Develop Reference

StreamReader with using statement difference?

I am using StreamReader as shown below in my code:
string json = await new StreamReader(context.Request.Body).ReadToEndAsync();
// ... use json variable here in some other code
And I stumbled upon using statement. Is there any difference between my first statement vs using the using statement with StreamReader?
Should I be using using statement with StreamReader here in prod code?
string json;
using (var reader = new StreamReader(context.Request.Body))
{
json = await reader.ReadToEndAsync();
}

Is there any difference between my first statement vs using the using
statement with StreamReader
Yes. The difference is that when you wrap StreamReader in a using statement it will clear up some resources directly instead of waiting for the garbage collector. More specifically it will call Dispose() on StreamReader. You should almost always use using when the class implements IDisposable.
If your app simply uses an object that implements the IDisposable
interface, you should call the object's IDisposable.Dispose
implementation when you are finished using it.
Thanks to .NET Core being open source we can take a look at the source for StreamReader:
protected override void Dispose(bool disposing)
{
if (m_stream != null)
{
if (disposing)
{
m_stream.Close();
}
m_stream = null;
m_buffer = null;
m_curBufPos = 0;
m_curBufLen = 0;
}
m_disposed = true;
}
As you can see it calls Close() on the stream, which (according to the docs) in turn will call Dispose() on the stream itself.
Correctly disposing objects can be crucial when working with larger objects or streams. However, I will try to target your other question.
Should I be using using statement with StreamReader here in prod code?
Yes, no and maybe. In your partical case you have a context.Request.Body as a Stream (which I assume is from HttpContext). There is no need for the StreamReader to close that particular stream. It will be disposed correctly (later) anyway. Also, there might be some other resource that need access to that particual stream later in the pipeline.
Generally, if the class implements IDisposable then you should wrap it in a using. But here I think that you have two better choices:
1.
If you actually have a json (as your variable suggest), you can deserialize it directly using JsonSerializer found in System.Text.Json.JsonSerializer:
YourModel model = await System.Text.Json.JsonSerializer.DeserializeAsync<YourModel>(context.Request.Body);
UPDATE: Or if you are using .NET 5 you have access to HttpResponseJsonExtensions and can use ReadFromJsonAsync. Then you can simply try the following:
YourModel model = await context.Request.ReadFromJsonAsync<YourModel>();
Thanks to caius-jard.
2.
Use the overload of StreamReader that doesn't close the stream.
string json;
using (var reader = new StreamReader(stream, Encoding.UTF8, true, -1, true))
{
json = await reader.ReadToEndAsync();
}
So, to sum up. Yes, there is a difference when using using. However, in your particular case you have better options.

Check out this link
https://www.c-sharpcorner.com/UploadFile/manas1/usage-and-importance-of-using-in-C-Sharp472/
In short: "using" statement ensures that managed/unmanaged resource object disposes correctly and you don't have to call "Dispose" method explicitly even there is any execeptions occured within the using block
You can read further from Microsoft official site too
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/using-statement#:~:text=The%20using%20statement%20calls%20the,t%20be%20modified%20or%20reassigned.

WPF code analyze : CA2202 Do not dispose objects multiple timesObject

in my WPF application code i got the following Warnings:
CA2202 Do not dispose objects multiple times Object 'fs' can be
disposed more than once in method
'MainWindow.TestResults_Click(object, RoutedEventArgs)'. To avoid
generating a System.ObjectDisposedException you should not call
Dispose more than one time on an object. : Lines:
429 yesMonitor MainWindow.xaml.cs 429
for code:
FileStream fs = new FileStream(System.AppDomain.CurrentDomain.BaseDirectory + "TestResult.htm", FileMode.Create);
using (fs)
{
using (StreamWriter w = new StreamWriter(fs, Encoding.UTF8))
{
w.WriteLine(GetTestResultsHtml());
}
}
what should be the reason for these warning?

Nested using statements can cause violations of the CA2202 warning. If the IDisposable resource of the nested inner using statement contains the resource of the outer using statement, the Dispose method of the nested resource releases the contained resource. When this situation occurs, the Dispose method of the outer using statement attempts to dispose its resource for a second time.
In the following example, a Stream object that is created in an outer using statement is released at the end of the inner using statement in the Dispose method of the StreamWriter object that contains the stream object. At the end of the outer using statement, the stream object is released a second time. The second release is a violation of CA2202.
using (Stream stream = new FileStream("file.txt", FileMode.OpenOrCreate))
{
using (StreamWriter writer = new StreamWriter(stream))
{
// Use the writer object...
}
}
To resolve this issue, use a try/finally block instead of the outer using statement. In the finally block, make sure that the stream resource is not null.
Stream stream = null;
try
{
stream = new FileStream("file.txt", FileMode.OpenOrCreate);
using (StreamWriter writer = new StreamWriter(stream))
{
stream = null;
// Use the writer object...
}
}
finally
{
if(stream != null)
stream.Dispose();
}

Personally in this case I would use:
public StreamWriter(
string path,
bool append
)
Initializes a new instance of the StreamWriter class for the specified
file by using the default encoding and buffer size. If the file
exists, it can be either overwritten or appended to. If the file does
not exist, this constructor creates a new file.
But there is NO good solution, see CA2202, how to solve this case

Pre-created Stream and "using" block

I'm really annoyed when "using" block tampered my pre-created object. I have this piece of code
class Asset {
public Stream FileStream { get; set; }
public Asset(string fileName) {
FileStream = ...open a file stream...;
}
}
// Somewhere else
Asset asset = new Asset("file.txt");
using (var reader = new StreamReader(asset.FileStream)) {
//blah blah blah
}
// Somewhere else else
using (var reader2 = new StreamReader(asset.FileStream))
=> throws this exception:
System.ArgumentException: Stream was not readable.
Debugging step-by-step in Visual Studio helped me know that asset.FileStream has been disposed after the first "using" block.
Please help me to save his life :((
How can I create a clone stream from a stream?

I agree that the fact that readers close the underlying stream is dumb. The approach outlined in this article is to create a decorator class that wraps the Stream and has a no-op for the Close and Dispose methods. It's probably not worth the overhead, though, so you should consider just not using using for these readers.

.NET - Replacing nested using statements with single using statement

If you came across some C# code like this with nested using statements/resources:
using (var response = (HttpWebResponse)request.GetResponse())
{
using (var responseStream = response.GetResponseStream())
{
using (var reader = new BinaryReader(responseStream))
{
// do something with reader
}
}
}
Is it safe to replace it with something like this?
using (var reader = new BinaryReader(((HttpWebResponse)request.GetResponse()).GetResponseStream()))
{
// do something with reader
}
The example above is just an example of nested disposable resources, so forgive me if it's not exactly correct usage. I'm curious if when you dispose the outermost resource (the BinaryReader in this case), if it will recursively dispose its children for you, or if you need to explicitly dispose each "layer" with separate using statements? E.g. if you dispose the BinaryReader, is it supposed to dispose the response stream, which in turn disposes the response? Thinking about that last sentence makes me think you actually do need the separate using statements, because there's no way to guarantee that a wrapper object would dispose of the inner object. Is that right?

You should just stack your using statements - it has the desired effect you are looking for:
using (var response = (HttpWebResponse)request.GetResponse())
using (var responseStream = response.GetResponseStream())
using (var reader = new BinaryReader(responseStream))
{
// do something with reader
}

You need the separate using statements.
In your second example, only the BinaryReader will get disposed, not the objects used to construct it.
In order to see why, look at what the using statement actually does. It takes your second code, and does something equivalent to:
{
var reader = new BinaryReader(((HttpWebResponse)request.GetResponse()).GetResponseStream());
try
{
// do something with reader
}
finally
{
if (reader != null)
((IDisposable)reader).Dispose();
}
}
As you can see, there would never be a Dispose() call on the Response or ResponseStream.

FWIW, here's another way to spell your original example which may satisfy any dismay over nesting:
using (var response = (HttpWebResponse)request.GetResponse())
using (var responseStream = response.GetResponseStream())
using (var reader = new BinaryReader(responseStream))
{
// do something with reader
}
Whether the reader disposes of the stream is really a function of the reader, not the 'using'. As I recall that is often what the behavior with readers is--they take ownership of the stream and dispose of it when the reader is itself closed. But the form I've provided above should be fine.

According to the documentation, BinaryReader.Close will close the underlying stream. http://msdn.microsoft.com/en-us/library/system.io.binaryreader.close.aspx
Also, according to the documentation for HttpWebResponse, you either have to close the underlying stream or dispose the response. http://msdn.microsoft.com/en-us/library/system.net.httpwebresponse.aspx
So the second example you provided would work.

I posted this elsewhere, but separating your declarations by comma seems to treat each statement separated this way as a new declaration and disposes of them.
using (IType1 a = new Type1(), b = new Type1()){}
This however means your objects must be of the same type. You could call them like
using (IDisposable a = new Type1(), b = new Type2()){}
But then of course, you only have access to IDisposable methods, without casting the object, which is kinda dumb. So instead, I believe you can use
using (var a = new Type1(), b = new Type2()){}
This appears to give you the correctly typed object references allowing you access to the proper method of the allocated type, and disposes of both objects created. If anyone knows why I'm not right, please let me know because this appears to work for me? (I know this question is really old n stuff, but it's all I could find while searching for this answer myself)

"Object can be disposed of more than once" error

When I run code analysis on the following chunk of code I get this message:
Object 'stream' can be disposed more than once in method 'upload.Page_Load(object, EventArgs)'. To avoid generating a System.ObjectDisposedException you should not call Dispose more than one time on an object.
using(var stream = File.Open(newFilename, FileMode.CreateNew))
using(var reader = new BinaryReader(file.InputStream))
using(var writer = new BinaryWriter(stream))
{
var chunk = new byte[ChunkSize];
Int32 count;
while((count = reader.Read(chunk, 0, ChunkSize)) > 0)
{
writer.Write(chunk, 0, count);
}
}
I don't understand why it might be called twice, and how to fix it to eliminate the error. Any help?

I struggled with this problem and found the example here to be very helpful. I'll post the code for a quick view:
using (Stream stream = new FileStream("file.txt", FileMode.OpenOrCreate))
{
using (StreamWriter writer = new StreamWriter(stream))
{
// Use the writer object...
}
}
Replace the outer using statement with a try/finally making sure to BOTH null the stream after using it in StreamWriter AND check to make sure it is not null in the finally before disposing.
Stream stream = null;
try
{
stream = new FileStream("file.txt", FileMode.OpenOrCreate);
using (StreamWriter writer = new StreamWriter(stream))
{
stream = null;
// Use the writer object...
}
}
finally
{
if(stream != null)
stream.Dispose();
}
Doing this cleared up my errors.

To illustrate, let's edit your code
using(var stream = File.Open(newFilename, FileMode.CreateNew))
{
using(var reader = new BinaryReader(file.InputStream))
{
using(var writer = new BinaryWriter(stream))
{
var chunk = new byte[ChunkSize];
Int32 count;
while((count = reader.Read(chunk, 0, ChunkSize)) > 0)
{
writer.Write(chunk, 0, count);
}
} // here we dispose of writer, which disposes of stream
} // here we dispose of reader
} // here we dispose a stream, which was already disposed of by writer
To avoid this, just create the writer directly
using(var reader = new BinaryReader(file.InputStream))
{
using(var writer = new BinaryWriter( File.Open(newFilename, FileMode.CreateNew)))
{
var chunk = new byte[ChunkSize];
Int32 count;
while((count = reader.Read(chunk, 0, ChunkSize)) > 0)
{
writer.Write(chunk, 0, count);
}
} // here we dispose of writer, which disposes of its inner stream
} // here we dispose of reader
edit: to take into account what Eric Lippert is saying, there could indeed be a moment when the stream is only released by the finalizer if BinaryWriter throws an exception. According to the BinaryWriter code, that could occur in three cases
If (output Is Nothing) Then
Throw New ArgumentNullException("output")
End If
If (encoding Is Nothing) Then
Throw New ArgumentNullException("encoding")
End If
If Not output.CanWrite Then
Throw New ArgumentException(Environment.GetResourceString("Argument_StreamNotWritable"))
End If
if you didn't specify an output, ie if stream is null. That shouldn't be a problem since a null stream means no resources to dispose of :)
if you didn't specify an encoding. since we don't use the constructor form where the encoding is specified, there should be no problem here either (i didn't look into the encoding contructor too much, but an invalid codepage can throw)
if you don't pass a writable stream. That should be caught quite quickly during development...
Anyway, good point, hence the edit :)

The BinaryReader/BinaryWriter will dispose the underlying stream for you when it disposes. You don't need to do it explicitly.
To fix it you can remove the using around the Stream itself.

A proper implementation of Dispose is explicitly required not to care if it's been called more than once on the same object. While multiple calls to Dispose are sometimes indicative of logic problems or code which could be better written, the only way I would improve the original posted code would be to convince Microsoft to add an option to BinaryReader and BinaryWriter instructing them not to dispose their passed-in stream (and then use that option). Otherwise, the code required to ensure the file gets closed even if the reader or writer throws in its constructor would be sufficiently ugly that simply letting the file get disposed more than once would seem cleaner.

Your writer will dispose your stream, always.

Suppress CA2202 whenever you are sure that the object in question handles multiple Dispose calls correctly and that your control flow is impeccably readable. BCL objects generally implement Dispose correctly. Streams are famous for that.
But don't necessarily trust third party or your own streams if you don't have unit tests probing that scenario yet. An API which returns a Stream may be returning a fragile subclass.