C# 如何防止System.Timers.Timer停止后引发已用事件？

我有以下代码：

using (FileStream sourceStream = sourceFile.Open())
{
    using (FileStream targetStream = File.Create(targetFilePath))
    {
        using (Timer timer = new Timer(250))
        {
            timer.Elapsed += (sender, e) =>
            {
                if (this.FileCopyProgressChanged != null)
                {
                    //Here the ObjectDisposedException appears
                    this.FileCopyProgressChanged(this,
                        new CopyProgressEventArgs(
                            sourceStream.Length,
                            targetStream.Length));
                }
            };

            timer.Start();

            sourceStream.CopyTo(targetStream);

            timer.Stop();
        }
    }
}

我的计时器每250毫秒运行一次，并引发一个包含流复制进度信息的事件。 问题是，有时在计时器事件中，会抛出ObjectDisposedException，因为流不再打开

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如何防止在处理流后计时器引发已用事件？

为什么不这样做，不需要计时器：

using (FileStream sourceStream = sourceFile.Open())
{
    using (FileStream targetStream = File.Create(targetFilePath))
    {
        int bytesTotal = sourceStream.Length;
        int bytesCurrent = 0;
        var buffer = new byte[32*1024];
        int bytes;
        while ((bytes = sourceStream.Read(buffer, 0, buffer.Length)) > 0)
        {
            targetStream.Write(buffer, 0, bytes);
            bytesCurrent += bytes;
            FileCopyProgressChanged(this,
                new CopyProgressEventArgs(bytesTotal, bytesCurrent));
        }
    }
}

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为什么不这样做，不需要计时器：

using (FileStream sourceStream = sourceFile.Open())
{
    using (FileStream targetStream = File.Create(targetFilePath))
    {
        int bytesTotal = sourceStream.Length;
        int bytesCurrent = 0;
        var buffer = new byte[32*1024];
        int bytes;
        while ((bytes = sourceStream.Read(buffer, 0, buffer.Length)) > 0)
        {
            targetStream.Write(buffer, 0, bytes);
            bytesCurrent += bytes;
            FileCopyProgressChanged(this,
                new CopyProgressEventArgs(bytesTotal, bytesCurrent));
        }
    }
}