This question already has answers here:
Is there any way to close a StreamWriter without closing its BaseStream?
(5 answers)
Closed 4 years ago.
Inside a using I'm generating a Stream:
using (var generatedStream = GenerateStream(str))
{
var streamContent = generatedStream;
}
GenerateStream(string s) looks like this:
private static Stream GenerateStream(string s)
{
var stream = new MemoryStream();
using (var writer = new StreamWriter(stream))
{
writer.Write(s);
writer.Flush();
stream.Position = 0;
return stream;
}
}
When I get to assigning generatedStream to streamContent, more specifically, when I exit the using statement in GenerateStream, it says that stream is disposed. What am I doing wrong in making sure I dispose my writer?
UPDATE:
The solution posted in the linked question seems to be the way to go for this particular problem.
The using statement in your method GenerateStream() shouldn't be present
using (var writer = new StreamWriter(stream))
{
the using statement helps in disposing the stream already. It disposes at the end of the code block.
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-statement
Related
I have 2 pieces of code.
Take a look at this one.
static void Main(string[] args)
{
using (FileStream fs = new FileStream("temp.txt", FileMode.OpenOrCreate, FileAccess.ReadWrite))
{
using (StreamWriter writer = new StreamWriter(fs))
{
writer.WriteLine("Hello World");
using (StreamReader reader = new StreamReader(fs))
{
Console.WriteLine(reader.ReadToEnd());
} // Line-1
} //Line-2 -> System.ObjectDisposedException: 'Cannot access a closed file.'
}
}
At the Line-2 I get System.ObjectDisposedException occurs.
Why do I get this exception here?
After a little bit of thinking I thought that the exception occurred at Line-2 because the Stream associated with reference 'fs' is already closed at Line-1. And at Line-2 it is trying to close the same stream again. And this answer made sense. Hence, I thought I had found the right answer to my question but then I tried something a little bit different.
Now look at this piece of code.
static void Main(string[] args)
{
using (FileStream fs = new FileStream("temp.txt", FileMode.OpenOrCreate, FileAccess.ReadWrite))
{
using (StreamReader reader = new StreamReader(fs))
{
Console.WriteLine(reader.ReadToEnd());
using (StreamWriter writer = new StreamWriter(fs))
{
writer.WriteLine("Hello World");
} // Line-1
} // Line-2 -> No Exception occurred.
}
}
In this piece of code I didn't get any exception. Even though I am doing the same thing again. Here also the stream associated with reference 'fs' is already closed at Line-1 and it is also trying to close the stream at Line-2.
So why didn't I get any exception here?
My question is different from this Is there any way to close a StreamWriter without closing its BaseStream? because I am trying to do the same thing with the second code and despite the fact (in second code) that using statement already closed the base stream (fs) in Line-1. I still don't get any exception in Line-2.
Since #mjwills and #evk said the exception occurred because of flushing the contents of the stream after the file had been closed.
I tried this.
static void Main(string[] args)
{
using (FileStream fs = new FileStream("temp.txt", FileMode.OpenOrCreate, FileAccess.ReadWrite))
{
using (StreamWriter writer = new StreamWriter(fs))
{
writer.WriteLine("Hello World");
writer.Flush(); // Flushed the contents explicitly.
using (StreamReader reader = new StreamReader(fs))
{
Console.WriteLine(reader.ReadToEnd());
} // Line-1
} // Line-2 -> System.ObjectDisposedException: 'Cannot access a closed file.'
}
}
And the result is same I still have that exception even though I have explicitly flushed the contents. Please help.
StreamWriter's Dispose method flushes to disk.
StreamReader's Dispose method does not (why would it? it is just a reader...).
As such, it is "kind of" safe to dispose the writer then the reader (in the sense that it works), but not vice versa.
Consider using leaveOpen on your StreamWriter and StreamReader to avoid this class of problem. By using leaveOpen, you are then able to manually Dispose the Stream which means you can be 100% confident the code will work - and not have to concern yourself with flushing behaviour etc etc.
So I haven't written the code I'm dealing right now and I'm looking how to best handle this.
Right now I have this.
public static void WriteSymbol(Stream stream, Symbol symbol)
{
using (var streamWriter = new StreamWriter(stream))
{
JsonSerializer.Create(SerializerSettings).Serialize(streamWriter, symbol);
}
}
I'd like to be able to read the content of the stream after this is done in my test so that I can check the integration. The problem is that right now after Serialize the stream is closed and I can't read anymore from it.
I see that JsonWriter public bool CloseOutput { get; set; } but I don't see something for this static JsonSerializer.
What would be the best way to go about this?
How do I prevent the Serializer from closing the stream?
Is there some way I should check the content of the Stream?
From .net 4.5 upwards you can use the LeaveOpen constructor argument of StreamWriter.
The default buffer size used by the StreamWriter is 1024 as visible when decompiling the type.
So you can do the following.
using (var streamWriter = new StreamWriter(stream, Encoding.UTF8, 1024, true))
{
// TODO: do something
}
Try something like this:
public static void WriteSymbol(Stream stream, Symbol symbol)
{
using (var streamWriter = new StreamWriter(stream))
{
JsonSerializer.Create(SerializerSettings).Serialize(streamWriter, symbol);
// test stream here
}
}
Or don't surround this call with a using statement and then close the stream outside of this method.
Is there any chance that fileStream object will likely be destroyed before its call to the Close method as below?
FileStream fileStream = new FileStream(xxx);
StreamReader txtReader = new StreamReader(fileStream);
curLog = txtReader.ReadToEnd();
txtReader.Close();
fileStream.Close();
Is there any chance that fileStream object will likely be destroyed
before its call to the Close method as below?
No.
But you should never write code like that. You should always wrap IDisposable resources in using statements to ensure that they will be disposed even if an exception is thrown and that you won't be leaking handles.
using (FileStream fileStream = new FileStream(xxx))
using (StreamReader txtReader = new StreamReader(fileStream))
{
curLog = txtReader.ReadToEnd();
}
But for the purpose of this specific example you could simply use the ReadAllText method.
string curLog = File.ReadAllText(xxx);
No, there isn't any chance that it is closed before that. And i would recommend using it like this
FileStream fileStream = new FileStream(xxx);
using (StreamReader txtReader = new StreamReader(fileStream))
{
curLog = txtReader.ReadToEnd();
}
This question already has answers here:
Using various types in a 'using' statement (C#)
(4 answers)
Closed 8 years ago.
I have a method that returns like this:
using (var response = request.GetResponse())
using (var stream = response.GetResponseStream())
using (var reader = new StreamReader(stream))
return reader.ReadToEnd();
Can I combine the three usings like this and have the same level of safety?
using (var reader = new StreamReader(request.GetResponse().GetResponseStream()))
return reader.ReadToEnd();
Or since this is inside a privately scoped function, can I return safely without a using?
return new StreamReader(request.GetResponse().GetResponseStream()).ReadToEnd();
The object that contains this method is not IDisposable. I'm guessing no to both my questions but I am curious on other viewpoints.
For the general case, you can't make this assumption. For your specific case, some consolidation is possible. In particular, disposing the StreamReader will also close the underlying stream, because the documentation for StreamReader.Close() makes this guarantee:
Closes the StreamReader object and the underlying stream
(Emphasis mine). Therefore, you can skip middle using block from your sample. However, the same guarantee is not made from the Stream to the HttpResponse object, meaning you still want to keep that. So you can do this:
using (var response = request.GetResponse())
using (var reader = new StreamReader(response.GetResponseStream()))
return reader.ReadToEnd();
Additionally, there is one other situation where you can combine stacked using blocks into a single block. When all of the items managed by the using blocks are of the same type, you can separate each object with a comma. For example, opening one file for reading and processing the output to another file can be done like this:
using (var infile = new FileStream("infile"), outfile = new FileStream("outfile"))
{
//...
}
In general, you need the separate statements to ensure that everything gets cleaned up properly:
using (var response = request.GetResponse())
using (var stream = response.GetResponseStream())
using (var reader = new StreamReader(stream))
return reader.ReadToEnd();
Otherwise, an error partway through might leave resources undisposed. E.g. if GetResponseStream() threw an exception in this code, you'd end up with an undisposed response:
using (var reader = new StreamReader(request.GetResponse().GetResponseStream()))
return reader.ReadToEnd();
Im trying to prompt a downloadable text file (.txt), but I get this error:
Cannot access a closed Stream.
I have looked at simular questions in here:
Cannot Access Closed Stream
But it was not very useful.
Here is my code:
private FileStreamResult Export()
{
string name = "filename.txt";
MemoryStream stream = new MemoryStream();
using (StreamWriter writer = new StreamWriter(stream))
{
StringBuilder sb = new StringBuilder();
sb.Append("A text...");
writer.WriteLine(sb.ToString());
}
return File(stream, "text/plain", name);
}
UPDATE (working copy):
This gives me an blank text file.
private FileResult Export()
{
string name = "filename.txt";
MemoryStream stream = new MemoryStream();
StreamWriter writer = new StreamWriter(stream);
StringBuilder sb = new StringBuilder();
sb.Append("A text...");
writer.WriteLine(sb.ToString());
writer.Flush();
stream.Seek(0, SeekOrigin.Begin);
return File(stream, "text/plain", name);
}
That is correct, when you wrap a stream in another stream, calling .Close() or .Dispose() on any of them will dispose the whole stream. In this case, wrapping the MemoryStream in a StreamWriter means that when the using statement completes the StreamWriter and MemoryStream are both disposed.
My guess is since you are returning a FileStreamResult the encapsulating File will close the stream for you after the stream is no longer used. In this case, you do not want to use the using statement and will want to leave the stream open when returning it.
UPDATE
Since a stream is forward access you'll need to see the stream back to the beginning to allow the data to be read back out.
stream.Seek(0, SeekOrigin.Begin);
Just remove that using statement - you are passing disposed object reference to File method and you that's the reason why you get exception. From MSDN,
The StreamWriter object calls Dispose on the provided Stream object
when StreamWriter.Dispose is called.
I believe File will dispose stream by itself after usage (not verified by looking at source code).
UPDATE:
writer.Flush(); before return statement should help you
You have to set the position of the memorystream to 0 before using it in your FileStreamResult, otherwise it will be read from current position (IE the end of the stream).
stream.Position = 0;
return File(stream, "text/plain", name);
Just had the same thing.
I know this thread is ancient, just hoping to aid others having the same issue.
Replace the FileStreamResult type on your action with FileResult.