如何限制.NET中StreamReader.ReadLine()读取的字符数？

Question

我正在用 C# 编写一个 Web 服务器应用程序，并使用 StreamReader 类从底层 NetworkStream 读取数据：

 NetworkStream ns = new NetworkStream(clientSocket);
 StreamReader sr = new StreamReader(ns);
 String request = sr.ReadLine();

此代码很容易受到 DoS 攻击，因为如果攻击者从不断开连接，我们将永远无法读完该行。 有没有办法限制.NET中StreamReader.ReadLine()读取的字符数？

Answer 1

您必须使用 Read(char[], int, int) 重载（这确实限制了长度）并进行自己的行尾检测； 不应该太棘手。

对于稍微懒惰的版本（使用单字符阅读版本）：

static IEnumerable<string> ReadLines(string path, int maxLineLength)
{
    StringBuilder currentLine = new StringBuilder(maxLineLength);
    using (var reader = File.OpenText(path))
    {
        int i;
        while((i = reader.Read()) > 0) {
            char c = (char) i;
            if(c == '\r' || c == '\n') {
                yield return currentLine.ToString();
                currentLine.Length = 0;
                continue;
            }
            currentLine.Append((char)c);
            if (currentLine.Length > maxLineLength)
            {
                throw new InvalidOperationException("Max length exceeded");
            }
        }
        if (currentLine.Length > 0)
        {
            yield return currentLine.ToString();
        }                
    }
}

Answer 2

您可能需要 StreamReader.Read 重载之一：

取自 http://msdn.microsoft.com/en-us/library/9kstw824.aspx

    using (StreamReader sr = new StreamReader(path)) 
    {
        //This is an arbitrary size for this example.
        char[] c = null;

        while (sr.Peek() >= 0) 
        {
            c = new char[5];
            sr.Read(c, 0, c.Length);
            //The output will look odd, because
            //only five characters are read at a time.
            Console.WriteLine(c);
        }
    }

重点关注 sr.Read(c, 0, c.Length) 行。 这仅从流中读取 5 个字符并将其放入 c 数组中。 您可能需要将 5 更改为您想要的值。

Answer 3

这是我自己的解决方案，基于 Marc Gravell 的解决方案：

using System;
using System.IO;
using System.Text;

namespace MyProject
{
    class StreamReaderExt : StreamReader
    {

        public StreamReaderExt(Stream s, Encoding e) : base(s, e)
        {            
        }

        /// <summary>
        /// Reads a line of characters terminated by CR+LF from the current stream and returns the data as a string
        /// </summary>
        /// <param name="maxLineLength">Maximum allowed line length</param>
        /// <exception cref="System.IO.IOException" />
        /// <exception cref="System.InvalidOperationException">When string read by this method exceeds the maximum allowed line length</exception>
        /// <returns></returns>
        public string ReadLineCRLF(int maxLineLength)
        {
            StringBuilder currentLine = new StringBuilder(maxLineLength);

            int i;
            bool foundCR = false;
            bool readData = false;

            while ((i = Read()) > 0)
            {

                readData = true;

                char c = (char)i;

                if (foundCR)
                {
                    if (c == '\r')
                    {
                        // If CR was found before , and the next character is also CR,
                        // adding previously skipped CR to the result string
                        currentLine.Append('\r');
                        continue;
                    }
                    else if (c == '\n')
                    {
                        // LF found, finished reading the string
                        return currentLine.ToString();
                    }
                    else
                    {
                        // If CR was found before , but the next character is not LF,
                        // adding previously skipped CR to the result string
                        currentLine.Append('\r');
                        foundCR = false;
                    }
                }
                else // CR not found
                {
                    if (c == '\r')
                    {
                        foundCR = true;
                        continue;
                    }
                }

                currentLine.Append((char)c);
                if (currentLine.Length > maxLineLength)
                {
                    throw new InvalidOperationException("Max line length exceeded");
                }
            }

            if (foundCR)
            {
                // If CR was found before, and the end of the stream has been reached, appending the skipped CR character
                currentLine.Append('\r');
            }

            if (readData)
            {
                return currentLine.ToString();
            }

            // End of the stream reached
            return null;

        }
    }
}

这段代码“按原样”提供，没有任何保证。

Answer 4

您始终可以使用“.Read(...)”，并且 MSDN 建议针对像您这样的情况这样做。

http://msdn.microsoft.com/en-us/library/system.io。 Streamreader.readline.aspx