IOException: 文件“d:\a.txt”正由另一进程使用，因此该进程无法访问此文件。
StackTrace:
   在 System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
   在 System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
   在 System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, String msgPath, Boolean bFromProxy, Boolean useLongPath, Boolean checkHost)
   在 System.IO.StreamReader..ctor(String path, Encoding encoding, Boolean detectEncodingFromByteOrderMarks, Int32 bufferSize, Boolean checkHost)
   在 System.IO.File.InternalReadAllText(String path, Encoding encoding, Boolean checkHost)
   在 DynamicClass.<>c__DisplayClass1.<OnWindowLoaded>b__0(Object sender, FileSystemEventArgs args)
   在 System.IO.FileSystemWatcher.OnChanged(FileSystemEventArgs e)
   在 System.IO.FileSystemWatcher.CompletionStatusChanged(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* overlappedPointer)
   在 System.Threading._IOCompletionCallback.PerformIOCompletionCallback(UInt32 errorCode, UInt32 numBytes, NativeOverlapped* pOVERLAP)