尝试此解决方案，可能会有所帮助

using (StreamWriter stream = new StreamWriter(attachment,false))
        {
            stream.WriteLine("some text here");
        }

问题出在MailMessage实例上，它使文件保持打开状态。处理邮件实例对我有用。

mail.Dispose();

string directory = Application.StartupPath + @"\Data Base";//create this folder
string fileName = "write here file name" + ".dat";//you can change type of file like .txt
string memoryPath = Path.Combine(directory,fileName);
using (StreamWriter sw = File.CreateText(memoryPath))
{
     sw.WriteLine("write here what you want");
}

使用此方法：

           private bool WriteToDisk(string content,string filePath)
            {
                using (FileStream sw = new FileStream(filePath,FileMode.Append,FileAccess.Write,FileShare.ReadWrite))
                {
                    byte[] infos = Encoding.UTF8.GetBytes(content);
                    sw.Write(infos,infos.Length);
                }

                return true;
            }

通知：如果您的文件不存在，请使用以下命令创建：

            private static void CreateFileIfNotExist(string filePath)
            {
                if (!File.Exists(filePath))
                {
                    var folderPath = Path.GetDirectoryName(filePath);
                    if (!Directory.Exists(folderPath))
                    {
                        Directory.CreateDirectory(folderPath);
                    }
                    File.Create(filePath).Dispose();
                }

尝试这个

 using (StreamWriter str = new StreamWriter(attachment,false))
            {
                str.WriteLine("Heyy!! How are you");
            }

为避免锁定问题，可以使用诸如对象或ReaderWriter之类的锁： 另外，可以使用File.AppendAllText或FileStream和StreamWriter代替 AppendAllLines。

public static var lock = new ReaderWriterLock();
    public static void WriteToFile(string text)
    {
        try
        {
            string fileName = "";
            lock.AcquireWriterLock(int.MaxValue); 

            File.AppendAllLines(fileName);
        }
        finally
        {
            lock.ReleaseWriterLock();

        }
    }