using (StreamWriter filenew = new StreamWriter(fileDetail.TempFileName, true, Encoding.ASCII))
                                {
                                    filenew.WriteLine(newContent);
                                }

要知道正确的编码，只需创建一个新的临时文件，在编辑器中写入一些内容并保存即可。在记事本中打开文件并执行“另存为”。这将在底部显示当前编码。在 .NET 代码中设置该编码。

如果这不起作用，请尝试(按照 shr 的建议):

using (StreamWriter filenew = new StreamWriter(fileDetail.TempFileName, true, Encoding.ASCII))
                                {
                                    filenew.Write(newContent + "\r\n");
                                }

长版:(可能与您的代码略有不同):

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            DateTime LastChecked = DateTime.Now;

            IDictionary<string, FileDetails> fileDetails = new Dictionary<string, FileDetails>(StringComparer.OrdinalIgnoreCase);
            IList<string> tempFileList = new List<string>();

            try
            {
                string[] files = System.IO.Directory.GetFiles(@"C:\Test", "*.jrn", System.IO.SearchOption.AllDirectories);

                foreach (string file in files)
                {
                    string currentfilename = file;
                    string currentcontent = string.Empty;

                    if (!fileDetails.Keys.Contains(file))
                    {
                        fileDetails[file] = new FileDetails(copywarehouse(file));
                        //do_some_processing();
                    }

                    try
                    {
                        using (StreamReader sr = new StreamReader(file))
                        {
                            currentcontent = sr.ReadToEnd();
                        }
                    }
                    catch (Exception)
                    {
                        // Let the user know what went wrong.
                    }

                    fileDetails[file].AddContent(currentcontent);
                }

                //TODO: Check using the file modified time. Avoids unnecessary reading of file.
                foreach (var fileDetail in fileDetails.Values)
                {
                    //checking
                    try
                    {
                        string tempFileContent = string.Empty;
                        string currentcontent = fileDetail.GetContent();

                        using (StreamReader sr = new StreamReader(fileDetail.TempFileName))
                        {
                            tempFileContent = sr.ReadToEnd();
                            sr.Close();
                        }

                        if (!(0 == string.Compare(tempFileContent, currentcontent)))
                        {
                            if (currentcontent.Contains(tempFileContent))
                            {
                                string newContent = tempFileContent.Substring(currentcontent.Length);

                                using (StreamWriter filenew = new StreamWriter(fileDetail.TempFileName, true, Encoding.ASCII))
                                {
                                    filenew.WriteLine(newContent);
                                }
                            }
                        }
                    }
                    catch (Exception)
                    {
                        // Let the user know what went wrong.
                    }

                }
            }
            catch (Exception)
            {
            }
        }

        private static string copywarehouse(string filename)
        {
            string sourcePath = @"C:\Test";
            string targetPath = @"C:\Test";

            string sourceFile = System.IO.Path.Combine(sourcePath, filename);
            string destFile = System.IO.Path.Combine(targetPath, filename+ "tempfile.txt");

            try
            {
                System.IO.File.Copy(sourceFile, destFile, true);
            }
            catch (Exception)
            {
            }

            return destFile;
        }

        internal class FileDetails
        {
            public string TempFileName { get; private set; }
            private StringBuilder _content;

            public FileDetails(string tempFileName)
            {
                TempFileName = tempFileName;
                _content = new StringBuilder();
            }

            public void AddContent(string content)
            {
                _content.Append(content);
            }

            public string GetContent()
            {
                return _content.ToString();
            }
        }
    }
}

编辑 2: 您能否将 copywarehouse 更改为此并看到问题仍然存在:

         private void copywarehouse(string filename)
        {
            const string sourcePath = @"C:\Test";
            const string targetPath = @"C:\Test";
            try
            {
                string sourceFile = Path.Combine(sourcePath, filename);
                string destFile = Path.Combine(targetPath, "tempfile.txt");


                string currentcontent;
                using (var sr = new StreamReader(sourceFile))
                {
                    currentcontent = sr.ReadToEnd();
                }

                using (var wr = new StreamWriter(destFile, false, Encoding.ASCII))
                {
                    wr.WriteLine(currentcontent);
                }
            }
            catch (Exception)
            {

            }
        }