How Can I Efficiently Access a Specific Line in a Large Text File Using C#?

C# efficiently reads specific lines in large text files

Need to read specific lines from a large text file, avoiding the memory-intensive solution of storing the entire file as an array of strings. The following methods can be achieved:

Method 1: Byte Offset Method

• Calculate the byte offset of the target line in the file.
• Position the file stream to the calculated offset using the FileStream.Seek method.
• Use StreamReader.ReadLine() to read the line.

Method 2: LINQ method (applicable to .NET 4.0 and above)

• Use File.ReadLines(fileName).Skip(line - 1).Take(1).First().
• This method allows direct access to individual lines without reading the entire file.

Method three: Custom line reader

• Create a custom line reader that skips a specified number of bytes before reading a line. This method is suitable for small line numbers and can be processed efficiently.

Example (Method 1)

public static string GetLine(string fileName, int line)
{
    using (var sr = new StreamReader(fileName))
    {
        // ... (代码略，此处需补充计算字节偏移量和读取行的逻辑) ...
    }
}

Processing structured documents

For structured documents, you can use the following strategies:

• Use File.ReadLines(fileName).Take(25).ToArray() to read the first 25 lines.
• Parse line 17 to determine the value of variable X.
• Use File.ReadLines(fileName).Skip(25).Take(X).ToArray() to read the next X lines.

These methods allow you to efficiently read specific lines from a large text file without loading the entire file into memory. Which method you choose depends on the file's specific size, structure, and .NET Framework version.

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