C＃ - 在发送到电子邮件System.ObjectDisposedException时附加FileStream

Question

美好的一天，

我正在尝试将文件流文件附加到电子邮件发送附件。场景是，我需要使用FileStream创建一个文件，然后将其附加到我的电子邮件中。

到目前为止，这是我的示例代码：

用于创建文件：

public FileStream CreateFileStream(){
    using (var ms = new MemoryStream())
    {
    var writer = new StreamWriter(ms);

    writer.WriteLine("file content blah blah blahh");
    writer.Flush();

    //You have to rewind the MemoryStream before copying
    ms.Seek(0, SeekOrigin.Begin);

       using (var fs = new FileStream("Filename.txt", FileMode.Create))
       {
          ms.CopyTo(fs);
          return fs;
       }
    }
}

这是我发送电子邮件的示例代码

// attachment
var fileStreamFile = dto.FileStreamFile;

var contentType = new ContentType(MediaTypeNames.Text.Plain);
var attach = new Attachment(dto.FileStreamFile, contentType);
attach.ContentDisposition.FileName = "File.txt";

mail.Attachments.Add(attach);
...
// code for sending here
...
dto.FileStreamFile.Dispose(); // for disposing File Stream

fileStreamFile不是null但它抛出一个错误，说，

Handle = 'dto.FileStreamFile.Handle' threw an exception of type 'System.ObjectDisposedException'

当我注释掉附加代码时，无论如何发送电子邮件都很好。当我包含用于附加FileStream的代码时，电子邮件发送失败

有什么帮助吗？先感谢您

Answer 1

你的代码

using (var fs = new FileStream("Filename.txt", FileMode.Create))
{
    ms.CopyTo(fs);
    return fs; // you are returning here
} // hang on, this is a using statement which will dispose fs!

将转化为此

FileStream fileStream = new FileStream("Filename.txt", FileMode.Create);
try
{
   return fileStream;
}
finally
{
   if (fileStream != null)
   {
      ((IDisposable)fileStream).Dispose();
   }
}

重要的是要注意，如果你在return中使用try finally，它会将返回值存储在局部变量中，以便在finally之后返回。看起来像这样

FileStream fileStream = new FileStream("Filename.txt", FileMode.Create);
try
{
   temp = fileStream;
}
finally
{
   if (fileStream != null)
   {
      ((IDisposable)fileStream).Dispose();
   }
}

return temp;

我们可以看一下IL来看看发生了什么

IL_0000: ldstr "someFile"
IL_0005: ldc.i4.2
IL_0006: newobj instance void [mscorlib]System.IO.FileStream::.ctor(string, valuetype[mscorlib]System.IO.FileMode)
// stloc.0 Pops the current value from the top of the evaluation stack and stores it in the 
// local variable list at a specified index. This is your fs reference
IL_000b: stloc.0 
   .try
{
   // ldloc.0 Loads the local variable at a specific index onto the evaluation stack.
   // This is your fs reference
   IL_000c: ldloc.0
   // stloc.1 Pops the current value from the top of the evaluation stack and stores it in 
   // the local variable list at a specified index. This is your fs reference
   IL_000d: stloc.1
   IL_000e: leave.s IL_001a
} // end .try
finally
{
   IL_0010: ldloc.0
   IL_0011: brfalse.s IL_0019     
   IL_0013: ldloc.0
   // oh no we just Disposed fs!!!
   IL_0014: callvirt instance void [mscorlib]System.IDisposable::Dispose()
   IL_0019: endfinally
} // end handler

// ldloc.1 Loads the local variable at a specific index onto the evaluation stack.
// This is your fs reference
IL_001a: ldloc.1
//ret Returns from the current method, pushing a return value (if present) from 
//the callee's evaluation stack onto the caller's evaluation stack.
IL_001b: ret

简而言之，不要从IDisposable声明中返回using引用，因为它将被处置。

您将需要在另一个上下文中创建FileStream（没有using）和Dispose，或者重构您的代码。

更新

但是，在我发布一个包含使用流发送电子邮件的使用声明后，我的电子邮件发送功能无效

你可能会过度复杂化，尝试这样的事情

using (var stream = new MemoryStream())
   using (var writer = new StreamWriter(stream)) // using UTF-8 encoding by default
      using (var mailClient = new SmtpClient("localhost", 25))
         using (var message = new MailMessage("[email protected]", "[email protected]", "Just testing", "See attachment..."))
         {
            writer.WriteLine("file content blah blah blahh");
            writer.Flush();
            stream.Position = 0; // read from the start of what was written

            message.Attachments.Add(new Attachment(stream, "filename.csv", "text/csv"));

            mailClient.Send(message);
         }