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Use Using!

, 8 Oct 2010
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The c# using syntax is often overlooked
This is probably an obvious tip to most seasoned C# developers, but I often come across people posting code that doesn't handle cleanup properly in the event of an error. (or not at all!)
Any object that implements IDisposable can be wrapped with a using statement as follows:
using(IDisposable x = ...)
   // do something with x
The using statement guarantees that the object will be disposed, even if something goes wrong or if the scope is otherwise exited.
A sample mistake is something like this (simplified for demonstration, this is NOT good code!)
public string ReadHtml(string url) {
        System.Net.HttpWebRequest wr = (System.Net.HttpWebRequest)WebRequest.Create(url);
        System.Net.HttpWebResponse resp = (System.Net.HttpWebResponse)wr.GetResponse();
        System.IO.StreamReader sr = new System.IO.StreamReader(resp.GetResponseStream());
        return sr.ReadToEnd();
Now, because this is such a simple demo, you and I can see that the return statement happens before the stream is closed, but when conditional logic is used, this can be much less obvious.
If, instead, this code was written as
public string ReadHtml(string url) {
        System.Net.HttpWebRequest wr = (System.Net.HttpWebRequest)WebRequest.Create(url);
        System.Net.HttpWebResponse resp = (System.Net.HttpWebResponse)wr.GetResponse();
        using(System.IO.StreamReader sr = new System.IO.StreamReader(resp.GetResponseStream())) {
            return sr.ReadToEnd();
then the stream would be properly disposed.
It's an easy syntax to use and can avoid performance problems when your code goes into production.
using(...) {} is functionally equivalent to
IDisposable x = ...;
try {
} finally { x.Dispose(); }


This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)


About the Author

Nathan Staudt
Web Developer
United Kingdom United Kingdom
I've been a web application developer since the late 1990s, working for eCommerce, media and telecoms companies across europe and america.
I have a BSc (Hons) in computing from Middlesex University and took a few MCP exams several years ago too.
Born in America, I grew up in Ireland. My father encouraged my interest in computers as a child when he would bring home his "portable" ibm pc... (20kg or something like that) and I first wrote some stuff in QBasic aged seven or eight, but my real interest in computers took off when my family got internet access at home in the early 90s.
Feel free to email me Smile | :)

Comments and Discussions

Answer"using" can even deal with null objects and it does not need to be assigned neither... PinmemberAndreas Gieriet27-Jun-12 10:35 
There might be situations where you don't know if an object is IDisposable or where the IDisposable object at hand might be null. This is gracefully handled by the using construct.
Dealing with null objects: works gracefully - no need to treat them differently
using (var x = null) // usually, this null is rather a result of some function 
As a result of that, one can handle the situation where an object might or might not be IDisposable.
What if not known if an object is ready for using construct? No problem, use the as keyword.
using (var x = GetSomeObject(...) as IDisposable) // if not IDisposable, null is passed to using
Finally, there may be situations where the object is not accessed at all in the using-block. You may leave the explicit declaration away.
You may leave the declaration away: declaration not needed if the object is not accessed in the block.
using (AcquireResource()) // the Disopse() function will release the resource
See also RAII (Resource Acquisition Is Initialization) C# Helper Classes[^] for some example where resource acquisition is the only purpose of the using block.

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