在 C# 中读取第 100 万行

Question

I have a very long text file. All rows have the same length. I want to read in C# the millionth line without first reading the previous 999999 lines because otherwise the program becomes too slow. How can I do?

Answer 1

Try this

const int BYTES_PER_LINE = 120;
static void Main(string[] args)
{
    StreamReader reader = new StreamReader("FileName", Encoding.UTF8);
    long skipLines = 999999;

    reader.BaseStream.Position = skipLines * BYTES_PER_LINE;
}​

Answer 2

Do you know the number of bytes in each line?

NB Knowing the number of characters is not sufficient.

If you know it's a fixed number of bytes use:

using( Stream stream = File.Open(fileName, FileMode.Open) )
{
    stream.Seek(bytesPerLine * (myLine - 1), SeekOrigin.Begin);
    using( StreamReader reader = new StreamReader(stream) )
    {
        string line = reader.ReadLine();
    }
}

if not, then:

string line = File.ReadLines(FileName).Skip(999999).Take(1).First();

While this second option still requires the lines to be enumerated, it avoids reading the whole file into memory all at once in order to do so.

Of course, if by the millionth line you really mean the end of the file, a different approach would make sense. Find the size of the file, and use that to read lines off the end.

Answer 3

streamReader.BaseStream.Seek(skip_lines_offset, SeekOrigin.Begin);

string line = streamReader.ReadLine();

Seek method avoids reading the whole file. You can read more here . skip_lines_offset is the byte offset of the line, so number_of_skipped_lines * bytes_In_Line