Application Domain

One of the badly behaved DLLs/components of your application can bring your whole application or everything else down. One of the things we can do to resolve this issue is isolate the DLL/component from everything else. What does this isolation means? In pre-.Net days this only means isolating the code through processes.

If you don’t put an extra effort to isolate the components in your application, when you run the application, your whole application will run within a context of a single process. Since windows isolates process from each other through memory address, it’ll share the same memory space. All the components in the application have access to this common memory space shared though out the application. Because of this, a badly behaved piece of code can bring the whole application down.

How can you isolate processes? In pre.Net days COM is one of the technologies that enabled you to isolate processes by allowing a process to call a COM component which is an executable. But the main disadvantage of this method is since the processes can’t share memory or use the same address space, a complex marshalling process has to be used to copy data between the processes. Tough processes are great by considering security, the disadvantage is the performance. Because often number of processes will normally working together and you have to develop data marshalling processes to ensure the communication between those.

So the two main problems need to be addressed were the isolation of the processes and to ensure the marshalling process between processes to copy data between them.

In .Net, application domains are designed in a way that separating the processes without resulting performance problems with passing data between them. The whole idea behind applications domains are a process can be divided in to several application domains(containers). Most probably application domain corresponds to single application. Even though there are different executables, if they are running in the context of the same process, theoretically they can directly see each other’s data and it should share the same address space. But CLR makes sure that it does not happen. .Net Remoting is one of the areas application domains come in to action.