Platform/Invoke allows the coder to use standard (unmanaged) C/C++ DLLs. If you need to have access to any function inside the extensive Windows APIs (which hold basically everything the operating system can perform) and there's no available wrapper to call the same functionality from the CLR, then this is the choice.
From the developer's perspective, by Platform/Invoke, we understand a feature of the CLR that allows a program to interact with the functionality that is unique to the system in which the application runs, thus allowing managed code to call native code and vice versa.
The assembly responsible for calling the APIs will define how the native code is called and accessed, via metadata embedded inside, which usually requires attribute decorations. These attributes are defined inside the class containing the caller methods in order to indicate the compiler the correct way to do the marshaling between the two worlds (managed and unmanaged).
The...