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Use Using!

, 8 Oct 2010 CPOL 9.3K 10
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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.

Note:
using(...) {} is functionally equivalent to
IDisposable x = ...;
try {
 
} finally { x.Dispose(); }

License

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

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About the Author

Nathan St
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.

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Comments and Discussions

 
Answer"using" can even deal with null objects and it does not need to be assigned neither... Pin
Andreas Gieriet27-Jun-12 10:35
memberAndreas 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.

Cheers
Andi

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