files, the fastest way of reading them I have found is this.

 MemoryMappedFile mmf = MemoryMappedFile.CreateFromFile(file);
 MemoryMappedViewStream mms = mmf.CreateViewStream();
 using (BinaryReader b = new BinaryReader(mms))
 {
 }

In my tests it\'s hundreds of times faster.

回答5:

Use a background worker and read only a limited number of lines. Read more only when the user scrolls.

And try to never use ReadToEnd(). It\'s one of the functions that you think \"why did they make it?\"; it\'s a script kiddies\' helper that goes fine with small things, but as you see, it sucks for large files...

Those guys telling you to use StringBuilder need to read the MSDN more often:

Performance Considerations
The Concat and AppendFormat methods both concatenate new data to an existing String or StringBuilder object. A String object concatenation operation always creates a new object from the existing string and the new data. A StringBuilder object maintains a buffer to accommodate the concatenation of new data. New data is appended to the end of the buffer if room is available; otherwise, a new, larger buffer is allocated, data from the original buffer is copied to the new buffer, then the new data is appended to the new buffer. The performance of a concatenation operation for a String or StringBuilder object depends on how often a memory allocation occurs.
A String concatenation operation always allocates memory, whereas a StringBuilder concatenation operation only allocates memory if the StringBuilder object buffer is too small to accommodate the new data. Consequently, the String class is preferable for a concatenation operation if a fixed number of String objects are concatenated. In that case, the individual concatenation operations might even be combined into a single operation by the compiler. A StringBuilder object is preferable for a concatenation operation if an arbitrary number of strings are concatenated; for example, if a loop concatenates a random number of strings of user input.

That means huge allocation of memory, what becomes large use of swap files system, that simulates sections of your hard disk drive to act like the RAM memory, but a hard disk drive is very slow.

The StringBuilder option looks fine for who use the system as a mono-user, but when you have two or more users reading large files at the same time, you have a problem.

回答6:

This should be enough to get you started.

class Program
{        
    static void Main(String[] args)
    {
        const int bufferSize = 1024;

        var sb = new StringBuilder();
        var buffer = new Char[bufferSize];
        var length = 0L;
        var totalRead = 0L;
        var count = bufferSize; 

        using (var sr = new StreamReader(@\"C:\\Temp\\file.txt\"))
        {
            length = sr.BaseStream.Length;               
            while (count > 0)
            {                    
                count = sr.Read(buffer, 0, bufferSize);
                sb.Append(buffer, 0, count);
                totalRead += count;
            }                
        }

        Console.ReadKey();
    }
}

回答7:

Have a look at the following code snippet. You have mentioned Most files will be 30-40 MB. This claims to read 180 MB in 1.4 seconds on an Intel Quad Core:

private int _bufferSize = 16384;

private void ReadFile(string filename)
{
    StringBuilder stringBuilder = new StringBuilder();
    FileStream fileStream = new FileStream(filename, FileMode.Open, FileAccess.Read);

    using (StreamReader streamReader = new StreamReader(fileStream))
    {
        char[] fileContents = new char[_bufferSize];
        int charsRead = streamReader.Read(fileContents, 0, _bufferSize);

        // Can\'t do much with 0 bytes
        if (charsRead == 0)
            throw new Exception(\"File is 0 bytes\");

        while (charsRead > 0)
        {
            stringBuilder.Append(fileContents);
            charsRead = streamReader.Read(fileContents, 0, _bufferSize);
        }
    }
}

Original Article

回答8:

You might be better off to use memory-mapped files handling here.. The memory mapped file support will be around in .NET 4 (I think...I heard that through someone else talking about it), hence this wrapper which uses p/invokes to do the same job..

Edit: See here on the MSDN for how it works, here\'s the blog entry indicating how it is done in the upcoming .NET 4 when it comes out as release. The link I have given earlier on is a wrapper around the pinvoke to achieve this. You can map the entire file into memory, and view it like a sliding window when scrolling through the file.

回答9:

An iterator might be perfect for this type of work:

public static IEnumerable<int> LoadFileWithProgress(string filename, StringBuilder stringData)
{
    const int charBufferSize = 4096;
    using (FileStream fs = File.OpenRead(filename))
    {
        using (BinaryReader br = new BinaryReader(fs))
        {
            long length = fs.Length;
            int numberOfChunks = Convert.ToInt32((length / charBufferSize)) + 1;
            double iter = 100 / Convert.ToDouble(numberOfChunks);
            double currentIter = 0;
            yield return Convert.ToInt32(currentIter);
            while (true)
            {
                char[] buffer = br.ReadChars(charBufferSize);
                if (buffer.Length == 0) break;
                stringData.Append(buffer);
                currentIter += iter;
                yield return Convert.ToInt32(currentIter);
            }
        }
    }
}

You can call it using the following:

string filename = \"C:\\\\myfile.txt\";
StringBuilder sb = new StringBuilder();
foreach (int progress in LoadFileWithProgress(filename, sb))
{
    // Update your progress counter here!
}
string fileData = sb.ToString();

As the file is loaded, the iterator will return the progress number from 0 to 100, which you can use to update your progress bar. Once the loop has finished, the StringBuilder will contain the contents of the text file.

Also, because you want text, we can just use BinaryReader to read in characters, which will ensure that your buffers line up correctly when reading any multi-byte characters (UTF-8, UTF-16, etc.).

This is all done without using background tasks, threads, or complex custom state machines.

回答10:

My file is over 13 GB:

The bellow link contains the code that read a piece of file easily:

Read a large text file

More information

回答11:

All excellent answers! however, for someone looking for an answer, these appear to be somewhat incomplete.

As a standard String can only of Size X, 2Gb to 4Gb depending on your configuration, these answers do not really fulfil the OP\'s question. One method is to work with a List of Strings:

List<string> Words = new List<string>();

using (StreamReader sr = new StreamReader(@\"C:\\Temp\\file.txt\"))
{

string line = string.Empty;

while ((line = sr.ReadLine()) != null)
{
    Words.Add(line);
}
}

Some may want to Tokenise and split the line when processing. The String List now can contain very large volumes of Text.