I recently maintained an ASP .Net application where Xml Serialization was used to read and persist configuration data during page requests. When site traffic increased, users encountered System.IO.IOExceptions with the message "The process cannot access the file "c:\SomeFolder\SomeFile.xml" because it is being used by another process. The exception was not always encountered 100% of the time. The problem could be easily reproduced by opening two browser windows, opening the page in each browser, and quickly refreshing each browser side by side.

I was able to narrow down the problem to a method in a class where deserialization takes place on each page request. The following code was used:

XmlSerializer serializer = new XmlSerializer(typeof(Foo));
TextReader reader = new StreamReader(serializedFooPath);
myFoo = (Foo)serializer.Deserialize(reader);
reader.Close();

Basically the TextReader locks the file until it is done using it.  You can easily avoid locking a file by changing how it is accessed. Simply introduce a FileStream object that can control how the file is accessed rather than allowing the TextReader to control it for you. The FileStream object can accomplish this by using the FileShare.Read enumeration value:

XmlSerializer serializer = new XmlSerializer(typeof(Foo));
FileStream fs = new FileStream(serializedFooPath, FileMode.Open, FileAccess.Read, FileShare.Read);
TextReader reader = new StreamReader(fs);
myFoo = (Foo)serializer.Deserialize(reader);
reader.Close();
fs.Close();

According to MSDN, any request to open the file will fail until the file is closed if FileShare.Read is not specified [1].

Making this change eliminated the problem and the IOException no longer occurred. It was an easy quick fix - perhaps a better deserialization strategy would come in handy but this fix was able to fix the bug quickly. Thanks to Jake for helping out by supplying the MSDN reference.

[1] Refer to http://msdn2.microsoft.com/en-us/library/system.io.fileshare.aspx for full documentation of the FileShare enumeration and http://msdn2.microsoft.com/en-us/library/system.io.filestream.aspx for full documentation of the FileStream class.