Platform Invocation Services - Pitfalls

Pitfalls

Writing P/Invoke wrappers can be difficult and error prone. Using native DLLs means that the programmer can no longer benefit from type safety and garbage collection as is usually provided in the .NET environment. When they are used improperly this may cause problems such as segmentation faults or memory leaks. Getting the exact signatures of the legacy functions for use in the .NET environment can be hard, which can result in such problems. For this purpose tools and websites exist to obtain such signatures, helping to prevent signature problems.

Other pitfalls include:

• Incorrect data alignment of user-defined types in the managed language: there are different ways data can be aligned depending on compilers or compiler directives in C and care must be taken to explicitly tell the CLR how to align data for non-blittable types. A common example of this is when trying to define a data type in .NET to represent a union in C. Two different variables overlap in memory, and defining these two variables in a type in .NET would cause them to be in different locations in memory, so special attributes must be used to correct the issue.
• Interference with the location of data by the managed language's garbage collector: if a reference is local to a method in .NET and is passed to a native function, when the managed method returns, the garbage collector may reclaim that reference. Care should be taken that the object reference is pinned, preventing it from being collected or moved by the garbage collector, which would result in an invalid access by the native module.

When using C++/CLI, emitted CIL is free to interact with objects located on the managed heap and simultaneously any addressable native memory location. A managed heap resident object may be called, modified or constructed, using simple "object->field;" notation to assign values or specify method calls. Significant performance gains result from having eliminated any needless context switching, memory requirements are reduced (shorter stacks).

This comes with new challenges:

• Code is prone to Double Thunking if not specifically addressed
• The Loader Lock issue

These references specify solutions for each of these issue if they are encountered. A primary benefit is the elimination of the structure declaration, the order of field declaration and alignment issues are not present in the context of C++ Interop.