为什么变量 FileStream writer 为空？

Question

我在两种情况下使用 writer。

private void downloadFile(Int32 fileNr)
        {
            FileStream writer = null;
            m_currentFileSize = 0;
            fireEventFromBgw(Event.FileDownloadAttempting);

            FileInfo file = this.Files[fileNr];
            FileInfo fileFirstDateTime = this.Files[0];
            FileInfo fileLastDateTime = this.Files[19];
            Int64 size = 0;

            Byte[] readBytes = new Byte[this.PackageSize];
            Int32 currentPackageSize;
            System.Diagnostics.Stopwatch speedTimer = new System.Diagnostics.Stopwatch();
            Int32 readings = 0;
            Exception exc = null;

            if (form1.checkBox1.Text == "Download satellite images")
            {
                LocalDirectorySettings(file, writer);
            }
            else
            {
                writer = new FileStream(this.LocalDirectory + "\\" + file.Name, System.IO.FileMode.Create);
            }


            HttpWebRequest webReq;
            HttpWebResponse webResp = null;

            try
            {
                webReq = (HttpWebRequest)System.Net.WebRequest.Create(this.Files[fileNr].Path);
                webResp = (HttpWebResponse)webReq.GetResponse();

                size = webResp.ContentLength;
            }
            catch (Exception ex) { exc = ex; }

            m_currentFileSize = size;
            fireEventFromBgw(Event.FileDownloadStarted);

            if (exc != null)
            {
                bgwDownloader.ReportProgress((Int32)InvokeType.FileDownloadFailedRaiser, exc);
            }
            else
            {
                m_currentFileProgress = 0;
                while (m_currentFileProgress < size && !bgwDownloader.CancellationPending)
                {
                    while (this.IsPaused) { System.Threading.Thread.Sleep(100); }

                    speedTimer.Start();

                    currentPackageSize = webResp.GetResponseStream().Read(readBytes, 0, this.PackageSize);

                    m_currentFileProgress += currentPackageSize;
                    m_totalProgress += currentPackageSize;
                    fireEventFromBgw(Event.ProgressChanged);
                    try
                    {
                        writer.Write(readBytes, 0, currentPackageSize);
                    }
                    catch(Exception eee)
                    {
                        string myeee = eee.ToString();
                    }
                    readings += 1;

                    if (readings >= this.StopWatchCyclesAmount)
                    {
                        m_currentSpeed = (Int32)(this.PackageSize * StopWatchCyclesAmount * 1000 / (speedTimer.ElapsedMilliseconds + 1));
                        speedTimer.Reset();
                        readings = 0;
                    }
                }

                speedTimer.Stop();
                writer.Close();
                webResp.Close();
                if (!bgwDownloader.CancellationPending) { fireEventFromBgw(Event.FileDownloadSucceeded); }
            }
            fireEventFromBgw(Event.FileDownloadStopped);
        }

在这部分中，如果它执行 else 部分，它工作正常：

if (form1.checkBox1.Text == "Download satellite images")
            {
                LocalDirectorySettings(file, writer);
            }
            else
            {
                writer = new FileStream(this.LocalDirectory + "\\" + file.Name, System.IO.FileMode.Create);
            }

如果它在做这条线：没关系：

writer = new FileStream(this.LocalDirectory + "\\" + file.Name, System.IO.FileMode.Create);

但是一旦我更改了复选框，它就会执行 IF 的第一部分：

LocalDirectorySettings(file, writer);

这是 LocalDirectorySettings 方法代码：

public void LocalDirectorySettings(FileInfo file, FileStream writer)
        {
            try
            {
                var startIndex = file.Path.IndexOf("region=") + "region=".Length;
                var length = file.Path.IndexOf("&", startIndex) - startIndex;
                var code = file.Path.Substring(startIndex, length);
                var countryName = form1.codeToFullNameMap[code];

                string firstDT = ParseDateTime(this.Files[0]);
                string lastDT = ParseDateTime(this.Files[19]);
                string subDirectoryDT = firstDT + "---" + lastDT;

                string countryPath = Path.Combine(form1.mainPath, countryName);
                LocalDirectory = countryPath + "\\" + subDirectoryDT;

                if (!Directory.Exists(LocalDirectory))
                {
                    Directory.CreateDirectory(LocalDirectory);
                }

                string fileName = this.LocalDirectory + "\\" + "test.png"; //file.Name + "---" + countFilesNames++.ToString("D6") + ".png";
                writer = new FileStream(fileName, System.IO.FileMode.Create); // Set a breakpoint here.
            }
            catch (Exception err)
            {
                string ggg = err.ToString();
            }
        }

即使在 LocalDirectorySettings 方法中有一行带有 writer 实例的行：

writer = new FileStream(fileName, System.IO.FileMode.Create);

我正在使用一个断点，它确实到达了 LocalDirectorySettings 中的这一行，但是当它继续到该部分时：

try
                    {
                        writer.Write(readBytes, 0, currentPackageSize);
                    }
                    catch(Exception eee)
                    {
                        string myeee = eee.ToString();
                    }

到了 catch 部分，说 writer 为空。并且使用断点我看到 writer 为空。

为什么当它到达 LocalDirectorySettings 之后，作者是空的？

Answer 1

设置参数，LocalDirectorySettings()方法中名为writer的局部变量，对other局部变量没有影响，也叫writer，在调用者中找到，@987654324 @。

我强烈建议反对使用类字段来共享此对象（如其他答案中所建议的那样）。没有理由这样做，这样做只会让你的课堂变得混乱。相反，要么通过引用传递参数，要么更好地让LocalDirectorySettings() 方法返回对象。例如：

void downloadFile(int fileNr)
{
    FileStream writer;

    //...

    if (form1.checkBox1.Text == "Download satellite images")
    {
        writer = LocalDirectorySettings(file);
    }
    else
    {
        writer = new FileStream(this.LocalDirectory + "\\" + file.Name, System.IO.FileMode.Create);
    }

    //...
}

public FileStream LocalDirectorySettings(FileInfo file)
{
    try
    {
        //...

        string fileName = this.LocalDirectory + "\\" + "test.png"; //file.Name + "---" + countFilesNames++.ToString("D6") + ".png";
        return new FileStream(fileName, System.IO.FileMode.Create);
    }
    catch (Exception err)
    {
        string ggg = err.ToString();
        throw; // you don't actually handle the exception, so rethrowing
               // is the appropriate action here. Your only alternative
               // would be to return "null" and have the caller check for
               // that return value explicitly.
    }
}

Answer 2

在LocalDirectorySettings 中，您正在为函数参数分配一个新的FileStream。这不会影响 downloadFile 中的 writer 变量。

将writer 设为成员变量，以便两个方法可以共享它。

FileStream writer = null;
private void downloadFile(Int32 fileNr)
{
    // Do stuff with writer
}

public void LocalDirectorySettings(FileInfo file)
{
    // Do stuff with writer
}