XmlSerializer may cause memory leak at 2:22 PM

In my previous post, I mentioned XmlSerializer.dll is needed to serialize/deserialize types during web service call. But I did not know a potential memory leak problem until today from an excellent MSDN article: Do not use the overload of XmlSerializer constructor that takes the XML root element name as its second parameter!

As you may know, once a .NET assembly is loaded into memory, it will not be unloaded until the hosting AppDomain is unloaded. XmlSerializer constructor generates a temporary assembly for the type to be serialized using reflection. Because the code generation is expensive, the assembly is cached in memory on a per-type basis.

For example, the following code will create a cached assembly for type Employee:

XmlSerializer serializer = new XmlSerializer(typeof(Employee));

Whenever an Employee object is to be serialized, the cached assembly will be used.

But sometimes, we may want to change XML root name in the serialized XML message in a web service. An option is to call an overloaded constructor with XML root name as a parameter:

XmlSerializer serializer = new XmlSerializer(typeof(Employee), 
new XmlRootAttribute("Manager"));

Because the root name parameter is supposed to be dynamic, XmlSerializer will not cache the generated temporary assembly. It will generate a new assembly every time you create a new XmlSerializer with that parameter, and the generated assembly will stay in memory unless AppDomain is unloaded. So more and more generated assembly will stay in memory -- memory is being leaked!

If the root name is static, you can use XmlRootAttribute on the class to change root name of the serialized type; if the root name is dramatically dynamic, there is no easy way to solve the leak problem yet ...