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)
Related
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.
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
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.
I have lots of code like this:
FileStream fs = File.Open(#"C:\Temp\SNB-RSS.xml", FileMode.Open);
using (XmlTextReader reader = new XmlTextReader(fs))
{
/* Some other code */
}
This gives me the following Code Analysis warning:
CA2000 : Microsoft.Reliability : In method 'SF_Tester.Run()', object 'fs' is not disposed along all exception paths. Call System.IDisposable.Dispose on object 'fs' before all references to it are out of scope.
If I follow the suggestion and I put the File.Open in a using statement, I get this:
CA2202 : Microsoft.Usage : Object 'fs' can be disposed more than once in method 'SF_Tester.Run()'. To avoid generating a System.ObjectDisposedException you should not call Dispose more than one time on an object.: Lines: 39
I'm using VS2010 and I can't help but think I'm doing something wrong but I don't see it.
What am I doing wrong?
Sigh, exhausting isn't it. Avoid all this by using the recommended Create() method:
using (var reader = XmlReader.Create(#"C:\Temp\SNB-RSS.xml")) {
//...
}
As nobody provided a solution that solves this issue yet, I'm writing my working solution down here:
FileStream fs = new FileStream(fileName, FileMode.Truncate, FileAccess.ReadWrite, FileShare.ReadWrite);
try
{
using (var fileWriter = new StreamWriter(fs, encoding))
{
fs = null;
fileWriter.Write(content);
}
}
finally
{
if (fs != null)
fs.Dispose();
}
This removes CA2000.
I am only guessing; don't have time to go through a full analysis now.
Suppose the XmlTextReader constructor 'takes ownership' of the stream passed in, and so disposing the XmlTextReader will also Dispose the underlying stream. That would explain the behavior you see. Perhaps XmlTextReader constructor can throw, and in that instance, the original warning about fs would make sense. However, given that hypothesis, this code
var fs = File.Open(#"C:\Temp\SNB-RSS.xml", FileMode.Open);
XmlTextReader reader = null;
try
{
reader = new XmlTextReader(fs);
}
finally
{
if (reader== null)
{
fs.Dispose();
}
}
if (reader != null)
{
using (reader)
{
/* Some other code */
}
}
is, I think, correct, but still yields a spurious warning. This smells like a nice example that demonstrates the limitations of static analysis tools.
As someone else said, there is another API to directly create the reader from the filename (XmlReader.Create()), which avoids all this (and shows how well-designed scenario-focused APIs are a good thing for a surprising variety of reasons).
It's a known issue
http://connect.microsoft.com/VisualStudio/feedback/details/535118/ca2000-and-ca2202-offer-contradictory-warnings
If you're using a StreamWriter rather than XmlTextReader (as in the solution above) you could use a similar method via the relevant constructor; e.g.
var sw = new StreamWriter("filename.txt");
or
var sw = new StreamWriter("filename.txt", /*append to file = */ false );
It is not clear from the documentation whether the first form of constructor will overwrite or append to a file.
As mentioned in this answer, the only way to work around it correctly is to do as recommended in CA2202 and use an outer try-finally block instead of an outer using block. Inside the inner using, set the outer IDisposable object to null to prevent it from being accessed once the inner using has finished.
Here's a generic wrapper that does it "correctly", i.e. works around the badly designed XmlReader (maybe it should not have taken ownership of the stream it receives? Not sure what the right way to do it would be)
Disclaimer: Not really tested
public static TResult SafeNestedUsing<TOuter, TInner, TResult>(Func<TOuter> createOuterDisposable, Func<TOuter, TInner> createInnerDisposable, Func<TInner, TResult> body)
where TInner : IDisposable
where TOuter : class, IDisposable
{
TOuter outer = null;
try
{
outer = createOuterDisposable();
using (var inner = createInnerDisposable(outer))
{
var result = body(inner);
outer = null;
return result;
}
}
finally
{
if (null != outer)
{
outer.Dispose();
}
}
}
Example usage:
SafeNestedUsing<MemoryStream, XmlReader, XmlDocument>(
() => new MemoryStream(array),
(memStream) => XmlReader.Create(memStream, xmlReaderSettings),
(xmlReader) =>
{
XmlDocument xmlDoc = new XmlDocument();
xmlDoc.Load(xmlReader);
return xmlDoc;
});
This is quite clunky, and you may argue that it's better to repeat the try/set null/finally pattern instead. But for a repeating pattern of nested usings I'd rather do it this way than repeat the full thing each time.
just use 'using' for the filestream
using(FileStream fs = new FileStream(fileName, FileMode.Truncate, FileAccess.ReadWrite, FileShare.ReadWrite))
{
// some codes here
}
Don't modify fs and don't use fs.close() inside using curly braces.
Use the using statement also on the FileStream itself just like on the XmlTextReader.
http://msdn.microsoft.com/en-us/library/system.io.filestream(VS.71).aspx.
Grz, Kris.
I've have some code similar to this:
HttpWebRequest req;
HttpWebResponse response;
Stream receiveStream = null;
StreamReader readStream = null;
try
{
req = (HttpWebRequest)WebRequest.Create("someUrl"));
req.Credentials = CredentialCache.DefaultCredentials;
req.Method = "GET";
response = (HttpWebResponse)req.GetResponse();
receiveStream = response.GetResponseStream();
readStream = new StreamReader(receiveStream, Encoding.Default);
return readStream.ReadToEnd();
}
catch
{
return "Error";
}
finally
{
readStream = null;
receiveStream = null;
response = null;
req = null;
}
Should this code have readStream.Dispose() and responseStream.Dispose() instead of setting both to null?
It's almost always a mistake to set local variables to null, unless you want to actually use that value later on. It doesn't force garbage collection any earlier - if you're not going to read from the variable later, the garbage collector can ignore the reference (when not in debug mode).
However, it's almost always correct to close streams - ideally in a using statement for simplicity.
It's also almost always wrong to have a bare "catch" block like that. Do you really want to handle anything going wrong, including things like OutOfMemoryException?
I would rewrite your code as:
HttpWebRequest req = (HttpWebRequest) WebRequest.Create("someUrl"));
req.Credentials = CredentialCache.DefaultCredentials;
req.Method = "GET";
using (WebResponse response = req.GetResponse())
{
using (StreamReader reader = new StreamReader(response.GetResponseStream(),
Encoding.Default))
{
return reader.ReadToEnd();
}
}
Now if something goes wrong, the exception will be propagated to the caller. You might want to catch a few specific exceptions, but it's generally not a good idea to represent errors using a value which could have been a valid "normal" response.
Finally, are you really sure you want Encoding.Default? That's the default encoding of the local machine - you normally want the encoding indicated by the response itself.
It should have using [which calls Dispose()].
Yes, Dispose() them.
Even better to do something like
using (HttpWebResponse response = (HttpWebResponse)req.GetResponse() )
using (Stream receiveStream = response.GetResponseStream() )
using (readStream = new StreamReader(receiveStream, Encoding.Default) )
{
return readStream.ReadToEnd();
}
A using(x) {} block will be rewritten (by the compiler)
as a try {} finally {x.Dispose();}
Note that the WebRequest is not IDisposable.
Also note that the following lines accomplish the same thing as all of your code:
using (var client = new System.Net.WebClient())
{
client.Encoding = ...;
client.Credentials = ...;
return client.DownloadString("SomeUrl");
}
Yes. Pretty much anything that implements a Dispose() method must have its Dispose() method called. You can implicitly call it in a with the 'using' statement:
using(StreamReader stream = GetStream())
{
stream.DoStuff();
}
Yes.
When you set to null, it only nulls the reference. It doesn't run any cleanup code the creator of the Stream class wrote.
You may also want to consider the using(){ } statement which handles this for you on IDisposable types.
Example:
using (MyDisposableObject mdo = new MyDisposableObject)
{
// Do some stuff with mdo
// mdo will automatically get disposed by the using clause
}
No, you should call Dispose or Close
Safest method:
try {
HttpWebRequest request = (HttpWebRequest) WebRequest.Create("someUrl");
request.Credentials = CredentialCache.DefaultCredentials;
request.Method = "GET";
using (HttpWebResponse response = (HttpWebResponse) request.GetResponse()) {
using (StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.Default)) {
return reader.ReadToEnd();
}
}
} catch {
return "Error";
}
There's no need to dispose of the response.GetResponseStream() stream explicitly because the attached StreamReader will dispose it for you.
EDIT: I agree with the other answers - catching exceptions like that is very bad practice. I just left it in for comparison purposes. :-)
Yes - you should explicitly call Dispose() on classes that implement IDisposable after you have used them - this ensures all their resources get cleaned up in a timely fashion. Wrapping the variable in a using() does the same thing (which adds wrapping code that calls Dispose for you):
using (StreamReader reader = new StreamReader()) {
// do stuff with reader
}
There are a few gotchas in the .net libraries. Stream is one, and the other is much of the Imaging API. These entities that use certain system resources don't garbage collect the attached system resources.
If anything uses the IDisposable API, the best thing to do is wrap it in a using block, as people have pointed out above.
Read up on "using", and keep it in mind whenever you're dealing with file handles or images.
Really, the question has been answered but I do want to elaborate on one thing.
Any time an object implements the IDisposable interface, you should dispose it with the Dispose method or (even better) use the using statement.
If you are ever faced with this question in the future, just find out which interfaces it implements. Once you see IDisposable you know to dispose it.
If you need to clear the stream use null; Otherwise, use the Dispose(); method if your application no longer requires the use of the stream.