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();
Related
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
I have been trying to get out a demo for MVC 6.0 and I find that I can't read a file anymore using StreamReader class as it doesn't accept a string anymore.
So code like this
StreamReader reader= new StreamReader("filename.txt")
is not valid?
I am using .NET Core 1.0
I think they've removed it as a StreamReader shouldn't be responsible for creating streams - it's a violation of the Single Responsibility Principle.
You'll need to create a FileStream or similar in order to get the same functionality
using (var stream = new FileStream(#"C:\temp\test.txt", FileMode.Open))
using (var reader = new StreamReader(stream))
{
// do stuff.
}
Is the compiler able to merge any of the following code into a single try/catch?
using (FileStream stream = new FileStream(filePath, FileMode.Open))
{
using (BinaryReader reader = new BinaryReader(stream))
{
a try/catch around my actual code...
}
}
If not, would there be any functional difference if I do this instead?
FileStream stream = null;
BinaryReader reader = null;
try
{
stream = new FileStream(filePath, FileMode.Open);
reader = new BinaryReader(stream)
// do my stuff
}
catch (stuff)
{
}
finally
{
if (stream != null)
stream.Close();
if (reader != null)
reader.Close();
}
Since the overhead of having an extra try/finally is virtually zero, there is no difference if the compiler combines two blocks in one or keeps them separate. Your translated code would have a single try-catch embedded inside two layers of try-finally, which are virtually free in terms of performance.
Converting the using code manually has a different implication - the visibility of variables is different. Your translation of nested using statements leaves stream and reader variables accessible after the block. They refer to a closed stream and a closed reader. Regular using, on the other hand, keeps its variables inside its scope, as if an additional pair of curly braces were placed around the whole block:
{ // <<==
FileStream stream = null;
BinaryReader reader = null;
try {
stream = new FileStream(filePath, FileMode.Open);
reader = new BinaryReader(stream)
// do my stuff
} finally {
if (stream != null)
stream.Close();
if (reader != null)
reader.Close();
}
} // <<==
using is better in this way, because it lets you avoid an extra level of nesting while keeping its variable in local scope. Moreover, you could reduce the level of nesting in your source by placing two consecutive using blocks together at the same level of indentation, with no curly braces:
using (FileStream stream = new FileStream(filePath, FileMode.Open))
using (BinaryReader reader = new BinaryReader(stream)) {
a try/catch around my actual code...
}
Is the compiler able to merge any of the following code into a single try/catch?
No, the compiler is not able to make your code behave differently from what you've written. And it would be different even without the possibility of Close() and Dispose() having different effects, but let's pretend you called Dispose() yourself.
If not, would there be any functional difference if I do this instead?
Yes. Putting both Dispose() invocations in a single finally block means that reader won't be disposed if stream.Dispose() throws an exception.
No well-written IDisposable's Dispose() method throws an exception, but the compiler cannot know or assume that all implementations are well-written.
I have a lot of text files and want to read them all by once, how do I do this?
This is my code till now:
List<StreamReader> lijst = new List<StreamReader>();
using (StreamReader qwe = new StreamReader("C:\\123.txt"))
using (StreamReader qwer = new StreamReader("C:\\1234.txt"))
lijst.Add(qwe);
lijst.Add(qwer);
But I get an ObjectDisposedException(Cannot read from a closed TextReader.) when doing this:
lijst[0].Readline();
Any idea how to fix this? Thansk in advance
You are not using curly braces, so you cannot see where the object is disposed. You code is identical to this code:
List<StreamReader> lijst = new List<StreamReader>();
using (StreamReader qwe = new StreamReader("C:\\123.txt"))
{
using (StreamReader qwer = new StreamReader("C:\\1234.txt"))
{
lijst.Add(qwe);
}
}
lijst.Add(qwer);
This means that when you get to the last line of this code your stream readers are already disposed. In your case you should not use using, but you need to make sure to dispose the stream readers afterwards:
try
{
List<StreamReader> lijst = new List<StreamReader>();
StreamReader qwe = new StreamReader("C:\\123.txt");
StreamReader qwer = new StreamReader("C:\\1234.txt");
lijst.Add(qwe);
lijst.Add(qwer);
// Use the stream readers
}
// you can use or not use catch here, it depends
finally
{
qwe.Dispose();
qwer.Dispose();
}
The using statement also defines the scope of the variable. As soon as the statement block in each using statement finishes, qwe and qwer go out of scope.
To solve this, don't use using:
StreamReader qwe = new StreamReader("C:\\123.txt");
lijst.Add(qwe);
or just
lijst.Add(new StreamReader("C:\\123.txt"));
Note though, that the file will immediately be opened, and will only be closed when the list and all its readers goes out of scope (or if you explicitly close the readers).
Anyway, it's not very efficient, and you shouldn't do this with a list of hundreds of files. A better solution would be to store a list of filesnames, and keeps the actual files open as short as possible, and as few as possible at a time.
Example:
variable = new StreamReader( file ).ReadToEnd();
Is that acceptable?
No, this will not close the StreamReader. You need to close it. Using does this for you (and disposes it so it's GC'd sooner):
using (StreamReader r = new StreamReader("file.txt"))
{
allFileText = r.ReadToEnd();
}
Or alternatively in .Net 2 you can use the new File. static members, then you don't need to close anything:
variable = File.ReadAllText("file.txt");
You should always dispose of your resources.
// the using statement automatically disposes the streamreader because
// it implements the IDisposable interface
using( var reader = new StreamReader(file) )
{
variable = reader.ReadToEnd();
}
Or at least calling it manually:
reader = new StreamReader(file);
variable = reader.ReadToEnd();
reader.Close();
You need to Dispose of objects that implement IDisposable. Use a using statement to make sure it gets disposed without explicitly calling the Dispose method.
using (var reader = new StreamReader(file))
{
variable = reader.ReadToEnd();
}
Alternately, use File.ReadAllText(String)
variable = File.ReadAllText(file);
Yes, whenever you create a disposable object you must dispose of it preferably with a using statement
using (var reader = new StreamReader(file)) {
variable = reader.ReadToEnd(file);
}
In this case though you can just use the File.ReadAllText method to simplify the expression
variable = File.ReadAllText(file);
It's good practice to close StreamReader. Use the following template:
string contents;
using (StreamReader sr = new StreamReader(file))
{
contents = sr.ReadToEnd();
}
You're better off using the using keyword; then you don't need to explicitly close anything.