Under the Hood of .NET Memory Management - Simple Talk

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Chapter 6: A Few More Advanced Topics public void Parse () { XmlDocument document = null; if (_weak.IsAlive ) document = _weak.Target as XmlDocument; if (document == null) { // log that weak reference was lost // recreate the document } // handle the parsing } Listing 6.6: Testing to see if the weak reference is still valid. If you find that you have to recreate the object frequently, or if your original estimate for how much time it takes to recreate the data is inaccurate, you may want to revisit the choice to use weak references. In almost all cases, you will be better served with reviewing your instrumentation data periodically to ensure that you are still getting the results that you expect. If you don't get the results you expect, you will almost certainly simplify your application logic by removing the weak reference logic. Marshaling Occasionally you may need to access functionality that is exposed in a native DLL. This is not common, because the CLR is quite extensive, but it is a good idea to understand what happens behind the scenes. Also if you use .NET Reflector to browse the source code for the framework, you will find this crop up from time to time, and you need to know what is going on. Marshaling can definitely have a memory impact. The MSDN documentation makes it clear when it states that the Marshal class "provides a collection of methods for allocating unmanaged memory, copying unmanaged memory blocks, and converting 192
Chapter 6: A Few More Advanced Topics managed to unmanaged types, as well as other miscellaneous methods used when interacting with unmanaged code." (see http://msdn.microsoft.com/en-us/library/ system.runtime.interopservices.marshal.aspx). Unmanaged memory means no GC which, in turn, means that anything that you allocate, you must also explicitly de-allocate. When you start working with unmanaged memory, you are explicitly going out on a limb and leaving the safety net that .NET provides. Naturally, this is not for the faint of heart, and any code that uses these techniques must be carefully monitored, not just for memory leaks but also for type safety. Some data types are interoperable and will work the same in an unmanaged world as they do in the managed world, while others do not, and will need to be explicitly converted. The basic integral types are interoperable, and pointers are at least interoperable in unsafe mode. It may be surprising to find out that some other common types, such as Char, are not interoperable. In managed code, a char will be a 2-byte Unicode character, but in unmanaged code it will be a 1-byte ASCII character. Bool is another tricky type; in unmanaged code, a bool is typically an integral value with 0 meaning false and anything else being true. Any integer value can server in the place of a bool. For instance, the code in Listings 6.7 and 6.8 is valid in C++, but not in C#. int x = list.Count; if (x) // Invalid in C# { Console.WriteLine("The list is not empty."); } Listing 6.7: Booleans are treated differently in C++. int x = list.Count; if (x > 0) // Valid in C# { Console.WriteLine("The list is not empty."); } Listing 6.8: Integers are not booleans in C#. 193
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